Landscape Evolution and the Information Stored in Topography

2026

1. 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. 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.

2023

1. 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.

2. 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.

3. 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.

2020

1. 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

2. 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.

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.

2017

1. 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.

2014

1. Dynamic Reorganization of River Basins

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

   Science, Mar 2014

   DOI

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