Hillslope Morphology Drives Variability of Detrital 10Be Erosion Rates in Steep Landscapes.

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]