A RECORD OF RAPID FLUVIAL SEDIMENTATION IN THE SIERRA NEVADA HYDRAULIC-MINING REGION: PRELIMINARY RESULTS FROM THE 2002 ENGLEBRIGHT LAKE CORING PROJECT

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, in the Sierra Nevada of northern California. Englebright Lake is a narrow, 14-km-long reservoir completed in 1941 for the primary purpose of trapping sediment derived from mining operations in the Yuba River watershed. Possible management scenarios include lowering or removing Englebright Dam, which could cause the release of stored sediments and associated contaminants, such as mercury used extensively in 19th-century hydraulic gold mining. Transport of released sediment to downstream areas could augment existing hazards including flooding and mercury bioaccumulation in sport fish. To characterize the extent, grain size, and chemistry of this sediment, a coring campaign was done in Englebright Lake in May and June 2002. Results from this campaign will be presented, including sedimentologic, geochronologic, and geochemical analyses of reservoir stratigraphy from cores and downhole geophysical logs. Twenty holes were drilled at 6 different locations along the longitudinal axis of the reservoir, recovering complete sequences of post-reservoir deposition and progradation. The total length of sediment recovered in core liners was approximately 300 m. Sediment accumulation varied in thickness and grain size along the axis of the reservoir: 6-8 m dominated by silt near the dam; 31 m of layered silt and sand in the middle of the reservoir; and 22 m of sand and gravel at the site farthest upstream. These cored thicknesses were compared to an isopach map derived by differencing the present-day lakebed surface, determined in May 2001 by fathometry and acoustic-reflection surveys, and the pre-dam topography of the river valley. Initial analysis of the core stratigraphy indicates that periods of finer-grained sedimentation are interrupted by couplets of coarser-grained sediments and organic-rich layers, interpreted to represent flood events. The large sediment load of the Yuba River system provides an excellent, high-resolution record of fluvial transport and depositional events in an anthropogenically disturbed system.