Abstract
Movements of pelagic organisms in the tidal freshwater regions of estuaries are sensitive to the movements of water. In the Sacramento-San Joaquin Delta—the tidal freshwater reach of the San Francisco Estuary—such movements are key to losses of fish and other organisms to entrainment in large water-export facilities. We used the Delta Simulation Model-2 hydrodynamic model and its particle tracking model to examine the principal determinants of entrainment losses to the export facilities and how movement of fish through the Delta may be influenced by flow. We modeled 936 scenarios for 74 different conditions of flow, diversions, tides, and removable barriers to address seven questions regarding hydrodynamics and entrainment risk in the Delta. Tide had relatively small effects on fate and residence time of particles. Release location and hydrology interacted to control particle fate and residence time. The ratio of flow into the export facilities to freshwater flow into the Delta (export:inflow or EI ratio) was a useful predictor of entrainment probability if the model were allowed to run long enough to resolve particles’ ultimate fate. Agricultural diversions within the Delta increased total entrainment losses and altered local movement patterns. Removable barriers in channels of the southern Delta and gates in the Delta Cross Channel in the northern Delta had minor effects on particles released in the rivers above these channels. A simulation of losses of larval delta smelt showed substantial cumulative losses depending on both inflow and export flow. A simulation mimicking mark–recapture experiments on Chinook salmon smolts suggested that both inflow and export flow may be important factors determining survival of salmon in the upper estuary. To the extent that fish behave passively, this model is probably suitable for describing Delta-wide movement, but it is less suitable for smaller scales or alternative configurations of the Delta.
Highlights
In tidal river estuaries, freshwater flows affect hydrodynamic phenomena important to biotic communities
Four large water diversions owned by the U.S and State of California governments collectively export an average of nearly 7 cubic kilometers per year from the Delta (Table 1)
Temporary rock barriers are installed at various sites in the southern Delta to maintain water levels for agricultural diversions, and one barrier is placed at the head of Old River to prevent salmon smolts from entering it during their migration down the adjoining San Joaquin River
Summary
Freshwater flows affect hydrodynamic phenomena important to biotic communities. The landward reach of California’s San Francisco Estuary, known as the Sacramento-San Joaquin Delta, may be the only place in the world where significant freshwater is diverted from within a tidal estuary. Reservoir releases throughout the watershed are managed to maintain most of the Delta as a permanently freshwater ecosystem to support a significant redistribution of California’s water resources from north to south (Kimmerer 2002). More than 95% of the water exported from the Delta is taken by the two largest diversions: the Jones Pumping Plant of the federal Central Valley Project (hereafter, CVP) and the State Water Project’s Banks Pumping Plant (hereafter, SWP). The exported water is pumped to agricultural, municipal, and industrial users to the south and west; an estimated 22 million Californians use water exported from the Delta
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