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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 debris‐flow 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.

 

Abstract. Estimation of erosion rate is an important component of landscapeevolution studies, particularly in settings where transience or spatialvariability in uplift or erosion generates diverse landform morphologies.While bedrock rivers are often used to constrain the timing and magnitude of changes in baselevel lowering, hilltop curvature (or convexity), CHT, provides an additional opportunity to map variations in erosion rate given that average slope angle becomes insensitive to erosion rate owing to threshold slope processes. CHT measurement techniques applied in prior studies (e.g., polynomial functions), however, tend to be computationallyexpensive when they rely on high-resolution topographic data such as lidar,limiting the spatial extent of hillslope geomorphic studies to small studyregions. Alternative techniques such as spectral tools like continuouswavelet transforms present an opportunity to rapidly document trends inhilltop convexity across expansive areas. Here, we demonstrate howcontinuous wavelet transforms (CWTs) can be used to calculate the Laplacianof elevation, which we utilize to estimate erosion rate in three catchmentsof the Oregon Coast Range that exhibit varying slope angle, slope length,and hilltop convexity, implying differential erosion. We observe thatCHT values calculated with the CWT are similar to those obtained from2D polynomial functions. Consistent with recent studies, we find thaterosion rates estimated with CHT from both CWTs and 2D polynomialfunctions are consistent with erosion rates constrained with cosmogenicradionuclides from stream sediments. Importantly, our CWT approachcalculates curvature at least 103 times more quickly than 2Dpolynomials. This efficiency advantage of the CWT increases with domainsize. As such, continuous wavelet transforms provide a compelling approachto rapidly quantify regional variations in erosion rate as well aslithology, structure, and hillslope sediment transport processes, which areencoded in hillslope morphology. Finally, we test the accuracy of CWT and 2Dpolynomial techniques by constructing a series of synthetic hillslopesgenerated by a theoretical nonlinear transport model that exhibit a range oferosion rates and topographic noise characteristics. Notably, we find thatneither CWTs nor 2D polynomials reproduce the theoretically prescribedCHT value for hillslopes experiencing moderate to fast erosion rates,even when no topographic noise is added. Rather, CHT is systematicallyunderestimated, producing a power law relationship between erosion rate andCHT that can be attributed to the increasing prominence of planarhillslopes that narrow the zone of hilltop convexity as erosion rateincreases. As such, we recommend careful consideration of measurement lengthscale when applying CHT to estimate erosion rate in moderate tofast-eroding landscapes, where curvature measurement techniques may be prone to systematic underestimation. 

Bedrock landsliding, including the formation of landslide dams, is a predominant geomorphic process in steep landscapes. Clarifying the importance of hydrologic and seismic mechanisms for triggering deep‐seated landslides remains an ongoing effort, and formulation of geomorphic metrics that predict dam preservation is crucial for quantifying secondary landslide hazards. Here, we identify >200 landslide‐dammed lakes in western Oregon and utilize dendrochronology and enhanced¹⁴C dating (“wiggle matching”) of “ghost forests” to establish slope failure timing at 20 sites. Our dated landslide dataset reveals bedrock landsliding has been common since the last Cascadia Subduction Zone earthquake in January 1700 AD. Our study does not reveal landslides that date to 1700 AD. Rather, we observe temporal clustering ofat leastfour landslides in the winter of 1889/1890 AD, coincident with a series of atmospheric rivers that generated one of the largest regionally recorded floods. We use topographic and field analyses to assess the relation between dam preservation and topographic characteristics of the impounded valleys. In contrast to previous studies, we do not observe systematic scaling between dam size and upstream drainage area, though dam stability indices for our sites correspond with “stable” dams elsewhere. Notably, we observe that dams are preferentially preserved at drainage areas of ∼1.5 to 13 km²and valley widths of ∼25 to 80 m, which may reflect the reduced downstream influence of debris flows and the accumulation of mature conifer trees upstream from landslide‐dammed lake outlets. We suggest that wood accumulation upstream of landslide dams tempers large stream discharges, thus inhibiting dam incision.

 

Understanding sedimentation patterns in small coastal watersheds due to landscape perturbations is critical for connecting hillslope and fluvial processes, in addition to managing aquatic habitats for anadromous fish and other aquatic species in the Oregon Coast Range (OCR). Changes in sedimentation patterns spanning the last 250 years are preserved in two landslide‐dammed lakes in small watersheds (< 10 km²) underlain by the Tyee Formation in the central OCR. Dendrochronology of drowned Douglas‐fir stumps in both lakes provided precise timing of the damming and formation of the lakes, with Klickitat Lake forming in winterad1751/52 and Wasson Lake in winterad1819/20. Perturbations from wildfires, logging and road development, and previously underappreciated snow events affect sedimentation rates in the lakes to different degrees, and are identified in the sediment record using cesium‐137 (¹³⁷Cs), high‐resolution charcoal stratigraphy, local fire records, and aerial photography. Each lake has variable sedimentation accumulation rates (0.05–4.4 cm yr⁻¹) and mass accumulation rates (0.02–1.42 g cm⁻²yr⁻¹). Sedimentation rates remained low from the landslide‐damming events until the mid‐19th century, when they increased following stand‐replacing wildfires. Aside from a sediment remobilization triggered by human modification of the landslide dam at Klickitat Lake around 1960, the largest peaks in mass accumulation rates in the mid‐20th century at both lakes in the early 1950s precede major road construction and logging activity in the watersheds. Subsequent sedimentation rates are lower, but variable, and possible effects of logging and road development might be exacerbated by abnormal precipitation and heavy snow events. A comparison of previous studies of landslide‐dammed lakes in larger watershed of the OCR are consistent with our findings of increased sedimentation in the mid‐20th century, as well as higher sedimentation rates in the debris‐flow dominated southern Tyee Formation than in the lower‐relief northern Tyee Formation.