Management hypotheses assert that water export operations in the south Delta create directional flows that pull migrating salmonids off their main migration routes at channel junctions and toward export facilities into areas where net flows are often negative and toward the export pumps where survival can be poor. This hypothesized effect has severed as justification for regulations restricting export operations during salmonid migration periods. To test for that effect on out-migrating salmon, we paired 41 acoustic telemetry-based estimates of juvenile Chinook Salmon routing at six Delta channel junctions with hydrologic conditions at those junctions when salmon were passing through. A linear model of routing as a function of the proportion of total flow entering the junction explained more than 70% of the variation in the realized salmon routing. The model then compared salmon routing at channel junctions over a range of outflow through the Delta and water export volumes. Model results revealed that exports have little effect on routing at junctions where flows are fully tidal driven or at riverine junctions on the Sacramento River. Routing was affected substantially by water exports at the Head of Old River (HOR) junction when outflow was particularly low and export volumes were high. These results indicate that tides and inflow have a much larger influence on routing relative to water exports from the south Delta. Operational changes to water exports are unlikely to significantly effect salmonid fates, except at specific channel junctions, and then under a narrow range of hydrologic conditions.
Cavallo B, Gaskill P, Melgo J, Zeug SC et al. 2015. Predicting juvenile Chinook routing in riverine and tidal channels of a freshwater estuary. Environmental Biology of Fishes 98:1571-1582.