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