State and federal water projects in the south Delta near Tracy, California. Credit: Virginia Macintosh.
Current estimates of young salmon lost to the south Delta pumps are based on a smattering of studies from the 1970s and should be updated, according to a new analysis.
"They don't represent current operations," says Ukiah-based consultant Andrew Jahn, lead author of the analysis reported in the September 2020 issue of San Francisco Estuary and Watershed Science. Current operations at the State Water Project (SWP) and the Central Valley Project (CVP) can reverse flows in the Old and Middle rivers, diverting salmon on their way to the ocean towards the projects. Existing salmon loss estimates also fail to account for a likely Old River hotspot for predators, drawn to the influx of salmon and other fish near the two water projects. Evidence of this hotspot lies in the numerous acoustic tags from young salmon to be found in the sediment, which were presumably defecated by striped bass, as well as the fact that it is a popular spot for striped bass fishing. The hotspot is a two-kilometer stretch of Old River between the radial tide gates at the entrance of Clifton Court Forebay, which leads to the SWP, and the trash boom just before the CVP. "The state says 'our impacts begin at the radial gates' and federal officials say 'our impacts being at the trash boom,' " Jahn explains. "But you know predators wouldn't concentrate upstream at the bend of Old River without a draw." Improving estimates of salmon loss in the south Delta will require new studies, the researchers say; Jahn envisions releasing tagged fish upstream of the predation hotspot. In the interim, Jahn and co-author William Kier, a San Rafael-based consultant, recommend incorporating data collected since the 1970s. "We could use the data we have better," Jahn says.