Geomorphic Response to Accelerated Sediment Transport and Erosion in Tectonically Active Streams.

Drainage basins and their network of streams affected by past and ongoing tectonic activity may be characterized by high gradients, re-directed (offset) channels and sediment loads that reflect hydrologically discontinuous transport and deposition processes. Further, watersheds that experience profound anthropomorphic changes such as removal of trees and generations of unregulated grazing, have resulted in hillslope and streamside instability owing to a decline of native riparian, arboreal, understory, and forb species. This study describes and evaluates patterns of sediment erosion, transport and deposition in the 126 km2 San Emigdio watershed in Kern County, California. Using an Unmanned Aerial Vehicle (UAV) in conjunction with an RTK GPS base station and rover along a 4 km reach, a series of high resolution (2 cm) images, a georeferenced orthomosaic, and digital terrain model (DTM) were generated to provide a baseline for sediment accumulation, bank erosion, and longitudinal profile changes over time. Channel cross sections and the longitudinal profile were surveyed with a total station and georeferenced with the UAV generated data. Results showed that while there were active hotspots of channel bank erosion at knickpoints which appear to be fault-controlled, there was also significant bank instability and addition of sediment into the active channel along the entire reach even under very low flow conditions. These processes combine to load sediment in the channel that apparently can be removed only by high discharge events. With increasing precipitation variability in the region during the past half century, punctuated by periods of drought, storage of sediment in the channel and its potential to be transported beyond the watershed into neighboring agricultural land suggests that continued measurement of channel and bank erosion and sediment transport and deposition is critical.