Water year 2019 has been a very wet year. Yet salmon and sturgeon survival was compromised by low flows and high water temperatures in the Sacramento River this spring.1 Young salmon survival has been further compromised by low flows, high exports, and high water temperatures in the Delta this past June.
Many of the wild smolts produced in Central Valley rivers this year entered the Delta in May and left (or died) by the end of June, as observed in Delta export salvage collections (Figure 1). Many of the wild smolts captured in the south Delta likely originated from San Joaquin tributaries. South Delta exports were near maximum at 10,000 cfs, about 70-80% of San Joaquin inflow to the Delta and 20% of total Delta inflow. The high exports caused lower flows and associated high water temperatures (>20oC) in the Delta channel of the lower San Joaquin River (Figure 2), and contributed to similarly high temperatures in the lower Sacramento River channel (Figure 3).
The high Delta water temperatures (>20oC) compromised the survival of the salmon smolts in June. Reducing the export limit to 5000-6000 cfs in June of this wet year would have kept the water temperature near a 20oC limit. The water quality standards in the 1980’s and 1990’s under D-1485 had a 6,000 cfs June export limit. In the past two decades under D-1641, the June export limit changed to 65% of total inflow.
New Delta water quality standards should provide export limits and inflow/outflow minimums that protect salmon through the spring months.