When the State Water Board gets around to finally updating decades-old Central Valley water quality standards, it should bring back some old spring standards, keep some good ones, and add some new ones to provide essential protection to salmon, steelhead, sturgeon, smelt, and many other native fish populations. One focus should be on improving survival of wild spring-run and fall-run salmon smolts migrating from Central Valley spawning rivers to the Delta, Bay, and ocean.
With its high spring water temperatures (Figure 1), 2020 is a good example of a solvable problem. The survival of wild spring-run and fall-run salmon smolts depends on sufficient flows and low water temperatures in the spring. This natural selection process, once tied to the natural spring snowmelt cycle, has been disrupted by reservoir storage and water diversions. Wild smolt emigration peaks in spring and extends into early summer (Figure 2). Sturrock et al. (2019) found that late spring smolt survival suffered from poor emigration habitat conditions. This affects population diversity because of the disproportionate loss of wild smolts in the late spring.
June Delta Exports
June exports in recent wet years (2011, 2017, and 2019) have averaged 9000-11,000 cfs under the State’s current State D-1641 standards. This is a new impact (since 1995) that has manifested itself in a decreased proportion of wild fish in the salmon runs, thus threatening the very integrity of the populations and commercial and sport fisheries. Under the previous D-1485 standards, south Delta project exports in June were limited to 6000 cfs in all year types.
June River Flows and Water Temperatures
June river flows should be sustained to help move smolts downstream and maintain water temperatures below stressful levels (less than 68°F/20°C). River flows need to be adequate to keep water temperatures in the lower sections of the Sacramento River below 68°F/20°C, as recognized in the Central Valley Basin Plan’s water quality standard. The flows needed to maintain water temperatures depend on air temperatures. Over the past decade, water temperatures have exceeded the target in June in the lower Sacramento River even in wet years 2011, 2017, and 2019 (Figures 3 and 4).
June Delta Inflow
June Delta inflows need to be of sufficient magnitude to help salmon smolts pass through the Delta in a timely fashion, and not get diverted off-course toward the south Delta export pumps or succumb to huge numbers of predator fishes. June flow entering the north Delta at Freeport needs to be maintained near 20,000 cfs to maintain water temperatures near 68°F/20°C (Figure 5).
June Delta Outflow
With 20,000+ cfs inflow and south Delta exports limited to 6,000 cfs, Delta outflow will be 10,000+ cfs (the other 4,000 cfs is from within-Delta diversions). This is sufficient to keep the Low Salinity Zone west of the Delta and salmon smolts moving toward the Bay and Ocean.
In conclusion, the present year-round water temperature standard for the lower Sacramento River, 68°F/20°C, should be sustained through June. New State Board standards should limit south Delta exports in June to 6,000 cfs to protect wild salmon smolts that are emigrating from Central Valley rivers.