Evolving Drainage Basins in the San Timoteo Badlands, Southern California

Pliocene to Pleistocene-age lacustrine and fluvial deposits are deformed and uplifted as a result of lateral passage through a restraining bend in the San Jacinto fault. The bend is associated with a bedrock high. Progressive exposure to erosion as the sediments move southeast past this high provides an opportunity to measure drainage basin development in a space-for-time substitution. Further displacement along the right-lateral fault system has juxtaposed these drainage basins with a structural releasing step, which has increased uplift and rejuvenated incision in the larger catchments located farther to the SE. Morphometric properties of the drainages (density, frequency, bifurcation ratio, first-order stream slope), and basins (area, length, relief, relief ratio, surface-to-planimetric ratios, denudation) were measured for 14 catchments. Hypsometric integrals were measured for selected basins, and SL Indices determined for the axial streams in these drainages. The development of these small drainage basins (0.048 to 10.42 km2) follows the model of initiation, elaboration and reduction, evidenced by morphometric properties. First-order stream slope, and the pattern of SL Index values both suggest increasing relief to the southeast, and defines the region affected by subsidence of the pull-apart basin. This extensional region is also defined by migrating knickpoints, stream piracy and deflection. The relief ratio of the basins decreases with increasing size, reflecting a very gradually changing height and dramatic lengthening. The denudation rate is determined for constant slip rate along the fault, and normalized for basin width. This value also decreases with increasing basin size. These basins are subjected to both ongoing structural controls and evolutionary processes. Bifurcation ratios, first-order stream slopes, and SL indices respond to tectonic uplift; drainage density and frequency, bifurcation ratios between first and second order stream segments, and first order stream length indicate drainage development as a function of time.