Unless there are high Delta inflows, south Delta export pumping creates a hydrologic “trap” for emigrating salmon and other Delta fishes. Even under moderate Delta inflows and outflows, as occurred in January 2019 (Figure 1), south Delta pumping traps salmon emigrating from both the Sacramento and San Joaquin river systems. This is because the pumps trap nearly all the water from the San Joaquin and about a third of the Sacramento River water the latter primarily via cross-Delta flow in Georgianna Slough (GS). In contrast, under high flows, the trap is confined only to the immediate area of the south Delta export pumps (Figure 2).
Susceptibility to the “trap” for Sacramento salmon under moderate early winter Delta flows is evident from the collection of smolts from the Coleman fish hatchery at south Delta fish salvage facilities (Figure 3, blue and green dots). Susceptibility of San Joaquin salmon smolts under moderate and high flows is evident from San Joaquin hatchery smolts salvage (Figure 3, orange dots).
Lower export limits in winter-spring since 2009 have helped to minimize the frequency of conditions in which salmon become trapped in the south Delta. Exports and salmon salvage are 30-50 % lower than pre-2009 levels, and sometimes lower still. However, political forces may eliminate these export restrictions and greatly increase the trapping of salmon in the south Delta. This would further limit the potential for salmon recovery in the Central Valley.
The State Water Board’s ongoing process of setting new Delta water quality standards should adopt more stringent measures to minimize the trapping of salmon in the south Delta. Such measures should include further restrictions on exports and increased flows in the lower San Joaquin River. Other options include:
- Constructing a barrier or fish screen at the head of Old River;
- Opening the Delta Cross Channel to increase net downstream flows in the lower San Joaquin channel within the Delta; and
- Increasing Delta outflow.