PROJECTED GEOMORPHIC RESPONSE TO FUTURE CLIMATE VARIABILITY AND CHANGE IN LOWLAND RIVERS, SAN FRANCISCO BAY-DELTA WATERSHED, CA

Climate variability and change alter fluvial processes in lowland floodplain rivers in California’s San Francisco Bay-Delta Watershed. Prior to anthropogenic modifications, geomorphic processes in the lowland portion of the Sacramento and San Joaquin River system were dominated by episodic avulsion, channel migration, erosion, and sedimentation that occurred during floods of a wide range of magnitudes. These dynamic processes created habitat heterogeneity that supported riparian system biodiversity. Several lines of paleoclimate evidence from the Bay-Delta watershed indicate that natural climate variability and changes capable of instigating geomorphic responses were common in the past 5,000 years. During the past 200 years, human activities have dramatically altered sediment supply and degraded lowland floodplain river ecology. Currently, extensive levees and many dams regulate flow. As a result, the amount of sediment transported from the Sacramento and San Joaquin River systems has progressively declined, especially due to the cessation of hydraulic mining in the late 1800’s and trapping of sediment upstream of dams built since the mid-1900’s. Now, human induced climate changes are projected that may increase magnitude, frequency, and variability of winter floods. These increases may cause thresholds of sediment mobility, erosion, and deposition to be exceeded, thus magnifying flood hazards and morphologic change. Potential geomorphic responses may include channel enlargement and more frequent levee breaching with associated floodplain inundation. Projected drier summers may result in loss of riparian vegetation and subsequent channel widening. Despite recent reductions in sediment supply to the lowland landscape, future climate variation and changes may induce new episodes of erosion from sediment stored in downstream floodplains and channels. Complex adjustments may occur as vulnerable flood and erosion-control infrastructure constructed over the past two centuries is threatened and as the fluvial system adjusts to more energetic and variable hydrologic regimes. In light of these projections, restoration efforts designed to accommodate highly dynamic hydrogeomorphic processes and increasing flood magnitudes may prove to be the most sustainable.