Comparative Rates of Uplift and Fluvial Erosion in the Santa Paula and Sespe Watersheds (Western Transverse Ranges), CA

Reviews of watershed-scale geology and sediment-yield data, combined with remotely sensed land-cover data and qualitative observations of sediment-production processes, have allowed us to develop sediment budgets for two watersheds of ~100 km², and to test the common assumption that erosion rates generally approximate landscape uplift rates. Data sources used for these analyses included published tectonic uplift rates, uplift rates from dated terraces, and GPS survey campaigns; sediment-production rates derived from county-recorded debris basin filling and excavation; mainstem sediment-transport and deposition rates from a measured sediment rating curve (for Sespe Creek) and multi-decadal gravel-extraction records (for Santa Paula Creek); and observations of the persistence of uplifted fluvial landforms in these watersheds.

Rates of catchment-scale sediment production from nearby debris basins range between ~2,000 and 20,000 t/km²/yr. Using GIS, we applied these data throughout the two study watersheds by evaluating sediment production from discrete “geomorphic landscape units,” defined by the various possible combinations of slope, land cover, and underlying geologic material. The resulting time-integrated, watershed-wide rates of sediment production equate to a landscape lowering rate of nearly 1 mm/yr, similar to but likely lower than reconstructed Holocene rates of landscape uplift. Available independent data with which to corroborate these results (sediment rating data, historical gravel extraction rates, and the spatial distribution of relict uplifted terraces) suggest that these results are broadly accurate, lending confidence both to comparisons with geologic uplift data and to management-driven analyses of potential land-cover changes.

Of those potential land-cover changes, most prominent is the episodic effects of fire in this steep, semiarid landscape. Fires result in temporary order-of-magnitude increases in total sediment load, but these increases are apparently size-selective and do not necessarily influence the coarse, channel-forming bedload as much as they increase the suspended sediment load, which is more voluminous but less morphologically significant.