EXPLOITING ANTHROPOGENIC “NATURAL” EXPERIMENTS TO MEASURE RIVER EROSION RATES AND PROCESSES

Human alteration of river systems can, in some instances, provide the opportunity to study fluvial erosion processes and rates over well-constrained temporal intervals. I present data from two anthropogenically altered river systems to illustrate the potential for exploiting these opportunities. Furnace Creek, the main drainage on the east side of central Death Valley, California, was diverted into Gower Gulch, a short, steep canyon in the northern Black Mountains, in 1941 to minimize flood damage downstream. Countless geologic fieldtrips have observed the diversion at Zabriskie Point. The diversion changed the drainage area at the mouth of Gower Gulch from 5.8 km² to 445 km², and the channel is now undergoing a dramatic morphologic transformation due to higher flood water and sediment discharges. I present new observations from field surveys, a time series of aerial photographs, and airborne laser swath mapping to discuss the erosional response of the system, which ranges from narrowing and incision by knickpoint migration to channel widening, depending on preexisting morphology and lithology. The second example investigates sedimentation behind Englebright Dam, an impoundment on the Yuba River in the northern Sierra Nevada, California constructed in 1940. Reservoirs provide an outstanding opportunity to measure transport rates because the boundary conditions and forcings can often be well constrained by pre-existing maps of the impoundment and time series of water discharge from nearby gauging stations. The Yuba River case illustrates opportunities to understand fluvial processes by studying reservoir sediments, as well as several key limitations of work in these systems. Specifically, I use cores collected in 2002 that sample the reservoir sedimentary package to document a decrease in watershed sediment yield during the 61-year interval since impoundment.