HR: 09:50h
AN: H51F-07    [Abstracts]
TI: Reconstructing Watershed History from Reservoir Stratigraphy: Englebright Lake, Yuba River, Northern California
AU: * Snyder, N P
EM: noah.snyder@bc.edu
AF: Dept. of Geology and Geophysics, Boston College, Chestnut Hill, MA 02467 United States
AU: Alpers, C N
EM: cnalpers@usgs.gov
AF: USGS, 6000 J St., Sacramento, CA 95819 United States
AU: Childs, J R
EM: jchilds@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Curtis, J A
EM: jacurtis@usgs.gov
AF: USGS, 6000 J St., Sacramento, CA 95819 United States
AU: Flint, L E
EM: lflint@usgs.gov
AF: USGS, 6000 J St., Sacramento, CA 95819 United States
AU: Holmes, C W
EM: cholmes@usgs.gov
AF: USGS, 600 Fourth St. South, St. Petersburg, FL 33701 United States
AU: Rubin, D M
EM: drubin@usgs.gov
AF: USGS, 400 Natural Bridges Dr., Santa Cruz, CA 95060 United States
AU: Wright, S A
EM: sawright@usgs.gov
AF: USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AB: Reservoirs provide the opportunity to study fluvial processes and rates in a controlled setting because they are effective traps of sediment and are often well monitored. An extensive sediment coring and sampling campaign was done in Englebright Lake on the Yuba River in northern California as part of a fish-habitat restoration study. The Yuba watershed (particularly the southern part) was the site of intensive hydraulic gold mining in the 19th and early 20th century, and Englebright Dam was built in 1940 to trap mining debris. Results of a bathymetric survey in 2001 indicate that the reservoir was 26% full (22x10$^{6}$ m$^{3}$ of material). The physical properties of the entire deposit were extrapolated from $\sim$300 m of cores collected at 7 sites along the longitudinal axis of the reservoir in 2002. The mass of the deposit is 26x10$^{6}$ metric tons, of which 3.2% is organic. The sediment is $\sim$65% sand and gravel, and distinct layers of differing grain size (sand-gravel, silt-clay, organics) are well preserved in the cores. The depositional chronology of the reservoir was established using $^{137}$Cs analysis and the relations between the cored stratigraphy and the hydrologic and impoundment history of the watershed. Deposits from three major flood events (1955, 1964, 1997; each with discharge $>$3,400 m$^{3}$/s) were identified in the stratigraphy of most of the coring sites. Observations of recent (post-1997) depositional patterns are guiding the development of a conceptual model of reservoir-sedimentation processes during floods, drawdowns, and intraflood periods. Enlargement of an upstream dam on the North Yuba River in 1970 caused a decrease in flood frequency in the Yuba River and changed management of Englebright Lake (ending annual drawdowns). A relict topset-foreset-bottomset sequence observed in the cored stratigraphy is interpreted to correlate with this change in watershed management; a second deltaic sequence was deposited on top of the first after 1970. Post-1970 average annual deposition rates appear to be $\sim$25% lower than pre-1970 rates. Although this change is small, it does appear to be robust (based on two methods for extrapolating cored sediment properties throughout the reservoir deposit: assumptions of constant and variable layer thickness). The decline in sediment deposition rates is expected for several reasons including (1) winnowing of Gold Rush-era mining debris; (2) fewer large flood events during the period; and (3) changed watershed management including construction of dams on tributaries.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1857 Reservoirs (surface)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting