Showing papers by "Lorraine E. Flint published in 2004"

Estimating accumulation rates and physical properties of sediment behind a dam: Englebright Lake, Yuba River, northern California

Abstract: [1] Studies of reservoir sedimentation are vital to understanding scientific and management issues related to watershed sediment budgets, depositional processes, reservoir operations, and dam decommissioning. Here we quantify the mass, organic content, and grain-size distribution of a reservoir deposit in northern California by two methods of extrapolating measurements of sediment physical properties from cores to the entire volume of impounded material. Englebright Dam, completed in 1940, is located on the Yuba River in the Sierra Nevada foothills. A research program is underway to assess the feasibility of introducing wild anadromous fish species to the river upstream of the dam. Possible management scenarios include removing or lowering the dam, which could cause downstream transport of stored sediment. In 2001 the volume of sediments deposited behind Englebright Dam occupied 25.5% of the original reservoir capacity. The physical properties of this deposit were calculated using data from a coring campaign that sampled the entire reservoir sediment thickness (6–32 m) at six locations in the downstream ∼3/4 of the reservoir. As a result, the sediment in the downstream part of the reservoir is well characterized, but in the coarse, upstream part of the reservoir, only surficial sediments were sampled, so calculations there are more uncertain. Extrapolation from one-dimensional vertical sections of sediment sampled in cores to entire three-dimensional volumes of the reservoir deposit is accomplished via two methods, using assumptions of variable and constant layer thickness. Overall, the two extrapolation methods yield nearly identical estimates of the mass of the reservoir deposit of ∼26 × 106 metric tons (t) of material, of which 64.7–68.5% is sand and gravel. Over the 61 year reservoir history this corresponds to a maximum basin-wide sediment yield of ∼340 t/km2/yr, assuming no contribution from upstream parts of the watershed impounded by other dams. The uncertainties and limitations of the estimates of overall sediment quantities are discussed. Implications for watershed management and future reservoir sedimentation studies are also presented.

Report on the May-June 2002 Englebright Lake Deep Coring Campaign

Abstract: This report describes the May-June 2002 Englebright Lake coring project. Englebright Lake is a 14km-long reservoir on the Yuba River of northern California, impounded by Englebright Dam, which was completed in 1940. The sediments were cored to assess the current conditions in the reservoir as part of the California Bay-Delta Authority’s Upper Yuba River Studies Program. Sediment was collected using both hydraulic-piston and rotational coring equipment mounted on a floating drilling platform. Thirty boreholes were attempted at 7 sites spaced along the longitudinal axis of the reservoir. Complete sedimentary sections were recovered from 20 boreholes at 6 sites. In total, 335 m of sediment was cored, with 86% average recovery. The core sections (each up to 1.5 m long) were processed using a standard set of laboratory techniques, including geophysical logging of physical properties, splitting, visual descriptions, digital photography, and initial subsampling. This report presents the results of these analyses in a series of stratigraphic columns. Using the observed stratigraphy as a guide, several series of subsamples were collected for various sedimentologic, geochemical, and geochronological analyses. The results of laboratory analyses of most of these subsamples will be presented in future reports and articles. INTRODUCTION Beginning in 2001, the California Bay-Delta Authority (CBDA), formerly the California-Federal BayDelta Program (CALFED), has sponsored the Upper Yuba River Studies Program (UYRSP, http://www.nasites.com/pam/yuba/), an investigation of the feasibility of introducing anadromous fish species to the Yuba River system upstream of Englebright Dam (Figure 1). The UYRSP has six scopes of work: sediment, water quality, habitat, flood risk, water supply and hydropower, and economics. To achieve the UYRSP objective of fish passage, some of the future management scenarios under consideration include lowering or removing the dam. Any reduction in size of the dam would result in some change in the sediment regime of the lower Yuba River, and could cause the release of material presently stored in the reservoir. This increased sediment load could exacerbate existing hazards in the Snyder and others, Open-File Report 2004-1061 2