BEDROCK CLIFFS, COLLUVIAL SLOPES, AND FLUVIAL RELIEF COUPLED VIA FRACTURE DENSITY AND GRAIN SIZE (Invited Presentation)

Quantifying how rock properties influence landscape evolution is an outstanding challenge that confounds attempts to generalize relationships between topography and erosion rate. Defining proxies for rock strength at the landscape scale is difficult because hillslope and channel erosion processes may differ in their sensitivity to rock material properties. Here we show how fracture density on bedrock hillslopes and the related grain size distribution of sediment delivered to channels exert a first order control on topography and erosion rate in steep, rocky landscapes. We analyzed two sites in southern California that share similar lithology and hydro-climate, but differ dramatically in bedrock fracture density, sediment grain size, and relief structure. Multiscale analysis of lidar point cloud data reveals that fracture density controls the mean slope and relief generated on individual bedrock cliffs. Furthermore, the relationship between fluvial relief and erosion rate indirectly records differences in fracture density via the grain size distribution observed in channels. By increasing the threshold for fluvial incision, coarser sediment decreases the frequency of erosive floods. Erosion thresholds also appear to control the extent of debris-flow channel networks. Taken together, these results demonstrate how bedrock fracturing can alter drainage density and systematically reduce the relief generated on bedrock cliffs, debris flow channels, and fluvial channels. The apparent connection between tectonic setting, fracturing, and landscape-scale erodibility implied by our data suggests an intriguing negative feedback whereby rapidly uplifting mountain ranges accelerate their own demise through fracturing and sediment grain-size reduction.