Publications

2026

1. Rapid ice-marginal lake growth in Alaska driven by glacier retreat through bed overdeepenings

   Daniel McGrath, Louis Sass, William H. Armstrong, and 2 more authors

   Proceedings of the National Academy of Sciences, Mar 2026

   Abs DOI

   The number and cumulative area of ice-marginal lakes have expanded globally in recent decades, with many lakes residing in glacier-bed overdeepenings, which are subglacial basins formed through preferential glacial erosion. However, current lake expansion rates, key drivers of expansion, and maximum future lake extents are poorly quantified. This is notable because glacial lakes pose hazards, alter hydrologic and ecological systems, and, in some cases, accelerate glacier flow and retreat. Here, we quantify recent ice-marginal lake growth and use existing ice thickness and topographic data to map glacier-bed overdeepenings in Alaska as a predictor of recent and future locations of lake growth. Ice-marginal lakes in the region grew by +156 km2 (26 km2 y−1) between 2018 and 2024, representing a 50% increase relative to the 2009–2018 rate. Eighty percent of lake growth since 2018 occurred in mapped glacier-bed overdeepenings. Approximately 4,250 km2 (2,966 to 5,503 km2 accounting for ± ice thickness uncertainty) of the overdeepened area is connected to an ice-marginal lake, indicating the potential for more than fourfold lake growth of existing lakes as glaciers retreat. An additional 14,500 km2 (12,469 to 17,134 km2) of remaining glacier area resides on glacier-bed overdeepenings not connected to existing lakes, highlighting the potential for substantial new lake development. Velocities from lake-terminating glaciers show clear passive and dynamic endmembers on a continuum of glacier–lake coupling. Glaciers with ice-marginal lakes thinned 23 to 54% more than glaciers of similar area without lakes, underscoring the critical importance of dynamic glacier–lake coupling on the evolution of glaciers in Alaska.

2. Equilibrium Ridge-Crest Morphology Records External Forcing

   Michael J. Robinson, Joel S. Scheingross, Scott W. McCoy, and 1 more author

   Journal of Geophysical Research: Earth Surface, 2026

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2025JF008670

   Abs DOI

   Ridgeline longitudinal profiles map elevation along drainage divides from headwaters to basin outlets. Despite being present in every landscape on Earth and across many planetary bodies, it is unclear whether ridgeline longitudinal profiles have a characteristic morphology and how that morphology adjusts in response to changes in climate, tectonics, or the competence of underlying bedrock. We show that ridgelines have a characteristic two-part form, consisting of a lower-gradient, near-linear “crest” extending from the headwaters to near the basin outlet and a steep-sloped “snout” that connects the crest to the mouth of the river. We develop a theory that predicts crest morphology as the distribution of relief generated from rivers and hillslopes, and predicts that crest slope, like river slope, is sensitive to basin area as well as rock uplift rate, climate, and lithology. We show that normalizing average ridge crest slope for basin area, a metric we term crest steepness, can isolate changes in external forcing and/or lithology in steady-state basins. We test these predictions using measurements of natural crests from two quasi-steady-state landscapes, where a nearly two-fold increase in rock uplift rate results in an increase of over a factor of three in crest steepness, consistent with theory. Our results suggest that ridge crest steepness represents a topographic proxy of rock uplift rate, which can be combined with, or used in lieu of, other topographic proxies (e.g., channel steepness, hillslope angle, and hilltop curvature) to estimate variation in rock uplift across landscapes.

2025

1. Cascading land surface hazards as a nexus in the Earth system

   Brian J. Yanites, Marin K. Clark, Joshua J. Roering, and 17 more authors

   Science, Jun 2025

   Abs DOI

   This Review synthesizes progress and outlines a new framework for understanding how land surface hazards interact and propagate as sediment cascades across Earth’s surface, influenced by interactions among the atmosphere, biosphere, hydrosphere, and solid Earth. Recent research highlights a gap in understanding these interactions on human timescales, given rapid climatic change and urban expansion into hazard-prone zones. We review how surface processes such as coseismic landslides and post-fire debris flows form a complex sequence of events that exacerbate hazard susceptibility. Moreover, innovations in modeling, remote sensing, and critical zone science can offer new opportunities for quantifying cascading hazards. Looking forward, societal resilience can increase by transforming our understanding of cascading hazards through advances in integrating data into comprehensive models that link across Earth systems.

2. Improved Prediction of Postfire Debris Flows Through Rainfall Anomaly Maps

   David B. Cavagnaro, Scott W. McCoy, Matthew A. Thomas, and 2 more authors

   Geophysical Research Letters, 2025

   Abs DOI

   Predicting where runoff-generated debris flows might occur during rainfall on steep, recently burned terrain is challenging. Studies of mass-movement processes in unburned areas indicate that event locations are well-predicted by rainfall anomaly, R*, in which peak observed rainfall is normalized by local rainfall climatology. Here, we use remote and field methods to map debris flows triggered within the 2020 Dolan Fire burn area in coastal California, demonstrate that a short-duration R* metric predicts debris-flow occurrence more effectively than absolute peak intensity or longer-duration rainfall metrics, and show that incorporating an R* criterion into an existing debris-flow likelihood model can reduce false positive predictions and improve accuracy. We test R* at three other climatically distinct fires in California, demonstrating its utility for mapping likely debris-flow locations in different climates. We also consider how R* can benefit postfire debris-flow prediction given recent increases in climatological variability within individual burn perimeters.

3. Rainfall Thresholds for Postfire Debris‐Flow Initiation Vary With Short‐Duration Rainfall Climatology

   David B. Cavagnaro, Scott W. McCoy, Donald N. Lindsay, and 3 more authors

   Journal of Geophysical Research: Earth Surface, Jun 2025

   Abs DOI

   The size, frequency, and geographic scope of severe wildfires are expanding across the globe, including in the Western United States. Recently burned steeplands have an increased likelihood of debris flows, which pose hazards to downstream communities. The conditions for postfire debris‐flow initiation are commonly expressed as rainfall intensity‐duration thresholds, which can be estimated given sufficient observational history. However, the spread of wildfire across diverse climates poses a challenge for accurate threshold prediction in areas with limited observations. Studies of mass‐movement processes in unburned areas indicate that thresholds vary with local climate, such that higher rainfall rates are required for initiation in climates characterized by frequent intense rainfall. Here, we use three independent methods to test whether initiation of postfire runoff‐generated debris flows across the Western United States varies similarly with climate. Through the compilation of observed thresholds at various fires, analysis of the spatial density of observed debris flows, and quantification of feature importance at different spatial scales, we show that postfire debris‐flow initiation thresholds vary systematically with short‐duration rainfall‐intensity climatology. The predictive power of climatological data sets that are readily available before a fire occurs offers a much‐needed tool for hazard management in regions that are facing increased wildfire activity, have sparse observational history, and/or have limited resources for field‐based hazard assessment. Furthermore, if the observed variation in thresholds reflects long‐term adjustment of the landscape to local climate, rapid shifts in rainfall intensity related to climate change will likely induce spatially variable shifts in postfire debris‐flow likelihood.

4. Dip Angles of Mountain-Front Facets Encode Long-Term Slip Rates Along the Wasatch Normal Fault, USA

   William T. Struble, Scott W. McCoy, Gregory E. Tucker, and 3 more authors

   Geophysical Research Letters, 2025

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2025GL117713

   Abs DOI

   Mountains bounded by seismogenic normal faults are commonly decorated with facet slopes: planar slopes made of bedrock, or bedrock mantled by regolith, that rise above the fault. The steepness of such slopes is thought to reflect a balance between fault slip and erosion rate. We show that facet dip angles along the Wasatch fault zone (WFZ), USA, positively correlate with fault slip rates, such that relative variations in slip rate can be estimated from facet angle, and that by constraining millennial-scale facet erosion rates, absolute slip rates can be estimated. We calculate {\vphantom}}sim }100-ka average vertical slip rates of }0.37_-0.1^+0.36} mm/yr (mean and 90% confidence interval) along the central WFZ, consistent with estimates from features offset over similar timescales, but lower than Holocene rates. Our results provide evidence of slip rate acceleration on the WFZ, and demonstrate the potential of facet-angle analysis to estimate slip rates on range-bounding normal faults.

2024

1. A Robust Quantitative Method to Distinguish Runoff-Generated Debris Flows From Floods

   David B. Cavagnaro, Scott W. McCoy, Jason W. Kean, and 4 more authors

   Geophysical Research Letters, 2024

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2024GL109768

   Abs DOI

   Debris flows and floods generated by rainfall runoff occur in rocky mountainous landscapes and burned steeplands. Flow type is commonly identified post-event through interpretation of depositional structures, but these may be poorly preserved or misinterpreted. Prior research indicates that discharge magnitude is commonly amplified in debris flows relative to floods due to volumetric bulking and increased frictional resistance. Here, we use this flow amplification to develop a metric (Q*) to separate debris flows from floods based on the ratio of observed peak discharge to the theoretical maximum water discharge from rainfall runoff. We compile 642 observations of floods and debris flows and demonstrate that Q* distinguishes flow type to ∼92% accuracy. Q* allows for accurate identification of debris flows through simple channel cross-section surveys rather than through qualitative interpretation of deposits, and therefore should increase the performance of models and engineered structures that require accurate flow-type observations.

2. Waterfalls Alter Reach-Scale Fluvial Erosion Rates: Evidence From Field Data and Process Modeling

   Sophie D. Rothman, Joel S. Scheingross, and Scott W. McCoy

   Journal of Geophysical Research: Earth Surface, 2024

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2024JF007792

   Abs DOI

   Waterfalls are often interpreted as transient, upstream-propagating features that mark changes in external conditions. Thus, waterfalls are commonly used to infer past tectonic and climatic forcing, making understanding the controls on waterfall erosion central to predicting how external perturbations move through landscapes. Surprisingly, there exist few direct field measurements of waterfall erosion, and existing waterfall retreat measurements are rarely paired with measurements of waterfall morphology and frequency, which, theory suggests, modulate retreat rates. This lack of data limits our ability to test existing theory and explore how waterfalls alter reach-scale bedrock erosion rates. Here, we use cosmogenic 10Be accumulated in bedrock riverbeds to measure erosion rates in fluvial reaches with varying waterfall frequency and morphology. We find that waterfall-rich reaches erode one to five times faster than the landscape average, and that reach-averaged erosion rates increase with increasing waterfall frequency. We develop a new, process-based model combining waterfall and planar-channel erosion to explore mechanistic controls on the relative erosion rate between waterfall-rich and waterfall-free reaches. This model predicts that reach-averaged erosion rates increase with waterfall frequency at low sediment supply, consistent with our field measurements, but that waterfalls can also slow reach-averaged erosion rates for high sediment supply, large grain sizes, low water discharge, or large plunge pools. Our work is consistent with previous suggestions that waterfall erosion rates may decrease in low drainage areas and can influence long-profile morphology.

3. Seasonal to decadal dynamics of supraglacial lakes on debris-covered glaciers in the Khumbu region, Nepal

   Lucas Zeller, Daniel McGrath, Scott W. McCoy, and 1 more author

   The Cryosphere, Feb 2024

   Abs DOI

   Supraglacial lakes (SGLs) play an important role in debris-covered glacier (DCG) systems by enabling efficient interactions between the supraglacial, englacial, and subglacial environments. Developing a better understanding of the short-term and long-term development of these features is needed to constrain DCG evolution and the hazards posed to downstream communities, ecosystems, and infrastructure from rapid drainage. In this study, we present an analysis of supraglacial lakes on eight DCGs in the Khumbu region of Nepal by automating SGL identification in PlanetScope, Sentinel-2, and Landsat 5–9 images. We identify a regular annual cycle in SGL area, with lakes covering approximately twice as much area during their maximum annual extent (in the pre-monsoon season) than their minimum annual extent (in the post-monsoon season). The high spatiotemporal resolution of PlanetScope imagery (∼ daily, 3 m) shows that this cycle is driven by the appearance and expansion of small lakes in the upper debris-covered regions of these glaciers throughout the winter. Decadal-scale expansion of large, near-terminus lakes was identified on four of the glaciers (Khumbu, Lhotse, Nuptse, and Ambulapcha), while the remaining four showed no significant increases over the study period. The seasonal variation in SGL area is of comparable or greater magnitude as decadal-scale changes, highlighting the importance of accounting for this seasonality when interpreting long-term records of SGL changes from sparse observations. The complex spatiotemporal patterns revealed in our analysis are not captured in existing regional-scale glacial lake databases, suggesting that more targeted efforts are needed to capture the true variability of SGLs on large scales.

2023

1. Debris‐Flow Process Controls on Steepland Morphology in the San Gabriel Mountains, California

   William T. Struble, Luke A. McGuire, Scott W. McCoy, and 2 more authors

   Journal of Geophysical Research: Earth Surface, Jul 2023

   Abs DOI

   Steep landscapes evolve largely by debris flows, in addition to fluvial and hillslope processes. Abundant field observations document that debris flows incise valley bottoms and transport substantial sediment volumes, yet their contributions to steepland morphology remain uncertain. This has, in turn, limited the development of debris-flow incision rate formulations that produce morphology consistent with natural landscapes. In many landscapes, including the San Gabriel Mountains (SGM), California, steady-state fluvial channel longitudinal profiles are concave-up and exhibit a power-law relationship between channel slope and drainage area. At low drainage areas, however, valley slopes become nearly constant. These topographic forms result in a characteristically curved slope-area signature in log-log space. Here, we use a one-dimensional landform evolution model that incorporates debris-flow erosion to reproduce the relationship between this curved slope-area signature and erosion rate in the SGM. Topographic analysis indicates that the drainage area at which steepland valleys transition to fluvial channels correlates with measured erosion rates in the SGM, and our model results reproduce these relationships. Further, the model only produces realistic valley profiles when parameters that dictate the relationship between debris-flow erosion, valley-bottom slope, and debris-flow depth are within a narrow range. This result helps place constraints on the mathematical form of a debrisflow incision law. Finally, modeled fluvial incision outpaces debris-flow erosion at drainage areas less than those at which valleys morphologically transition from near-invariant slopes to concave profiles. This result emphasizes the critical role of debris-flow incision for setting steepland form, even as fluvial incision becomes the dominant incisional process.

2. The spatial distribution of debris flows in relation to observed rainfall anomalies: Insights from the Dolan Fire, California

   David B. Cavagnaro, Scott W. McCoy, Matthew A. Thomas, and 2 more authors

   E3S Web of Conferences, 2023

   Abs DOI

   A range of hydrologic responses can be observed in steep, recently burned terrain, which makes predicting the spatial distribution of large debris flows challenging. Studies from rainfall-induced landslides in unburned areas show evidence of hydroclimatic tuning of landslide triggering, such that the spatial distribution of events is best predicted by the observed rainfall anomaly relative to climatic norms rather than by absolute rainfall. In this paper, we test whether the spatial distribution of debris flows in response to rainfall can be similarly predicted by rainfall anomaly. The 520 km\textlesssup\textgreater2\textlesssup/\textgreater Dolan Fire burn scar in Monterey County, California, USA, spans a sharp hydroclimatic gradient and experienced a widespread storm in January 2021 that triggered floods and debris flows, providing a natural experiment in which to test this hypothesis. In this study, we use remote and field methods to map debris-flow response and examine its spatial heterogeneity. Together with rainfall data, our mapping reveals that the observed anomalies in peak 15-min rainfall intensity (I\textlesssub\textgreater15\textlesssub/\textgreater) relative to the intensity of the 1-yr return interval storm predict debris-flow occurrence better than the absolute peak I\textlesssub\textgreater15\textlesssub/\textgreater. Our findings indicate that debris-flow processes and threshold rainfall required for debris-flow initiation may be tuned to local hydroclimate.

3. A robust method to identify the occurrence of a runoff-generated debris flow

   Scott W. McCoy, David B. Cavagnaro, Jason W. Kean, and 2 more authors

   E3S Web of Conferences, 2023

   Abs DOI

   Debris flows generated by rainfall runoff can occur in rocky alpine landscapes and burned steeplands. Runoff-generated debris-flow events are commonly composed of a series of dense granular surge fronts separated by water-rich flows. Owing to this intra-event variability in flow composition and mechanics, post-event interpretations of preserved sedimentary deposits, or lack thereof, can result in a dizzying mix of interpretations that range from clearwater flow to debris flow. Accurate identification of the presence or absence of a debris flow during a runoff event is critical for building empirical models used to predict likelihood of debris-flow occurrence, rainfall thresholds, and flow properties. Here, we propose a simple, quantitative method to identify the occurrence of a runoff-generated debris flow, based on a dimensionless discharge \textlessi\textgreaterQ\textlessi/\textgreater\textlesssub\textgreater*\textlesssub/\textgreater calculated as the ratio of the peak event discharge \textlessi\textgreaterQ\textlesssub\textgreaterp\textlesssub/\textgreater\textlessi/\textgreater to the theoretical maximum clearwater runoff rate \textlessi\textgreaterQ\textlesssub\textgreaterw\textlesssub/\textgreater\textlessi/\textgreater. Using a preliminary compilation of \textlessi\textgreaterQ\textlessi/\textgreater\textlesssub\textgreater*\textlesssub/\textgreater values from floods and runoff-generated debris flows, we find 98% of floods have \textlessi\textgreaterQ\textlessi/\textgreater\textlesssub\textgreater*\textlesssub/\textgreater values \textless 1.6, whereas 91% of debris flows have \textlessi\textgreaterQ\textlessi/\textgreater\textlesssub\textgreater*\textlesssub/\textgreater values greater than 1.6. Estimating \textlessi\textgreaterQ\textlessi/\textgreater\textlesssub\textgreater*\textlesssub/\textgreater is typically straightforward as part of standard post-event reconnaissance if suitable rainfall estimates are available, and appears to be a robust indicator that runoff-generated debris flows traversed a particular portion of a valley network.

4. Steady-state forms of channel profiles shaped by debris flow and fluvial processes

   Luke A. McGuire, Scott W. McCoy, Odin Marc, and 2 more authors

   Earth Surface Dynamics, Nov 2023

   Abs DOI

   Debris flows regularly traverse bedrock channels that dissect steep landscapes, but our understanding of bedrock erosion by debris flows and their impact on steepland morphology is still rudimentary. Quantitative models of steep bedrock channel networks are based on geomorphic transport laws designed to represent erosion by water-dominated flows. To quantify the impact of debris flow erosion on steep channel network form, it is first necessary to develop methods to estimate spatial variations in bulk debris flow properties (e.g., flow depth, velocity) throughout the channel network that can be integrated into landscape evolution models. Here, we propose and evaluate two methods to estimate spatial variations in bulk debris flow properties along the length of a channel profile. We incorporate both methods into a model designed to simulate the evolution of longitudinal channel profiles that evolve in response to debris flow and fluvial processes. To explore this model framework, we propose a general family of debris flow erosion laws where erosion rate is a function of debris flow depth and channel slope. Model results indicate that erosion by debris flows can explain the occurrence of a scaling break in the slope–area curve at low-drainage areas and that upper-network channel morphology may be useful for inferring catchment-averaged erosion rates in quasi-steady landscapes. Validating specific forms of a debris flow incision law, however, would require more detailed model–data comparisons in specific landscapes where input parameters and channel morphometry can be better constrained. Results improve our ability to interpret topographic signals within steep channel networks and identify observational targets critical for constraining a debris flow incision law.

5. Quantifying Variability of Incipient-Motion Thresholds in Gravel-Bedded Rivers Using a Grain-Scale Force-Balance Model

   Scott A. Feehan, Scott W. McCoy, Joel S. Scheingross, and 1 more author

   Journal of Geophysical Research: Earth Surface, 2023

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2023JF007162

   Abs DOI

   Predicting thresholds of sediment motion is critical for a range of applications involving sediment transport. However, thresholds for sediment motion can vary over an order of magnitude for a single characteristic flow and bed configuration. Lacking simple ways to incorporate this variability, many assume thresholds are constant for rough, turbulent flow. Here, we quantify variability of incipient-motion thresholds based on a commonly used grain-scale force-balance model, with model parameter distributions determined from published experiments. We show that variability in the threshold of motion within the 2D force-balance model occurs predominantly due to variability in the lift coefficient and grain protrusion, and secondarily due to drag coefficient variability. For a known grain size, the mean threshold of motion, and variability about the mean, can be predicted from a family of power laws. These power laws can be altered with site-specific parameter distributions, allowing for site-specific application to well-studied reaches and other planets. Using compiled flume and field data we show that constraining force-balance parameter distributions with independent data results in narrower distributions of the predicted threshold of motion, consistent with constrained flume experiments. This analysis highlights that while the threshold of sediment motion is variable, the magnitude of variability is predictable within the force-balance model based on site-specific physical constraints of local flow and bed conditions.

6. Unchanged frequency and decreasing magnitude of outbursts from ice-dammed lakes in Alaska

   B. Rick, D. McGrath, S. W. McCoy, and 1 more author

   Nature Communications, Oct 2023

   Number: 1

   Abs DOI

   Glacial lakes can form and grow due to glacial retreat, and rapid lake drainage can produce destructive floods. Outburst flood compilations show a temporal increase in frequency; however, recent studies highlight the role of observational bias, creating uncertainty about current and future glacial-lake hazards. Here, we focus on the Alaska region, which generated a third of previously documented outbursts globally. Using multitemporal satellite imagery, we documented 1150 drainages from 106 ice-dammed lakes between 1985 and 2020. Documented events became more frequent over time, however, accounting for increasing image availability reveals no significant increase occurred. Most lakes decreased in area and volume, suggesting a reduction in regional flood hazard. Our satellite-based approach documented 60% more events in a 35-year period than had previously been documented over 100 years. This suggests that outburst floods have historically been underreported and warrants systematic study of other regions.

7. Impacts of Spontaneous Waterfall Development on Bedrock River Longitudinal Profile Morphology

   Sophie D. Rothman, Joel S. Scheingross, Scott W. McCoy, and 1 more author

   Journal of Geophysical Research: Earth Surface, 2023

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2022JF007057

   Abs DOI

   River profiles are shaped by climatic and tectonic history, lithology, and internal feedbacks between flow hydraulics, sediment transport and erosion. In steep channels, waterfalls may self-form without changes in external forcing (i.e., autogenic formation) and erode at rates faster or slower than an equivalent channel without waterfalls. We use a 1-D numerical model to investigate how self-formed waterfalls alter the morphology of bedrock river longitudinal profiles. We modify the standard stream power model to include a slope threshold above which waterfalls spontaneously form and a rate constant allowing waterfalls to erode faster or slower than other fluvial processes. Using this model, we explore how waterfall formation alters both steady state and transient longitudinal profile forms. Our model predicts that fast waterfalls create km-scale reaches in a dynamic equilibrium with channel slope held approximately constant at the threshold slope for waterfall formation, while slow waterfalls can create local channel slope maxima at the location of slow waterfall development. Furthermore, slow waterfall profiles integrate past base level histories, leading to multiple possible profile forms, even at steady-state. Consistency between our model predictions and field observations of waterfall-rich rivers in the Kings and Kaweah drainages in the southern Sierra Nevada, California, supports the hypothesis that waterfall formation can modulate river profiles in nature. Our findings may help identify how bedrock channels are influenced by waterfall erosion and aid in distinguishing between signatures of external and internal perturbations, thereby strengthening our ability to interpret past climate and tectonic changes from river longitudinal profiles.

8. Postfire hydrologic response along the Central California (USA) coast: insights for the emergency assessment of postfire debris-flow hazards

   Matthew A. Thomas, Jason W. Kean, Scott W. McCoy, and 8 more authors

   Landslides, Jul 2023

   Abs DOI

   The steep, tectonically active terrain along the Central California (USA) coast is well known to produce deadly and destructive debris flows. However, the extent to which fire affects debris-flow susceptibility in this region is an open question. We documented the occurrence of postfire debris floods and flows following the landfall of a storm that delivered intense rainfall across multiple burn areas. We used this inventory to evaluate the predictive performance of the US Geological Survey M1 likelihood model, a tool that presently underlies the emergency assessment of postfire debris-flow hazards in the western USA. To test model performance, we used the threat score skill statistic and found that the rainfall thresholds estimated by the M1 model for the Central California coast performed similarly to training (Southern California) and testing (Intermountain West) data associated with the original model calibration. Model performance decreased when differentiating between “minor” and “major” postfire hydrologic response types, which weigh effects on human life and infrastructure. Our results underscore that the problem of false positives is a major challenge for developing accurate rainfall thresholds for the occurrence of postfire debris flows. As wildfire activity increases throughout the western USA, so too will the demand for the assessment of postfire debris-flow hazards. We conclude that additional collection of field-verified inventories of postfire hydrologic response will be critical to prioritize which model variables may be suitable candidates for regional calibration or replacement.

9. The Rainfall Intensity-Duration Control of Debris Flows After Wildfire

   Matthew A. Thomas, Donald N. Lindsay, David B. Cavagnaro, and 3 more authors

   Geophysical Research Letters, 2023

   _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2023GL103645

   Abs DOI

   Increased wildfire activity in the western United States has exposed regional gaps in our understanding of postfire debris-flow generation. To address this problem, we characterized flows in an unstudied area to test the rainfall intensity-duration control of the hazard. Our rainfall measurements and field observations from the northern Sierra Nevada (California, USA) show that debris flows resulted from a short burst rainfall during a low-accumulation storm. In contrast, a much higher accumulation storm (∼10 times more rainfall) with lower short-duration rainfall rates only produced low-hazard flooding. We conclude that total storm rainfall is not an ideal metric for identifying the rainfall conditions that initiate runoff-generated debris flows in the first year after wildfire. Rather, a focus on short-duration (\textless1 hr), high-intensity rainfall that can occur during localized thunderstorms, or bands of intense rainfall during prolonged rainstorms, is more beneficial for the purposes of hazard assessment and warning.

10. The Erosional Signature of Drainage Divide Motion Along the Blue Ridge Escarpment

    Maya F. Stokes, Isaac J. Larsen, Samuel L. Goldberg, and 3 more authors

    Journal of Geophysical Research: Earth Surface, 2023

    _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1029/2022JF006757

    Abs DOI

    The planform rearrangement of river basins is recognized as an important process for landscape evolution. The boundaries of river basins can shift either through gradual drainage divide migration or discrete river captures, but the methods for identifying these processes often rely on topographic evidence that remains otherwise untested. Moreover, efforts to understand the relative importance of either process are hampered by a lack of age constraints on river captures. We use 10Be-derived erosion rates to test whether, and how, divide motion is occurring at three locations along the Blue Ridge Escarpment in the Appalachian Mountains. In the Pee Dee River basin, we find that the escarpment is migrating inland up to 45 m/Myr, consistent with topographic evidence for gradual divide migration. In the Dan River basin, erosion rates support the topographic evidence for river capture, and we use a forward model of river incision to estimate that the capture likely occurred in the past 12.5 Myr. In the South Fork Roanoke River basin, where the presence of a knickzone has been interpreted as evidence that a river capture initiated a pulse of faster erosion, we instead measure nearly uniform tributary erosion rates above and within the mainstem knickzone. Simulations show that river incision into a more erodible layer of rock, with or without a river capture, could produce the observed topography and erosion rates in the South Fork Roanoke River. Our results show how multiple lines of evidence can illuminate the rates and mechanisms of river basin reorganization.

2022

1. Dam type and lake location characterize ice-marginal lake area change in Alaska and NW Canada between 1984 and 2019

   Brianna Rick, Daniel McGrath, William Armstrong, and 1 more author

   The Cryosphere, Jan 2022

   Abs DOI

   \textlessp\textgreater\textlessstrong class="journal-contentHeaderColor"\textgreaterAbstract.\textless/strong\textgreater Ice-marginal lakes impact glacier mass balance, water resources, and ecosystem dynamics and can produce catastrophic glacial lake outburst floods (GLOFs) via sudden drainage. Multitemporal inventories of ice-marginal lakes are a critical first step in understanding the drivers of historic change, predicting future lake evolution, and assessing GLOF hazards. Here, we use Landsat-era satellite imagery and supervised classification to semi-automatically delineate lake outlines for four \textlessspan class="inline-formula"\textgreater∼5\textless/span\textgreater-year time periods between 1984 and 2019 in Alaska and northwest Canada. Overall, ice-marginal lakes in the region have grown in total number (\textlessspan class="inline-formula"\textgreater+183\textless/span\textgreater lakes, 38 % increase) and area (\textlessspan class="inline-formula"\textgreater+483\textless/span\textgreater km\textlessspan class="inline-formula"\textgreater^\textrm2\textless/span\textgreater, 59 % increase) between the time periods of 1984–1988 and 2016–2019. However, changes in lake numbers and area were notably unsteady and nonuniform. We demonstrate that lake area changes are connected to dam type (moraine, bedrock, ice, or supraglacial) and topological position (proglacial, detached, unconnected, ice, or supraglacial), with important differences in lake behavior between the sub-groups. In strong contrast to all other dam types, ice-dammed lakes decreased in number (six fewer, 9 % decrease) and area (\textlessspan class="inline-formula"\textgreater−51\textless/span\textgreater km\textlessspan class="inline-formula"\textgreater^\textrm2\textless/span\textgreater, 40 % decrease), while moraine-dammed lakes increased (56 more, 26 % and \textlessspan class="inline-formula"\textgreater+479\textless/span\textgreater km\textlessspan class="inline-formula"\textgreater^\textrm2\textless/span\textgreater, 87 % increase for number and area, respectively) at a faster rate than the average when considering all dam types together. Proglacial lakes experienced the largest area changes and rate of change out of any lake position throughout the period of study and moraine-dammed lakes which experienced the largest increases are associated with clean-ice glaciers (\textlessspan class="inline-formula"\textgreater<19\textless/span\textgreater % debris cover). By tracking individual lakes through time and categorizing lakes by dam type, subregion, and topological position, we are able to parse trends that would otherwise be aliased if these characteristics were not considered. This work highlights the importance of such lake characterization when performing ice-marginal lake inventories and provides insight into the physical processes driving recent ice-marginal lake evolution.\textless/p\textgreater

2021

1. Mapping Potentially Acid Generating Material on Abandoned Mine Lands Using Remotely Piloted Aerial Systems

   Alison S. Cramer, Wendy M. Calvin, Scott W. McCoy, and 3 more authors

   Minerals, Apr 2021

   Number: 4

   Abs DOI

   Weathering and transport of potentially acid generating material (PAGM) at abandoned mines can degrade downstream environments and contaminate water resources. Monitoring the thousands of abandoned mine lands (AMLs) for exposed PAGM using field surveys is time intensive. Here, we explore the use of Remotely Piloted Aerial Systems (RPASs) as a complementary remote sensing platform to map the spatial and temporal changes of PAGM across a mine waste rock pile on an AML. We focus on testing the ability of established supervised and unsupervised classification algorithms to map PAGM on imagery with very high spatial resolution, but low spectral sampling. At the Perry Canyon, NV, USA AML, we carried out six flights over a 29-month period, using a RPAS equipped with a 5-band multispectral sensor measuring in the visible to near infrared (400–1000 nm). We built six different 3 cm resolution orthorectified reflectance maps, and our tests using supervised and unsupervised classifications revealed benefits to each approach. Supervised classification schemes allowed accurate mapping of classes that lacked published spectral libraries, such as acid mine drainage (AMD) and efflorescent mineral salts (EMS). The unsupervised method produced similar maps of PAGM, as compared to supervised schemes, but with little user input. Our classified multi-temporal maps, validated with multiple field and lab-based methods, revealed persistent and slowly growing ‘hotspots’ of jarosite on the mine waste rock pile, whereas EMS exhibit more rapid fluctuations in extent. The mapping methods we detail for a RPAS carrying a broadband multispectral sensor can be applied extensively to AMLs. Our methods show promise to increase the spatial and temporal coverage of accurate maps critical for environmental monitoring and reclamation efforts over AMLs.

2. Watershed Sediment Yield Following the 2018 Carr Fire, Whiskeytown National Recreation Area, Northern California

   Amy E. East, Joshua B. Logan, Peter Dartnell, and 4 more authors

   Earth and Space Science, Sep 2021

   Abs DOI

   Wildfire risk has increased in recent decades over many regions, due to warming climate and other factors. Increased sediment export from recently burned landscapes can jeopardize downstream infrastructure and water resources, but physical landscape response to fire has not been quantified for some at-risk areas, including much of northern California, USA. We measured sediment yield from three watersheds (13–29 km2) that drain to Whiskeytown Lake, California, within the area burned by the 2018 Carr Fire. Structure-from-Motion photogrammetry on aerial images combined with sonar bathymetric mapping of submerged areas indicated first-year post-fire sediment yields of 4,080 ± 598 t/km2 (Brandy Creek), 2,700 ± 527 t/km2 (Boulder Creek), and 305 ± 58.0 t/km2 (Whiskey Creek)—some of the first post-fire yields measured in northern California and 64, 42, and 4.8 times greater than pre-fire yields, respectively. These were measured during a wet year and resulted largely from rilling erosion and fluvial sediment transport, without post-fire debris flows. Rilling preferentially developed in contact with dirt roads, aided by thin soils and exposed bedrock, and on slopes vegetated by chaparral pre-fire. The second post-fire year (a dry year) was characterized by fluvial reworking and delta progradation of the first-year deposits and relatively little new sediment export. First-year sedimentation of 111,000 m3 represented minor loss of storage capacity in Whiskeytown Lake but would be detrimental to smaller reservoirs; in general, increased sediment yields from western US watersheds as fire and extreme rainfall increase will likely pose risks to water quality and storage.

2020

1. The shaping of erosional landscapes by internal dynamics

   Joel S. Scheingross, Ajay B. Limaye, Scott W. McCoy, and 1 more author

   Nature Reviews Earth & Environment, Dec 2020

   Number: 12

   Abs DOI

   Erosional landscapes transport sediment downstream, host natural hazards and are geologically active. While perturbations in external forcing, particularly climate and tectonics, sculpt erosional landscapes, similar landforms can be created by internal dynamics, that is, feedbacks between topography, erosion and sediment transport that occur independent of external perturbations. Internal system responses, termed autogenic dynamics, can remain active as landscapes adjust to perturbations in forcing, allowing for complex responses to external perturbations that potentially obscure links between external forcing, topographic form and sedimentary archives. Autogenic dynamics are being increasingly recognized in depositional systems, yet understanding of autogenic dynamics in erosional landscapes is nascent. In this Review, we discuss the mechanisms that contribute to internal dynamics in erosional landscapes. We use examples of autogenic terrace formation, knickpoint formation and river-basin reorganization to show how autogenic dynamics that occur over spatial scales of metres and temporal scales of hours can influence the evolution of mountain ranges over Myr periods. Unravelling the mechanics of autogenic processes allows the interplay of internal dynamics and external forcing to be explored and provides a framework to assess the influence of erosional processes in the geologic record.

2. Modeling the Shape and Evolution of Normal-Fault Facets

   Gregory E. Tucker, Daniel E. J. Hobley, Scott W. McCoy, and 1 more author

   Journal of Geophysical Research: Earth Surface, 2020

   _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2019JF005305

   Abs DOI

   Facets formed along the footwalls of active normal-fault blocks display a variety of longitudinal profile forms, with variations in gradient, shape, degree of soil cover, and presence or absence of a slope break at the fault trace. We show that a two-dimensional, process-oriented cellular automaton model of facet profile evolution can account for the observed morphologic diversity. The model uses two dimensionless parameters to represent fault slip, progressive rock weathering, and downslope colluvial-soil transport driven by gravity and stochastic disturbance events. The parameters represent rock weathering and soil disturbance rates, respectively, scaled by fault slip rate; both can be derived from field-estimated rate coefficients. In the model’s transport-limited regime, slope gradient depends on the ratio of disturbance to slip rate, with a maximum that represents the angle of repose for colluvium. In this regime, facet evolution is consistent with nonlinear diffusion models of soil-mantled hillslope evolution. Under the weathering-limited regime, bedrock becomes partly exposed but microtopography helps trap some colluvium even when facet gradient exceeds the threshold angle. Whereas the model predicts a continuous gradient from footwall to colluvial wedge under transport-limited behavior, fully weathering-limited facets tend to develop a slope break between footwall and basal colluvium as a result of reduced transport efficiency on the rocky footwall slope. To the extent that the model provides a reasonable analogy for natural facets, its behavior suggests that facet profile morphology can provide useful constraints on relative potential rates of rock weathering, soil disturbance, and fault slip.

3. Geomorphic signatures of the transient fluvial response to tilting

   Helen W. Beeson and Scott W. McCoy

   Earth Surface Dynamics, Feb 2020

   Abs DOI

   \textlessp\textgreater\textlessstrong\textgreaterAbstract.\textless/strong\textgreater Nonuniform rock uplift in the form of tilting has been documented in convergent margins, postorogenic landscapes, and extensional provinces. Despite the prevalence of tilting, the transient fluvial response to tilting has not been quantified such that tectonic histories involving tilt can be extracted from river network forms. We used numerical landscape evolution models to characterize the transient erosional response of a river network initially at equilibrium to rapid tilting. We focus on the case of punctuated rigid-block tilting, though we explore longer-duration tilting events and nonuniform uplift that deviates from perfect rigid-block tilting such as that observed when bending an elastic plate or with more pronounced internal deformation of a fault-bounded block. Using a model river network composed of linked 1-D river longitudinal profile evolution models, we show that the transient response to a punctuated rigid-block tilting event creates a suite of characteristic forms or geomorphic signatures in mainstem and tributary profiles that collectively are distinct from those generated by other perturbations, such as a step change in the uniform rock uplift rate or a major truncation of the headwater drainage area, that push a river network away from equilibrium. These signatures include (1) a knickpoint in the mainstem that separates a downstream profile with uniform steepness (i.e., channel gradient normalized for drainage area) from an upstream profile with nonuniform steepness, with the mainstem above the knickpoint more out of equilibrium than the tributaries following forward tilting toward the outlet, versus the mainstem less out of equilibrium than the tributaries following back tilting toward the headwaters; (2) a pattern of mainstem incision below paleo-topography markers that increases linearly up to the mainstem knickpoint or vice versa following back tilting; and (3) tributary knickzones with nonuniform steepness that mirrors that of the mainstem upstream of the slope-break knickpoint.\textless/p\textgreater \textlessp\textgreaterImmediately after a punctuated tilting event, knickpoints form at the mainstem outlet and each mainstem–tributary junction. Time since the cessation of rapid tilting is recorded by the mainstem knickpoint location relative to base level and by the upstream end of tributary knickzones relative to the mainstem–tributary junction. Tilt magnitude is recorded in the spatial gradient of mainstem incision depth and, in the forward tilting case, also by the spatial gradient in tributary knickzone drop height. Heterogeneous lithology can modulate the transient response to tilting and, post tilt, knickpoints can form anywhere in a stream network where more erodible rock occurs upstream of less erodible rock. With a full 2-D model, we show that stream segments flowing in the tilt direction have elevated channel gradient early in the transient response. Tilting is also reflected in network topologic changes via stream capture oriented in the direction of tilt. As an example of how these geomorphic signatures can be used in concert with each other to estimate the timing and magnitude of a tilting event, we show a sample of rivers from two field sites: the Sierra Nevada, California, USA, and the Sierra San Pedro Mártir, Baja California, Mexico, two ranges thought to have been tilted westward toward river outlets in the late Cenozoic.\textless/p\textgreater

2019

1. Post-fire debris-flow hazard analysis for Interstate 80, Truckee River Canyon, near the California-Nevada state line, USA.

   Garrett Felling, Andrew Myers, and Scott W McCoy

   2019

   Abs

   On July 9, 2017, west of the California-Nevada state line, USA, the 2.8 km2 Farad fire burned steep slopes above Interstate 80. On the evening of August 18, 2017, a localized convective storm produced short-duration, moderate-intensity rain, which in turn triggered debris flows in a historically inactive basin. These flows impacted Interstate 80. At least four additional debris flows, not related to wildfire, have occurred along this section of road since 2013, but during much higher intensity rainfall. Here we utilize the history of pre- and post-fire debris flows along this section of Interstate 80 to explore the impacts of wildfire on debris flows. Specifically, we combine pre- and post-fire rainfall data and field measurements with empirical debris-flow models to quantify the impacts wildfire had on debris-flow generation and to estimate the likelihood and magnitude of future events. A characteristic pre-fire debris flow occurred on July 25, 2013 in a susceptible path ~30 minutes after rainfall began and during peak 15-minute intensities of ~50 mm/hr. This event closed both east and westbound lanes of Interstate 80. No other nearby paths had debris flows during this rainstorm. In contrast, one month post-fire, on August 18, 2017 a debris flow occurred in a historically inactive path, but within an area of high burn severity. Debris-flow initiation occurred ~30 minutes after the beginning of rainfall, but with peak 15-minute intensities of only ~26 mm/hr. This amplified rainfall-runoff response is consistent with fire-induced changes in soil hydraulic properties for which we measured post-fire decreases of a factor of 2 in field-saturated hydraulic conductivity and a factor of 4 in sorptivity. From field measurements, total volume estimates for the August 18, 2017 post-fire debris-flow event ranged between 1270 and 4700 m3 depending on assumptions regarding pre-event channel geometry and volume of hillslope sediment transported. A shallow landslide that liquefied and flowed into the channel contributed ~450 m of material and was apparently triggered by concentrated overland flow off an old road into the toe slope of a much older deepseated landslide. Debris flows eroded most of the travel path above the fan to bedrock and contributed \textgreater850 m of debris, at a nearly uniform spatial rate, both of which suggest this event was likely limited by sediment availability. Just 100-150 m above Interstate 80 the flow transported boulders with maximum diameters in excess of 1 m, at peak velocities of ~2-5 m/s. We used the analysis of the August 18, 2017 debris-flow event to verify empirical equations developed by the USGS for predicting the probability, total volume, and runout distance of post-fire debris flows. We found good agreement between model output and observations and hence used these equations to predict characteristics of debris flows likely to occur in the near future.

2018

1. Transience of the North American High Plains landscape and its impact on surface water

   Sean D. Willett, Scott W. McCoy, and Helen W. Beeson

   Nature, Sep 2018

   Abs DOI

   The High Plains region of North America is in a transient state, with a younger, efficient network of river channels progressively cannibalizing an older, less efficient region, aiding water retention for wetlands and groundwater recharge.

2. A lattice grain model of hillslope evolution

   Gregory E. Tucker, Scott W. McCoy, and Daniel E. J. Hobley

   Earth Surface Dynamics, Jul 2018

   Abs DOI

   \textlessp\textgreater\textlessstrong\textgreaterAbstract.\textless/strong\textgreater This paper describes and explores a new continuous-time stochastic cellular automaton model of hillslope evolution. The Grain Hill model provides a computational framework with which to study slope forms that arise from stochastic disturbance and rock weathering events. The model operates on a hexagonal lattice, with cell states representing fluid, rock, and grain aggregates that are either stationary or in a state of motion in one of the six cardinal lattice directions. Cells representing near-surface soil material undergo stochastic disturbance events, in which initially stationary material is put into motion. Net downslope transport emerges from the greater likelihood for disturbed material to move downhill than to move uphill. Cells representing rock undergo stochastic weathering events in which the rock is converted into regolith. The model can reproduce a range of common slope forms, from fully soil mantled to rocky or partially mantled, and from convex-upward to planar shapes. An optional additional state represents large blocks that cannot be displaced upward by disturbance events. With the addition of this state, the model captures the morphology of hogbacks, scarps, and similar features. In its simplest form, the model has only three process parameters, which represent disturbance frequency, characteristic disturbance depth, and base-level lowering rate, respectively. Incorporating physical weathering of rock adds one additional parameter, representing the characteristic rock weathering rate. These parameters are not arbitrary but rather have a direct link with corresponding parameters in continuum theory. Comparison between observed and modeled slope forms demonstrates that the model can reproduce both the shape and scale of real hillslope profiles. Model experiments highlight the importance of regolith cover fraction in governing both the downslope mass transport rate and the rate of physical weathering. Equilibrium rocky hillslope profiles are possible even when the rate of base-level lowering exceeds the nominal bare-rock weathering rate, because increases in both slope gradient and roughness can allow for rock weathering rates that are greater than the flat-surface maximum. Examples of transient relaxation of steep, rocky slopes predict the formation of a regolith-mantled pediment that migrates headward through time while maintaining a sharp slope break.\textless/p\textgreater

2017

1. Hydrologic and geomorphic changes resulting from episodic glacial lake outburst floods: Rio Colonia, Patagonia, Chile

   J. Jacquet, S. W. McCoy, D. McGrath, and 5 more authors

   Geophysical Research Letters, Jan 2017

   Abs DOI

   Glacial lake outburst floods (GLOFs) are a prominent but poorly understood cryospheric hazard in a warming climate. We quantify the hydrologic and geomorphic response to 21 episodic GLOFs that began in April 2008 using multitemporal satellite imagery and field observations. Peak discharge exiting the source lake became progressively muted downstream. At ~40–60 km downstream, where the floods entered and traveled down the main stem Rio Baker, peak discharges were generally \textless 2000 m3 sÀ1, although these flows were still \textgreater1–2 times the peak annual discharge of this system, Chile’s largest river by volume. As such, caution must be applied to empirical relationships relating lake volume to peak discharge, as the latter is dependent on where this observation is made along the flood path. The GLOFs and subsequent periods of free drainage resulted in \textgreater 40 m of incision, the net removal of ~25 × 106 m3 of sediment from the source lake basin, and a nonsteady channel configuration downstream. These results demonstrate that GLOFs sourced from low-order tributaries can produce significant floods on major main stem rivers, in addition to significantly altering sediment dynamics.

2. Geometric disequilibrium of river basins produces long-lived transient landscapes

   Helen W. Beeson, Scott W. McCoy, and Amanda Keen-Zebert

   Earth and Planetary Science Letters, Oct 2017

   Abs DOI

   Although equilibrium has long been considered the attractor state for landscapes, the time required to reach equilibrium or even the possibility of reaching equilibrium is still debated. Using 10Be-based catchment-averaged denudation rates, topographic analysis, and analysis of the basin topology and geometry, including its area-channel length scaling relationship, we show that an ancient postorogenic dome on the North American Craton, the Ozark dome, is not in a state of equilibrium. The persistent state of disequilibrium on the Ozark dome is characterized by nonuniform erosion rates that vary by a factor of three, asymmetric drainage divides, and evidence for drainage rearrangement via stream capture. We find that planform geometric disequilibrium of river basins and drainage area exchange between adjoining basins can hold river networks in a disequilibrium state for potentially hundreds of million years and that, when sustained over time, erosion rate differences associated with drainage area exchange can lead to transient events such as stream capture and production of relief in the form of elevated, low-relief surfaces. Our results suggest that landscapes with slowly moving drainage divides might not reach equilibrium, and that river basin dynamics may contribute to setting the large-scale morphology of old cratonic landscapes.

2015

1. Estimating rates of debris flow entrainment from ground vibrations

   J. W. Kean, J. A. Coe, V. Coviello, and 3 more authors

   Geophysical Research Letters, 2015

   Abs DOI

   Debris flows generate seismic waves as they travel downslope and can become more dangerous as they entrain sediment along their path. We present field observations that show a systematic relation between the magnitude of seismic waves and the amount of erodible sediment beneath the flow. Specifically, we observe that a debris flow traveling along a channel filled initially with sediment 0.34 m thick generates about 2 orders of magnitude less spectral power than a similar-sized flow over the same channel without sediment fill. We adapt a model from fluvial seismology to explain this observation and then invert it to estimate the level of bed sediment (and rate of entrainment) beneath a passing series of surges. Our estimates compare favorably with previous direct measurements of entrainment rates at the site, suggesting the approach may be a new indirect way to obtain rare field constraints needed to test models of debris flow entrainment.

2. RESEARCH FOCUS: Infrequent, large-magnitude debris flows are important agents of landscape change

   Scott W McCoy

   Geology, May 2015

   DOI
3. Landscapes in the lab

   Scott W. McCoy

   Science, Jul 2015

   DOI

2014

1. Dynamic Reorganization of River Basins

   S. D. Willett, S. W. McCoy, J. T. Perron, and 2 more authors

   Science, Mar 2014

   DOI

2013

1. Field measurement of basal forces generated by erosive debris flows

   S. W. McCoy, G. E. Tucker, J. W. Kean, and 1 more author

   Journal of Geophysical Research: Earth Surface, Jun 2013

   Abs DOI

   It has been proposed that debris flows cut bedrock valleys in steeplands worldwide, but field measurements needed to constrain mechanistic models of this process remain sparse due to the difficulty of instrumenting natural flows. Here we present and analyze measurements made using an automated sensor network, erosion bolts, and a 15.24 cm by 15.24 cm force plate installed in the bedrock channel floor of a steep catchment. These measurements allow us to quantify the distribution of basal forces from natural debris‐flow events that incised bedrock. Over the 4 year monitoring period, 11 debris‐flow events scoured the bedrock channel floor. No clear water flows were observed. Measurements of erosion bolts at the beginning and end of the study indicated that the bedrock channel floor was lowered by 36 to 64 mm. The basal force during these erosive debris‐flow events had a large‐magnitude (up to 21 kN, which was approximately 50 times larger than the concurrent time‐averaged mean force), high‐frequency (greater than 1 Hz) fluctuating component. We interpret these fluctuations as flow particles impacting the bed. The resulting variability in force magnitude increased linearly with the time‐averaged mean basal force. Probability density functions of basal normal forces were consistent with a generalized Pareto distribution, rather than the exponential distribution that is commonly found in experimental and simulated monodispersed granular flows and which has a lower probability of large forces. When the bed sediment thickness covering the force plate was greater than ∼ 20 times the median bed sediment grain size, no significant fluctuations about the time‐averaged mean force were measured, indicating that a thin layer of sediment (∼ 5 cm in the monitored cases) can effectively shield the subjacent bed from erosive impacts. Coarse‐grained granular surges and water‐rich, intersurge flow had very similar basal force distributions despite differences in appearance and bulk‐flow density. These results demonstrate that debris flows can have strong control on rates of steepland evolution and contribute to a foundation needed for modeling debris‐flow incision stochastically. , Key Points We measured 36-64 mm of bedrock incision by debris flows over four years Basal force variability was large (\textgreater+/-10x mean) and increased with mean force Granular surge fronts and watery tails had similar basal force distributions

2012

1. Sediment entrainment by debris flows: In situ measurements from the headwaters of a steep catchment

   S. W. McCoy, J. W. Kean, J. A. Coe, and 3 more authors

   Journal of Geophysical Research: Earth Surface, Sep 2012

   Abs DOI

   Debris flows can dramatically increase their volume, and hence their destructive potential, by entraining sediment. Yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited. Using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris‐flow events. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time‐averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry. Bed sediment was entrained from the sediment‐surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water‐rich, inter‐surge flow. En masse failure of bed sediment along the sediment‐bedrock interface was never observed. Large‐magnitude, high‐frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment. Within this near surface layer, concomitant fluctuations in Coulomb frictional resistance are expected, irrespective of the influence of pore fluid pressure or fluctuations in shear stress. If the near‐surface sediment was wet as it was overridden by a flow, additional large‐magnitude, high‐frequency pore pressure fluctuations were measured in the near‐surface bed sediment. These pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner. The depth to which large excess pore pressures propagated was typically less than 10 cm, but scaled as ( D / f i ) 0.5 , in which D is the hydraulic diffusivity and f i is the frequency of a particular pore pressure fluctuation. Shallow penetration depths of granular‐normal‐stress fluctuations and excess pore pressures demonstrate that only near‐surface bed sediment experiences the full dynamic range of effective‐stress fluctuations, and as a result, can be more easily entrained than deeper sediment. These data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment. , Key Points Entrainment rates were 2‐10x faster for wet channel sediment than dry sediment Flow induced near‐surface pore pressures facilitated progressive entrainment Entrainment occurred during dense granular surges and water‐rich debris floods

2011

1. OSSERVAZIONI SUI DEBRIS FLOWS DI CHALK CLIFFS, COLORADO, USA: PARTE 1, MISURE IN SITU DELLA DINAMICA DI FLUSSO, TRACCIANTI DEL MOVIMENTO PARTICELLARE E VIDEO IMAGERY DELL’ESTATE 2009

   Scott W McCoy

   Italian Journal of Engineering Geology and Environment, Jun 2011

   Abs DOI

   Debris ows initiated by surface-water runoff during short duration, moderate- to high-intensity rainfall are common in steep, rocky, and sparsely vegetated terrain. Yet large uncertainties remain about the potential for a ow to grow through entrainment of loose debris, which make formulation of accurate mechanical models of debris-ow routing difcult. Using a combination of in situ measurements of debrisow dynamics, video imagery, tracer rocks implanted with passive integrated transponders (PIT) and pre- and post-ow 2-cm resolution digital terrain models (terrain data presented in a companion paper by STALEY et alii, 2011), we investigated the entrainment and transport response of debris ows at Chalk Cliffs, CO, USA. Four monitored events during the summer of 2009 all initiated from surface-water runoff, generally less than an hour after the rst measurable rain. Despite reach-scale morphology that remained relatively constant, the four ow events displayed a range of responses, from long-runout ows that entrained signicant amounts of channel sediment and dammed the main-stem river, to smaller, short-runout ows that were primarily depositional in the upper basin. Tracer-rock travel-distance distributions for these events were bimodal; particles either remained immobile or they travelled the entire length of the catchment. The longrunout, large-entrainment ow differed from the other smaller ows by the following controlling factors: peak 10-minute rain intensity; duration of signicant ow in the channel; and to a lesser extent, peak surge depth and velocity. Our growing database of natural debris-ow events can be used to develop linkages between observed debris-ow transport and entrainment responses and the controlling rainstorm characteristics and ow properties.

2. Geomorphic significance of postglacial bedrock scarps on normal-fault footwalls

   Gregory E. Tucker, Scott W. McCoy, Alexander C. Whittaker, and 3 more authors

   Journal of Geophysical Research: Earth Surface, Mar 2011

   DOI

2010

1. Evolution of a natural debris flow: In situ measurements of flow dynamics, video imagery, and terrestrial laser scanning

   Scott W. McCoy, Jason W. Kean, Jeffrey A. Coe, and 3 more authors

   Geology, 2010

1. Disequilibrium river networks dissecting the western slope of the Sierra Nevada, California, USA, record significant late Cenozoic tilting and associated surface uplift

   Helen W Beeson and Scott W McCoy

   Geological Society of America Bulletin

2024

1. Post-Wildfire Debris Flows

   Joseph E. Gartner, Jason W. Kean, Francis K. Rengers, and 3 more authors

   In Advances in Debris-flow Science and Practice, 2024

   Abs DOI

   Post-wildfire debris flows pose severe hazards to communities and infrastructure near and within recently burned mountainous terrain. Intense heat of wildfires changes the runoff characteristics of a watershed by combusting the vegetative canopy, litter, and duff, introducing ash into the soil and creating water repellant soils. Following wildfire, rainfall on bare ground is less able to infiltrate into the fire-altered soils and overland flow is less impeded by vegetation. Rainfall runoff in recently burned areas can erode hillslopes owing to the removal of soil binding organic matter near the soil surface by fire. In channels, loose, dry-ravel deposits composed of sand and gravel are readily entrained by concentrated runoff in channels. Entrainment of soil on hillslopes and in channels bulks up the sediment concentration of the rainfall runoff to generate debris flows capable of transporting boulders and large woody debris. Post-wildfire debris flows can be triggered by rainfall conditions that would typically produce little runoff during unburned conditions. The primary rainfall trigger for post-wildfire debris flows is high intensity rainfall during short duration convective rainstorms or periods of high rainfall intensity embedded within a long-duration frontal storm. Numerous observations of debris flows triggered by storms lasting less than an hour following periods of little to no rainfall indicate that antecedent rainfall is not a requirement for initiation of post-wildfire debris flows. Post-wildfire debris-flow hazard assessment entails estimating probability and magnitude of debris flows in the burned area, estimating debris-flow runout and intensity, and defining rainfall intensity-duration thresholds for debris-flow initiation. In the United States, probability and magnitude is estimated using empirically derived models largely based on data collected in southern California. The models provide maps to identify watersheds and drainage paths where post-wildfire hazards are most pronounced. Rainfall intensity-duration thresholds can be incorporated into flood hazard forecasting tools. Currently, work is underway to identify how to best implement debris-flow runout models in burned areas with efficiency and accuracy. Post-wildfire debris flows have been a long-recognized process in the Transverse Ranges of southern California; however, climate change is driving more frequent wildfires to burn more mountainous terrain throughout the western United States and worldwide. As a result, post-wildfire debris flows are becoming a more common threat in areas where they were once infrequent. As the threat of post-wildfire debris flow expands into new areas, evaluating the hazard becomes challenging because the degree to which wildfire increases debris-flow susceptibility varies from region to region. This chapter summarizes the knowledge to date for evaluating post-wildfire debris-flow susceptibility and hazard assessment. We summarize the characteristics of wildfire burn severity, topography, underlying soil and geology, and rainfall conditions that contribute to making a watershed most likely to produce post-wildfire debris flows. Methods for hazard assessment in the United States and other countries are summarized. We highlight knowledge gaps for how post-wildfire debris-flow susceptibility varies throughout the western United States and worldwide and identify research needs to improve hazard assessment methods in different geographies.

2010

1. Chalk Creek Valley: Colorado’s natural debris-flow laboratory

   Jeffrey A. Coe, Jason W. Kean, Scott W. McCoy, and 2 more authors

   In Through the Generations: Geologic and Anthropogenic Field Excursions in the Rocky Mountains from Modern to Ancient, 2010

   Abs DOI

   Recent debris flow studies in Colorado indicate that the state is most susceptible to debris flows that initiate from surface-water runoff that erodes and entrains hillslope and channel sediment. These runoff-initiated debris flows grow in size by entraining sediment along travel paths, thereby increasing their destructive potential. Yet, the mechanics of initiation, erosion, and entrainment processes for runoffinitiated debris flows are poorly understood. The steep, bedrock-dominated flanks of the formerly glaciated Chalk Creek Valley near Nathrop, Colorado, generate an average of two runoff-initiated debris flows per year, making the valley an ideal natural laboratory for debris-flow research. This two-day field trip to the Chalk Creek Valley will examine debris-flow initiation areas, transport zones, deposits, and the impact of large pulses of debris-flow sediment on the morphology of Chalk Creek. On the first day, participants will hike into a particularly active basin at Chalk Cliffs where debris flows are being monitored by the U.S. Geological Survey, the University of Colorado, and East Carolina University. The second day will focus on debris-flow deposits in Chalk Creek and on recent debris flows in and near the community of Alpine in the central part of the valley.

2024

1. Waterfall Erosion and Alteration of River Form

   Sophie D. Rothman

   2024

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Bedrock rivers are critical to landscape evolution because they transmit tectonic signals throughout the landscape, set the base level for hillslope erosion and remove eroded sediment. Because of their role at the center of landscape erosion, bedrock rivers can provide insight into the climatic and tectonic conditions within a drainage. In particular, bedrock river steepness can provide information about the relative erosion rate of a channel, and bedrock channel width can contain clues regarding sediment flux. However, it is possible that these signals may be altered due to waterfalls which are common features in steep bedrock rivers. While many waterfalls form as a part of a transient knickzone (an oversteepened river reach created through a change in external conditions), some waterfalls are known to self-form when Froude supercritical flows develop in steep bedrock channels. And although previous work has developed an understanding of knickzones retreat through a basin, it is not clear whether waterfalls which are not a part of a knickzones follow the same rules. Specifically, it is important to understand whether self-formed waterfalls are capable of altering long profile form and channel morphology independent from the influence of knickzone dynamics. This research addresses this knowledge gap by investigating waterfalls in many different settings, and through different methods. In Chapter 2, I use a finite difference model to examine how self-formed waterfalls alter longitudinal profiles. I find that self-formed waterfalls may create a reach with a constant slope in dynamic equilibrium, or alternatively, self-formed knickpoints, depending on the speed of waterfall erosion. Furthermore, I find these results consistent with some natural river profiles. In Chapter 3, I investigate how quickly waterfall-rich channels erode using 10-Be analysis. I find that waterfall-rich channels erode faster than the rest of the landscape on average and erode faster with increasing concentration of waterfalls. Through comparing my results with a physics-based model, I also find that waterfalls may erode slowly under conditions such as high sediment flux, large grain sizes, and low discharge. In Chapter 4, I extract channel widths and waterfall locations from lidar and use the data to show that waterfalls locally constrict channel width. I also demonstrate that the length scale of waterfall influence below a waterfall grows with increasing drainage areas, indicating an increase in waterfall erosion rates. Overall, this research provides a detailed look at the physical reality of waterfall erosion, which can be used to better understand knickzones dynamics, and channels in steady state.

2023

1. Understanding Post-wildfire Debris-Flow Activity Across Climates: Insights on Initiation Conditions and Flow Identification

   David B. Cavagnaro

   2023

   Publication Title: ProQuest Dissertations and Theses

   Abs

   The size, frequency, and geographic scope of severe wildfires are expanding in the western U.S. and globally, exposing an ever-larger population to fire-related hazards. Compared to unburned areas, recently burned steeplands have an increased likelihood of runoff-generated debris flows, which are triggered by heavy short-duration rainfall and pose hazards to downstream communities. As the geographic and climatic scope of severe wildfire expands, the degree to which the initiation conditions of these flows vary with local hydroclimate is unknown. This research aims to both investigate the relationship between postfire debris-flow initiation and climate at regional and local scales, and to develop tools for accurately identifying recent debris-flow events. I organize this research into three chapters. The first chapter presents a regional analysis of debris-flow initiation across the Western U.S. and use three independent methods to demonstrate that initiation thresholds vary systematically with local rainfall-intensity climatology. The second chapter takes advantage of this variation to develop and test a rainfall anomaly metric which delineates debris-flow locations that we mapped within individual burn perimeters. The third chapter develops and tests a quantitative flow-type diagnostic metric which accurately identifies the type of flow that occurred in a steep catchment, ensuring the reliability of future training datasets for predictive debris-flow models. Together, these chapters address existing knowledge gaps on how postfire debris-flow generation may inherently vary with local climate, how hydroclimate-based metrics may serve as an important predictors of debris-flow location and initiation conditions, and how debris-flow events can be accurately identified through quantitative methods. These findings mark a contribution to our understanding of postfire hydrologic hazards that is essential under modern fire regimes and will only increase in relevance as anthropogenic climate change continues to expose an ever-larger population to these unique hazards.

2. Controls on the Uncertainty of Sediment Transport Thresholds and the Implications for Interpreting River Processes

   Scott A. Feehan

   2023

   Publication Title: ProQuest Dissertations and Theses

   Abs

   The transport of sediment by fluid flow shapes landscapes through the redistribution of material as a response to changes in imposed flow. Rivers are a primary example of this process as sediment sourced from the headwaters is transported downstream by flowing water that is highly variable in space and time. When and where this sediment will be entrained or deposited is relevant to biota, human lives and infrastructure in river-adjacent areas. Despite the importance of sediment transport, the process of entraining sediment is still poorly understood. As a result, entrainment is commonly predicted based on a critical stress related to local flow conditions. This technique has been used for nearly a century; however attempts to refine this framework have done little to collapse the multi-order magnitude scatter of observations on the threshold for sediment motion in both laboratory experiments and natural systems. A necessary next step is to reframe the entrainment problem as one that directly incorporates inherent properties of the sediment and describes the conditions for entrainment as a flow metric rather than a state of stress, such that both can be directly measured and the processes associated with sediment transport can be better understood. I use a combination of existing theory and observations from natural systems to define a new scaling relationship between sediment grain size and flow velocity necessary for entrainment and investigate how entrainment reframed in terms of these properties can be leveraged to further investigate more complex processes. This framework is unique in that the incorporation of these observations provides an explicit account of natural uncertainty in the parameters controlling sediment entrainment. With this approach, I seek to (1) collapse and explain multi-order magnitude scatter in entrainment observations. I show that observed variability of parameters that both promote and resist entrainment of sediment explains nearly all variability in observations in laboratory experiments and natural systems. I will also (2) determine how variability in entrainment thresholds is expressed in natural rivers. I demonstrate that the grain size-flow velocity scaling relationship explains a large portion of data relating formative river discharges and the size distribution of sediment in gravel-bedded rivers. Lastly, I (3) use entrainment thresholds applicable to common river systems to estimate flow conditions of unconstrained high magnitude floods. I use the refined scaling parameters to quantify a set of outburst floods from glacial Lake Tahoe to test the limits of applicability of the scaling relationship. The results suggest that entrainment is a fundamentally variable process but it can be constrained with additional observations and used to interpret and explain critical processes.

2019

1. Quantification of Internal System Dynamics and External Forcing in Bedrock River Networks

   Helen W. Dow née Beeson

   2019

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Landscapes evolve in response to tectonic and climatic boundary conditions towards an equilibrium state in which rock uplift is balanced by erosion. In unglaciated mountainous terrain, bedrock rivers transmit tectonic and climatic conditions through the landscape, carving it into ridges and valleys and controlling the relief structure by setting the boundary condition for hillslopes. Despite the importance of bedrock rivers, the manner by which bedrock river networks respond to changes in tectonic and climatic conditions and the associated timescales for these responses remain poorly constrained. As a result, difficulties exist in reconstructing tectonic or climatic histories of landscapes and fundamental questions remain regarding the relative importance of external forcing versus internal system dynamics or complex system response in shaping landscape morphology. I use a combination of quantitative terrain and network analysis, geochronology, and numerical landscape evolution modeling applied to two postorogenic landscapes in North America–the Ozark dome and the Sierra Nevada–to investigate the complex system response to tectonic forcing in bedrock river networks. Specifically, I seek to contribute to the following outstanding questions in geomorphology: (1) Do the erosional dynamics between contiguous river basins (“river basin dynamics”) prevent landscapes from reaching steady-state conditions in which erosion everywhere balances uplift? (2) To what extent do internal system dynamics, particularly river basin dynamics, shape landscape morphology in bedrock landscapes? (3) To what degree can internal system dynamics obscure geomorphic signatures of external forcing, and what methods can be used to see through the noise of internal system dynamics to robustly recover histories of external forcing from bedrock landscape form? I show that planform geometric disequilibrium of river basins may hold landscapes in a persistent transient state for potentially hundreds of millions of years in which erosion gradients generate asymmetric divides, the formation of elevated, low-relief surfaces, and discrete events such as stream capture. Although the majority of river longitudinal profiles may approach a near-equilibrium state, if the river network is in geometric disequilibrium, landforms will not be time-invariant and the broad-scale morphology can reflect the dynamics between contiguous river basins. I demonstrate that the transient fluvial response to tectonic and climatic perturbations leaves signatures in bedrock river channels and networks that are distinct from signatures of topologic change. I use these signatures to support a robust reconstruction of the tectonic history of a rejuvenated postorogenic landscape, the Sierra Nevada, despite the presence of heterogeneous lithology, Pleistocene glaciation, active river reorganization, and major beheading of the majority of mainstem rivers. I show that while drainage area exchange in the form of both steady divide migration and discrete stream capture modifies bedrock longitudinal channel profiles in the Sierra, the signature of a major late Cenozoic tectonic perturbation can be recovered provided a systematic approach to reconstructing tectonic histories is taken in which river basin dynamics is taken into account. Further, I demonstrate through analysis of the Ozark dome and Sierra Nevada that while tectonic and climatic boundary conditions generally control the relief structure of landscapes, river basin dynamics can drive substantial deviations in relief.

2012

1. Controls on erosion and transport of mass by debris flows

   Scott W. McCoy

   UNIVERSITY OF COLORADO AT BOULDER, 2012

2024

1. Hydraulic Geometry of Debris-Flow Channel Networks With Implications for Discharge Estimation

   Caleb M. Ring

   University of Nevada, Reno, 2024

   Publication Title: ProQuest Dissertations and Theses

   Abs

   In steep landscapes, debris flows scour much of the upper channel network and significantly alter longitudinal profile form at drainage areas less than 1 to 5 km². The degree to which debris-flow processes alter other aspects of the upper channel network is largely unexplored. Here we compile a large data set of bedrock channel cross-sections following debris-flow scour to constrain the downstream hydraulic geometry in the upper channel network. We find that bedrock channel networks scoured by debris flows display systematic power-law relationships between discharge Q and channel width w ~ Q0.26-0.35 and mean depth d ~ Q0.40-0.46 and hence display hydraulic geometry akin to that found further downstream in bedrock and alluvial rivers. Observed w/d ratios are narrowly distributed (6.0+/- 2.5) despite w and d not having identical scaling with Q. However, the scaling observed when substituting upstream drainage area A for Q depends on initiation mechanism and network structure. For branching networks scoured by debris flows initiated from rainfall runoff, we find w ~ A0.31-0.33 as found in many bedrock rivers, whereas for debris flows initiated from failure of shallow landslides we find w ~ A0.08-0.16. In all cases, debris-flow channel widths are 2-10 times wider at A=105 km2 than that found in fluvial channels, highlighting that when present, debris flows are systematically larger at a given drainage area than rivers. If channel networks are nonbranching, we find almost no dependence of w on drainage area, w ~ A0.01-0.05. We use these relationships in combination with simple empirical velocity relationships and a constant w/d ratio to develop debris-flow discharge estimation techniques that require only measurements of scoured channel width or drainage area. We find good agreement when tested against debris-flow monitoring sites around the world. Hydraulic geometry relationships specific to bedrock channel networks scoured by debris flows should aid in developing more accurate models of landscape evolution that incorporate debris-flow processes, as well as provide first-order guidance on expected discharges emanating from these networks.

2022

1. Mapping Post-wildfire Erosion across California Using Very High-Resolution Multitemporal Satellite Imagery

   Ingrid H. Suter

   University of Nevada, Reno, 2022

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Exposure to post-wildfire debris-flow hazards and the need for accurate hazard assessments is increasing for many communities owing to the dramatic increase in the frequency and magnitude of wildfires and the expansion of values at risk into steeplands. Empirical observations to guide predictions of the post-fire landscape response to rainfall have not kept pace with rapidly changing fire regimes have introduced high-severity wildfires into new climates zones, vegetation types and landscapes. Central and northern California are such regions where we lack extensive observations of post-fire erosion and debris-flow responses and hence the extent to which fire affects the susceptibility to runoff-related erosion is unconstrained. In this work, we utilize multitemporal, very high-resolution satellite imagery to map post-wildfire erosion and debris-flow responses, or lack thereof, in a consistent manner across five burn scars from southern California to northern California. We then compare mapped erosion magnitudes to maps of debris-flow probability from the USGS emergency post-fire debris-flow hazard assessment, which is based on an empirical model trained with debris-flow responses observed in southern California. We focus on fires that were predicted to have high debris-flow hazard owing to high values of topographic steepness, burn severity, and soil erodibility, and that have received a stressing rainstorm with intensities greater than the predicted threshold intensity needed to trigger debris flows. To turn mapped erosion magnitude into maps of debris-flow occurrence, we used the database from Staley et al. (2017) which includes 334 field-confirmed debris flows. We determined that debris-flow occurrence was associated with our highest erosion magnitude class in ~90% of cases and our medium class in ~10% of cases. Our mapping across California shows that while metrics currently used to predict debris-flow hazard were approximately uniform across studied fires, erosional responses were notably nonuniform. In the Transverse ranges of southern California, erosion features were ubiquitous and commonly consistent with a debris-flow interpretation, whereas moving into central and northern California erosion features were generally smaller in magnitude, less ubiquitous, and more consistent with features found after fluvial scour in steep channel networks as opposed to catastrophic debris-flows, or simply absent at the mapped scale. Such heterogeneity in the post-fire erosional response highlights the need for more observations to develop and test the next generation of post-fire debris-flow models capable of accurate prediction in different landscapes. These results demonstrate that mapping erosion features from multitemporal, very high-resolution satellite imagery can provide a useful and robust response metric.

2. Quantification of Wildfire-Induced Perturbations to Soil-Water Retention Curves and Their Impact on Rainfall Runoff

   Nathan R. Delgado

   University of Nevada, Reno, 2022

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Increased frequency and magnitude of wildfires can cause elevated post-fire rainfall-runoff and debris-flow hazards in and around steeplands. Improved prediction of the severity and spatial extent of post-fire increases to these hazards requires understanding which soil properties are most strongly affected by wildfire and what role these changes play in runoff generation and debris-flow initiation. Previous studies have shown significant post-fire perturbations to saturated hydraulic conductivity (Ks) and sorptivity, and in some cases these perturbations appear to exert a strong control on the observed increase in post-fire runoff and debris flow. However, a variety of other soil properties also influence rainfall infiltration rates into unsaturated soils. The soil-water retention curve (SWRC), also called the soil-water characteristic curve, is the relationship between matric suction and soil-water content and determines many critical unsaturated soil properties such as unsaturated hydraulic conductivity. The SWRC is commonly described using the Van Genuchten equation, in which key parameters are the residual water content, θr, saturated water content, θs, the pore-size distribution index, n, and the inverse of the air-entry pressure head, α. To date, the degree to which SWRCs are perturbed by wildfire is less clear, and as a result, they are commonly assumed to be unaffected by wildfire and obtained based on soil texture from databases composed of unburned agricultural soils. To quantify potential wildfire perturbations to SWRCs, we collected a suite of 43 unburned and 70 burned soil cores from seven recent wildfires in Nevada and California. We measured burned and unburned SWRCs and found best-fit Van Genuchten parameters θr, θs, n, α, using ASTM standard methods and the Meter HYPROP system. We found high quality fits to SWRCs from both unburned and burned samples using the Van Genuchten equation. While the magnitude of fire perturbation varies across fire sites, in most cases we found post-wildfire reductions in soil water retention at most values of matric suction, as quantified by increases in n and α, and decreases in θs and θr relative to the unburned state. To generalize the importance of the observed perturbations to SWRC parameters for rainfall runoff generation, we conducted a suite of numerical rainfall-runoff simulations in which we solved Richards equation with HYDRUS 1-D and with parameters constrained from burned and unburned SWRCs. These simulations suggest that the most commonly observed perturbations to SWRCs decrease time to ponding and strongly increase runoff generation at the sub-hourly timescales relevant to flash flooding and debris-flow initiation by rainfall runoff. These results highlight that field constraints on soil-hydraulic properties beyond just the saturated hydraulic conductivity are critical to more accurate prediction of rapid rainfall runoff following fire. The large database of such measurements presented here provides an important step towards increasing our understanding of the magnitude of expected post-fire perturbations to soil properties.

2021

1. Mapping Temporal Changes of Erosion and Potentially Acid Generating Material on Abandoned Mine Lands Using Remotely Piloted Aerial Systems

   Alison S. Cramer

   University of Nevada, Reno, 2021

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Erosion, transport, and weathering of potentially acid generating material (PAGM) on abandoned mine lands (AMLs) can degrade downstream environments and contaminate surface and groundwater resources through exposure to acid mine drainage (AMD) and secondary sulfate minerals. Current AML monitoring techniques rely on field measurements, which can make monitoring the thousands of AMLs for exposed PAGM and erosion of cover material on mine waste rock piles time intensive. Remotely Piloted Aerial Systems (RPASs) provide a platform that can be combined with remote sensing and structure from motion photogrammetry techniques to quantify erosion from mine waste covers, and to map the temporal dispersion of PAGM. Here, we explore the use of RPASs as a complementary remote sensing platform to quantify erosion and map the spatial and temporal changes of PAGM on a mine waste rock pile remediated with a soil cover at the Perry Canyon, NV, USA AML. We carried out eleven flights over 29 months. We built five different mm resolution 3D point clouds using a RPAS equipped with a standard camera. We focused on quantifying erosion of the soil cover and erosion and deposition occurring in the creek adjacent to the mine waste rock pile. From these data, cm-scale surface changes on waste rock piles, soil covers, and subsequent erosion and deposition in the adjacent stream were mapped. Quantitative differencing of point clouds showed that 10.2 m3 of soil cover and waste rock were eroded over a 29-month time period. Additionally, we measured continued incision of the distinct rill network incised into the mine waste rock pile, indicating active and focused removal of cover material. Using a RPAS equipped with a 5-band multispectral sensor measuring in the visible to near infrared (400-1000 nm), we created six different 3 cm resolution orthorectified reflectance maps. We focused on testing the ability of established supervised and unsupervised classification algorithms to map PAGM on imagery with very high spatial resolution, but low spectral sampling. The unsupervised method produced similar maps of PAGM, as compared to supervised schemes, but with little user input. Our classified multi-temporal maps, validated with multiple field and lab-based methods, revealed persistent and slowly growing ’hotspots’ of jarosite, a secondary sulfate mineral, on the mine waste rock pile. The mapping methods we detail for RPASs carrying a broadband multispectral sensor can be applied extensively to AMLs. RPASs with standard cameras can provide high resolution data sets to calibrate and test erosion and transport models for more accurate predictions of cover maintenance and design. Our methods show promise to increase the spatial and temporal coverage of accurate maps critical for environmental monitoring and reclamation efforts over AMLs.

2016

1. Quantifying environmental controls on the magnitude of glacial lake outburst floods and the resulting impacts to hydrology and geomorphology: Lago Cachet Dos, Patagonia, Chile

   Jonathan D. Jacquet

   University of Nevada, Reno, 2016

   Publication Title: ProQuest Dissertations and Theses

   Abs

   The sudden release of water from an ice-dammed lake poses substantial hazard to the downstream environment, but the degree to which peak discharge depends on environmental variables that change with climate remains unclear. We combine in situ measurements of environmental variables with high-resolution discharge measurements from a glacier-dammed lake, Lago Cachet Dos (LC2), to evaluate environmental conditions that influence the peak discharge of observed glacial lake outburst floods (GLOFs). Since April 2008, 20 GLOFs have initiated out of LC2, located on the eastern edge of the Northern Patagonia Icefield, Chile and flooded areas along the Rio Colonia–Rio Baker system. GLOF frequency has averaged 1–3 events annually and calculated peak discharges exiting LC2 have ranged widely from 2,000 to \textgreater15,000 m3/s. We show that, consistent with physics-based theory, increasing water volume released, lake temperature, and the rate of meltwater input into the glacially dammed lake all increase the peak discharge of observed GLOFs. Additionally, we quantify the hydrologic and geomorphic response to episodic GLOFs using multi-temporal satellite imagery and field observations. Peak discharge exiting the source lake exceeded 15,000 m3/s, but became progressively muted downstream. At ~40 km downstream, peak discharges were generally \textless 2000 m3/s, but still \textgreater 15 times the median discharge. The GLOFs resulted in \textgreater 40 m of downcutting and erosion of ~25 × 106 m3 of sediment from the source lake basin and a non-steady channel configuration downstream. Our results suggest that more accurate predictions of GLOF magnitude from ice dammed lakes can be made by incorporating additional measurements of environmental conditions and that quantifying GLOF water and sediment fluxes in the Colonia system may provide insight into potential changes that similar fluvial systems could experience after the onset of large floods.

2. Numerical Investigation of Granular Flow Dynamics Over Complex Topography: Quantifying the Efficacy of In-Path Engineered Structures to Control Granular Flow Mobility and Impact Velocity

   Sara E. Jensen

   University of Nevada, Reno, 2016

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Rapid mass movement events pose a significant threat to people, infrastructure and property. These processes initiate suddenly, can reach speeds in excess of 10 m/s, and can travel distances greater than a kilometer beyond the steep terrain where they initiated. Rapid mass movements are especially dangerous when they initiate in close proximity to populated regions where they can cause a loss of life, damage property, and decrease land production. Due to the significant threats mass movements pose to populated areas, and the difficulties associated with source area stabilization, it is important to understand their mechanics such that accurate assessments of the hazards they pose to downslope communities can be made and effectively mitigated. Of key importance are accurate predictions of flow characteristics, such as runout distance and impact velocity, as these properties set the extent and severity of potential damage. Traditionally, empirical methods are used to relate path-averaged properties, such as total drop height or average slope, to these important flow runout characteristics. However, it is well known that work done by frictional processes that lead to flow resistance are dependent on the path taken and hence should depend on flow path shape. The following study investigates how the inclusion of flow path shape, as opposed to using path-average properties, effect the accuracy of flow runout predictions and to what degree the inclusion of engineered structures placed along the channel can decrease hazardous characteristics of the flow (e.g., runout distance, flow front impact velocity, and total system kinetic energy). A suite of numerical experiments, using the Discrete Element Method (DEM), were conducted in which dry gravity-driven granular flows were allowed to flow down flumes with check dams of varying heights and locations along the flow path. Confidence in the flow dynamics stimulated by the model was gained by first validating the model against a benchtop granular avalanche experiment conducted by Iverson et al. (2004). No model tuning was required because DEM input parameters were constrained using independently tested granular properties reported by Iverson. The validation study concluded that the DEM can accurately simulate the initiation of flow from a static state, the rapid granular flow down complex three-dimensional topography, and the resulting deposition patterns at the base of the slope. The addition of a sensitivity analysis that investigated particle stiffness, size, and shape highlighted the DEM’s dependency to particle diameter; where, smaller particle diameter simulations decreased particle runout distances. The DEM also had a slight dependency on particle shape; however, the sensitivity was notably higher for static particle conditions compared to dynamic particle conditions; however, the validation investigation demonstrated that a majority of the bulk dynamic properties of the flow were adequately represented by means of a constant directional torque rolling friction parameter. Lastly, DEM simulations were found to be insensitive to particle stiffness for Young’s Modulus values within the rigidity definition outlined by da Cruz et al. (2005). A reduction in stiffness values significantly reduced model runtimes allowing for subsequent investigations to increase model complexities. The results of the numerical experiments investigating the influence of check dam height and location along the flow path demonstrated that flow kinetic energy and impact velocity decreased with increasing check dam height and were sensitive to the proximity of the dam in relation to the base of the slope. Events that traversed a linear flume compared to those with a check dam, at full sediment capacity, encountered up to a 40% reduction of peak kinetic energy. Although runout distance had clear trends with increasing dam height and dam location, observed changes were comparatively small. These numerical experiments highlighted the importance of including the specifics of flow path shape, as opposed to just using path-average properties, when predicting runout characteristics. They also provided some first-order guidance for engineers if the situation allows for flexibility in check dam height or placement along the flow path. With increased knowledge of the capabilities that engineered structures have to decrease the hazardous nature of a flow, engineers can more successfully repurpose their design.

3. Analysis of fault rupture potential resulting from large-scale groundwater withdrawal: Application to Spring Valley, Nevada

   Brian Anderson

   University of Nevada, Reno, 2016

   Publication Title: ProQuest Dissertations and Theses

   Abs

   Hydrospheric mass changes create subsurface stress perturbations on a scale that can trigger seismic events or accelerate frequency of seismicity on proximal faults. For example, groundwater pumping has been implicated in the 2011 Mw 5.1 earthquake in Lorca, Spain and the 2010 M w 7.1 El-Mayor Cucapah earthquake in northern Baja California. Previous work on effects of pumping on seismicity is retrospective. We propose a method to assess changes in rupture potential on faults near areas of large-scale groundwater withdrawal ahead before pumping begins. Changes in potentiometric head due to pumping predicted by (MODFLOW) groundwater flow models can be used as the change in surface load input for analytical solutions from Boussinesq [1885] to resolve changes in the subsurface state of stress. Coulomb stress, which quantifies a fault’s tendency toward failure, is then resolved on proximal faults. These stress changes can be compared with a 10 kPa stress threshold developed in previous work from statistical correlation of aftershock occurrence with spatial patterns of post-seismic Coulomb stress change on surrounding faults. Stress changes on critical to near-critically stressed faults above the threshold represent a higher likelihood of seismic rupture. The method is applied to a proposed groundwater development project in Spring Valley, Nevada. Proposed pumping in excess of 50 years will result in stress change on the proximal normal fault exceeding the 10 kPa threshold. This change in Coulomb stress is in the realm of earthquake-inducing pumping. However, the low seismic hazard in the region determined from geodetic and paleo-seismic analysis does not suggest imminent rupture.