THE HISTORY AND FUTURE OF SEDIMENT DEPOSITION BEHIND ENGLEBRIGHT DAM, YUBA RIVER, NORTHERN CALIFORNIA

The Upper Yuba River Studies Program is a multi-disciplinary investigation of the feasibility of introducing anadromous fish species to the Yuba River system upstream of Englebright Dam, an 80-m-high structure built in 1941. Possible management scenarios include lowering or removing Englebright Dam, which could cause the release of stored sediments and associated contaminants, such as mercury used in 19th-century hydraulic gold mining. Transport of released sediment to downstream areas could increase existing hazards including flooding and mercury availability for bioaccumulation. In this study, reservoir sediment was profiled and cored in 2001 and 2002 to characterize its volume, grain size, stratigraphy, and chemistry. A gridded isopach map was made by differencing the present-day lakefloor surface (mapped by sonar), and the pre-dam topography of the river valley (digitized from 1939 contour maps). To evaluate the topographic datasets, the elevations of the lakefloor and pre-dam surface were subtracted from the corresponding elevations measured directly at each coring location. This analysis yielded a mean difference of –0.8 m for the lakefloor and –0.4 m for the pre-dam surface. The isopach data indicates that the reservoir contains approximately 21.9 million m³ of sediment, occupying 25.5% of the total capacity. In the reach of the present-day reservoir, the pre-dam Yuba River had a mean gradient of 0.48%. The post-dam deposit has a deltaic form with relatively gradual upper section (0.13%), a steep front (maximum gradient of 6.55% over 200 m in distance), and a lower section that reflects the pre-dam river gradient (0.45%). Using measurements and analyses of the properties of the reservoir sediment from the lake cores, calculations of the mass and grain size of the deposit will be presented. The longitudinal profile of the reservoir deposit will be compared to the discharge and lake-level histories, to place the stratigraphy in the context of watershed hydrologic history and deltaic depositional processes. Strategies for modeling downstream routing of the trapped sediment under various future dam-management scenarios will be discussed. The Englebright Dam case provides an example of the role of geoscience in aiding decision making within a stakeholder-driven, river-restoration process.