Bank Erosion as a Geomorphic Process: Dynamic Interactions between Channels, Banks, and Floodplains.
Joan L. Florsheim1, Brian A. Pellerin2, Brian Bergamaschi2, and Peter J. Hernes3. (1) Geology Department, University of California, One Shields Ave, Davis, CA 95616, (2) USGS, 6000 J Street, Placer Hall, Sacramento, CA 95819, (3) Land, Air, and Water Resources, University of California, One Shields Ave, Davis, CA 95616
Bank erosion is one component of the sediment transfer continuum responsible for creating and altering morphology in natural systems. Land use alteration and geologic conditions within a small tectonically active fluvial system draining the California Coast Ranges eastward to the Sacramento Valley, Willow Slough, influence bank erosion rates and contribute fine sediment, nutrients, and dissolved organic carbon that degrade water quality. Bank erosion in an upland tributary (gradient is 0.014) is dominated by mass failure of incised valley fill deposits remaining as terraces. Bank heights average 4.4 m, with the lower portion of vertical banks composed of friable shale and upper banks composed of relatively fine alluvium. Mass failures occur as both planar and rotational slides; bank erosion processes also include fluvial erosion of shale bedrock at the base of bank, sloughing, and ravel. Grazing in this upstream area accelerates erosion rates. Bank erosion in the lowland floodplain-channel areas of Willow Slough (gradient is ~0.0014) is dominated by fluvial processes during storm and irrigation flows that entrain fine bed and bank material in channels; bank erosion processes also include slides and sloughing. Construction of low levees composed of sediment dredged from channels increases bank heights, limits channel-floodplain interactions, and concentrates hydrologic forces within enlarged channels. Riparian vegetation removal and channel realignment also alter fluvial processes and accelerate fine sediment contributions. A conceptual sediment routing model relates erosion and transfer processes to sediment stored in fluvial deposits such as terraces, floodplains, and bars. Assessment of hydrologic and sediment load data from local gaging stations will help determine the timing and mechanisms of bank failures as a contribution to sediment transfer and basin export. Ongoing work documents the spatial distribution and quantifies watershed sediment sources that influence water quality in an effort to develop strategies to minimize erosion.