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The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been limited due to expectations of: (a) decoupling between erosion rate and "threshold" hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published 10Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in 10Be erosion rates in catchments as small as 0.09 km2, and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes. Plain Language Summary: In general, erosion rates increase as landscapes steepen. But where landslides are common, this relationship is thought to break down as hillslopes approach their angle of repose. The main tracer for measuring erosion rates, 10Be in sediment, can also be affected by landslides, and models predict it is unreliable for small watersheds in steep landscapes. Here, we compile an extensive data set of 10Be erosion rates from the San Gabriel Mountains of California. We show that slope and erosion rate are coupled well above the soil angle of repose due to systematic exposure of bedrock cliffs, supporting a new conceptual model for steep landscapes. The presence of landslides adds scatter but does not bias 10Be erosion rates, which yield robust results even in small, steep watersheds that have previously been avoided. Key Points: Progressive exposure of bare rock on steeper slopes leads to correlation of 10Be erosion rate and mean hillslope angle up to 47°Deep seated landslide deposits add scatter, but do not systematically bias 10Be erosion rate estimates in the San Gabriel MountainsNo evidence for drainage area dependence of 10Be erosion rates in upland catchments [ABSTRACT FROM AUTHOR]

Abstract: The successful quantification of long-term erosion rates underpins our understanding of landscape formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide such long-term erosion rates. Although deep-seated bedrock landsliding is an important erosional process in active orogens, its effect on CRN-derived erosion rates is largely unquantified. We present a numerical simulation of cosmogenic nuclide production and distribution in landslide-dominated catchments to address the effect of bedrock landsliding on erosion rates. Results of the simulation indicate that the temporal stability of erosion rates determined from CRN concentrations in sediment decreases with increased ratios of landsliding to bedrock weathering rates within a given catchment area, and that as the frequency of landsliding increases, larger catchment areas must be sampled in order to accurately evaluate long-term erosion rates. In addition, results of this simulation suggest that sediment sampling for CRNs is the appropriate method for determining long-term erosion rates in regions dominated by mass-wasting processes, whereas bedrock surface sampling for CRNs may generally underestimate long-term erosion rates. Response times of CRN concentrations to changes in erosion rate indicate that climatically driven cycles of erosion may be detected, but that complete equilibration of CRN concentrations to new erosional conditions may take tens of thousands of years. Comparison of simulated CRN-derived erosion rates with a new data set of such rates from the Nepalese Himalaya underscore the conclusions drawn from the model results. [Copyright &y& Elsevier]

The article discusses the study by D. McPhillips and colleagues regarding the generation of the coarse sediment in river deposits through landslides. Topics discussed include the determination of the erosional processes through the analysis of the abundance of cosmogenic nuclide in geologic surface materials, the observation of the concentration of cosmogenic nuclide, and the complementary information about process of basin-scale erosion which can be recorded by grain-size fractions.