GEOMORPHIC-BASED EVALUATION OF THE ANTHROPOGENIC DEVELOPMENT OF THE LOWER SACRAMENTO-SAN JOAQUIN DELTA, CALIFORNIA

Digital (GIS) geomorphic mapping based on interpretation of historic river surveys, vintage and modern topographic maps, and temporal sets of aerial photography delineates the distribution and composition of geomorphic landforms and associated deposits along the lower Sacramento and San Joaquin Rivers, including historic flood basins adjacent to the active, confined river channels. Prior to construction of artificial levees and repair of crevasses in the natural levees alongside the river, these flood basins were hydrologically connected to the major rivers feeding the Delta. Our research, building on previously published mapping by Brian Atwater and others, provides a record of latest Holocene changes along the lower Sacramento and San Joaquin Rivers that have shaped current conditions in the Delta, including the recent anthropogenic overprint of the Delta levee system. Historic changes in the location of river and stream channels, accompanied by localized erosion and historic fill placement, likely locally influences levee foundation stability and susceptibility of overlying levee fill to failure during extreme flood events and earthquakes. The mapped distribution of geologic deposits also provides information on possible local sources of materials used to construct historic levees during initial development of the Delta levee system. In addition, the mapping shows the distribution of remnant sediments from historic hydraulic mining that washed downstream into flood basins over the past centuries. These mercury-rich sediments are stored along the margins of, and within flood basins of, the Sacramento River, and are separated from the current river system by levees. Identification of these historic deposits is vital to preventing remobilization of mercury into the San Francisco Bay during future flood events or as a result of reconfiguration of the Delta levee system.