MORPHOLOGIC EFFECTS OF ANTHROPOCENE SEDIMENT PULSES ON THE SOUTH FORK EEL RIVER OF NORTHWESTERN CALIFORNIA

Modern sediment dynamics in the South Fork Eel River watershed has become a significant public policy concern due to their impact on several populations of anadromous salmonids. The decline in suitable alluvial rearing habitat is a particular concern. The watershed consists primarily of highly erodible marine sediments and is undergoing rapid, though variable, tectonic uplift. The river alternates between bedrock, colluvial, and alluvial reaches. In the years after 1945, logging and road building activities accelerated substantially through much of the watershed. Accelerated land use related sediment delivery has continued to the present. In 1955 and then more dramatically in 1964, floods introduced massive pulses of sediment into the fluvial system. In order to quantitatively measure changes in sediment storage and channel morphology, we have produced a time series of channel and floodplain topography with coverages from 1941 to 2009. Topography was generated using stereo photogrammetry, historically surveyed topographic cross sections, and an airborne LIDAR coverage. The floods disrupted in stream wood structures and removed riparian forests from flood plains. The reduction in channel cohesion, increased fine sediment, and increased rates of channel migration resulted in significant adjustment to alluvial reaches. Pool frequency and size has been significantly reduced. Sedimentary deposits have been mapped using oblique structure from motion photogrammetry. Overbank deposits from the 1964 flood in excess of 1 meter thickness are still widely distributed, however significant declines in stored sediment is evident. Laser particle size analysis has been conducted on sediment samples to determine hydraulic characteristics of the depositional environment. Channel incision and dissection of depositional surfaces observed since 1964 are compared to the persistence rates of older elevated terrace deposits.