GEOMORPHIC AND ECOLOGICAL RESPONSES TO ANTHROPOGENIC DISTURBANCE, MOKELUMNE RIVER-DRY CREEK-COSUMNES RIVER CONFLUENCE, CALIFORNIA

Anthropogenic disturbances in the Mokelumne River-Dry Creek-Cosumnes River tributary systems that drain the west slope of the Sierra Nevada and its foothills, CA affected geomorphic processes and ecological functions in the floodplain ecotone upstream of the Sacramento-San Joaquin River Delta. The pre-disturbance anastomosing channels converged in the Sacramento flood basin, a low lying area between the natural levees of the Sacramento River and Sierran Pleistocene glacial outwash fans. The geologic record of the flood basin deposit, historical data, and remnant riparian patches adjacent to former channels suggest that prior to disturbance, flooding tributary and Sacramento River flows inundated a floodplain marsh, while adjacent levees, splay deposits, and an associated riparian ecosystem remained drier. Disturbance from gold mining, agriculture, and logging deposited a locally variable, reddish sandy and silty clay layer that overlies the flood basin deposit. Today, flow is regulated on the Mokelumne and Sacramento Rivers and the tributary confluence is characterized by constructed levees that concentrate flow into fewer channels and that minimize overbank flow onto surrounding agricultural fields in the flood basin.

Prior to these anthropogenic disturbances, geomorphic processes and sediment deposits in the avulsion-inundation dominated confluence zone formed heterogeneous topography and physical habitat that supported ecological diversity in the dynamic floodplain ecotone. Erosion, sedimentation, avulsion, and flood processes promoted riparian vegetation establishment and succession, input of woody debris, and diversity of habitat for aquatic biota in this pre-disturbance system. Land use alteration in the system during the past 150 years prompted geomorphic responses including reduction in the number of channel segments, changes in hydrology, leveling of topography, and minimization of floodplain sedimentation and inundation. The ecological response to this simplification of habitat combined with introduction of exotic species was the loss of ecosystem function over a large spatial scale. Understanding the coupled geomorphic and ecological response to landuse disturbance is critical in developing management strategies for restoring or rehabilitating habitat.