Sediment Fluxes at Mallard Island

Sediment supply to a subembayment of an estuary is determined by watershed sediment input and the sediment exchange with adjacent embayments. Sediment supply is a critical variable for investigations of habitat stability, restoration potential, and contaminant fate/transport. Suspended sediment is needed to create and sustain valuable estuarine habitats such as tidal wetlands (Pont et al., 2002; Reed, 2002; Temmerman et al., 2003; Zedler and Callaway, 2001), though sediment-associated contaminants can also accumulate wherever sediment preferentially deposits (Arzayus et al., 2002; Hornberger et al., 1999; Taylor et al., 2004). In addition, nutrients and biota accumulate near estuarine turbidity maxima (ETM), where high suspended-sediment and contaminant concentrations are found (Peterson et al., 1975; Jassby and Powell, 1994). Enhanced biological activity in these areas may increase contaminant uptake by the food web (Kimmerer et al., 1998). These issues converge in Suisun Bay, California. Over 90% of marsh area has been lost in San Francisco Bay since the 19th century, and current management goals in Suisun Bay include marsh restoration. Deposited sediment in Suisun Bay is thought to be high in mercury concentrations due to 19th century gold mining in the watershed (Hornberger et al., 1999), leading to concerns about resuspension of these deposits over long timescales (>10 y). Jaffe et al. (1998) show net erosion in Suisun Bay since the first bathymetric surveys in the 19th century. The net sediment budget of Suisun Bay in the current era may shed light on the viability of habitat restoration as well as the magnitude of mercury introduction to the water column. In order to quantify this sediment budget, sediment flux at the upstream and downstream boundaries of the subembayment must be determined.

This study demonstrates the use of SSC data collected at Mallard Island as a means of determining suspended-sediment load entering San Francisco Bay from the Sacramento and San Joaquin River watersheds. Optical backscatter data were collected every 15 minutes during water years 1995 to 2003 and converted to SSC. Daily fluvial advective sediment load was estimated by combining estimated Delta outflow with daily averaged SSC. On days when no data were available, SSC was estimated using linear interpolation. A model was developed to estimate the landward dispersive load using velocity and SSC data collected during WYs 1994 and 1996. The advective and dispersive loads were summed to estimate the total load. Annual suspended-sediment load at Mallard Island averaged 1.2± 0.4 million metric tones (Mt). Given that the average water discharge for the 1995-03 period was greater than the average discharge for the last decade, it seems likely that the average suspended-sediment load may be less than 1.2± 0.4 Mt. Average landward dispersive load was 0.24 Mt/yr, 20 percent of the total. On average during the wet season, 88 percent of the annual suspended-sediment load was discharged through the Delta and 43 percent occurred during the wettest 30-day period. The January 1997 flood transported 1.2 Mt of suspended sediment or about 11 percent of the total 9-year load (10.9 Mt). Previous estimates of sediment load at Mallard Island are about a factor of 3 greater because they lacked data downstream from riverine gages and sediment load has decreased. Decreasing suspended-sediment loads may increase erosion in the Bay, help to cause remobilization of buried contaminants, and reduce the supply of sediment for restoration projects. (from McKee et al., 2006)

Publications

Leatherbarrow, J.E., McKee, L.J., Schoellhamer, D.H., Ganju, N.K., and Flegal, A.R., 2005. Concentrations and loads of organic contaminants and mercury associated with suspended sediment discharged to San Francisco Bay from the Sacramento-San Joaquin River Delta, California. Report prepared for the Sources Pathways and Loading Workgroup of Regional Monitoring Program for Trace Substances in the San Francisco Estuary, San Francisco Estuary Institute, Oakland, California, 93 p.

Link to Report

McKee, L., Ganju, N., Schoellhamer, D., Davis, J., Yee, D., Leatherbarrow, J., and Hoenicke, R., 2002, Estimates of suspended sediment flux entering San Francisco Bay from the Sacramento and San Joaquin Delta: Report prepared for the Sources Pathways and Loading Workgroup of Regional Monitoring Program for Trace Substances in the San Francisco Estuary, San Francisco Estuary Institute, Oakland, California, 28 p.
Link to Report

McKee, L., Ganju, N.K., and Schoellhamer, D.H., 2006, Estimates of suspended sediment entering San Francisco Bay from the Sacramento and San Joaquin Delta, San Francisco Bay, California. Journal of Hydrology, 323, 335-352.
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Link to Project Photos

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