Sacramento River Delta Regional Salmon Out-migration Study
Study: Delta Cross Channel (DCC)
Statement of the Problem:
Delta Cross Channel (DCC) gate operations alter tidal and riverine flows throughout the Sacramento River Delta. These changes in flow can alter the migration pathways and survival of out-migrating juvenile Chinook salmon (Oncorhynchus tshawytscha). It is generally believed that juvenile salmon passing through the DCC experience higher rates of mortality in the central Delta than salmon that remain in the Sacramento River during seaward migration. However, it is not known what proportion of out-migrants enter the central Delta via the DCC or other pathways, or how gate operations or other variables (e.g., discharge, tidal phase, diel cycle) influence the rate of entrainment into the central Delta. Gate operations also change the amount of Sacramento River water entering the central Delta which, in turn, alters the position, structure, and movement of the salt field. For example, closure of the DCC gates is often required by regulation in the fall of each year to protect salmon out-migrants. When DCC gates are closed, water exports at the Central Valley and the State Water projects are typically reduced to prevent salinity intrusion and meet these water quality standards, resulting in reduced surface water supplies to areas south of the Delta. Thus, operation of the DCC affects route selection and survival of juvenile salmon outmigrants, water quality in the central and south Delta, and water deliveries south of the Delta.
The overall goal of this salmon outmigration study is to build tools (e.g. models) that can be used to predict the impacts of proposed management actions on overall salmon survival. Our objectives are to determine the proportion of fish using various routes (e.g. proportion of fish that enter the central Delta via the DCC or Geogiana Slough versus proportion that remain in the main stem of the Sacramento River) and measure differences in survival among these routes. We will also use three-dimensional acoustic telemetry to explore fish behavior and identify the mechanism of entrainment within junctions of the Sacramento River with the DCC and Georgiana Slough.
We plan to meet our objectives using acoustic telemetry techniques. Juvenile Chinook salmon will be acoustically tagged and released into the Sacramento River, upstream from the study area. Fish will be released under three DCC gate operation scenarios: open, closed and closed only at night. The U.S. Geological Survey will establish, maintain, and download about 50 acoustic receiver sites in the study area. Primary data will focus on detection times of individual transmitters at each acoustic receiver; migratory pathways selected by salmon smolts, reach and region specific survival estimates and fish behavior in relation to different flow conditions. A statistical model of route selection probability and survival rates will be developed based on acoustic telemetry data collected. This analysis will be foundational in terms of understanding the natural and human-induced impacts on salmon outmigrants throughout the Delta.