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Delta Pumps Throttled Back – December 2021

A January 1 article in the Bee noted federal and state Delta pumping plants were “throttled back” in late December to protect the nearly extinct Delta smelt and other fish.  Exports had reached 9000 cfs from December 17-19, 2021, and then cut to 2000 cfs on December 20, despite high Delta inflows from the Sacramento River (Figure 1).  With the Delta Cross Channel closed in December and the False River Barrier in place, exports were drawing from the interior Delta via the Old and Middle River channels (Figure 2).  Most of that water was replaced via Georgianna Slough from the Sacramento River (7000 cfs) and San Joaquin River (1000 cfs).  The overall pattern from December 19 is shown in the map below.

Map of daily average flow conditions in the Delta, December 19, 2021.

Export cutbacks in December were prompted by the federal ESA biological opinions to protect Delta smelt, as well as winter-run and spring-run salmon from the Sacramento River.  Adult smelt move up into the interior Delta from the Bay with the first late-fall, early-winter runoff events.  Juvenile salmon move downstream to the Delta during these same early storm events and move into the interior Delta via Georgianna Slough.

Historically, exports were maxed-out near 11,000 cfs during December storm flows to refill San Luis Reservoir in the San Joaquin Valley after the irrigation season.  But such high exports resulted in heavy losses of smelt and salmon at the south Delta pumping plants (Figures 3 and 4), prompting the mandates in the biological opinions for export reductions.

San Luis Reservoir at the end of December 2021 was only at 30% of its 2 million acre-feet capacity; when the historical average for December has been 60%.  There is strong pressure to refill the reservoir.  On the other hand, the smelt and salmon are endangered species on the verge of extinction after several multi-year droughts in the past 15 years.  The smelt and salmon are in the Delta now, and need protection, whereas San Luis still has a chance of filling this winter under the allowed exports if 2022 continues normal or wet.  Because Federal and state laws mandate protecting the endangered fish and the biological opinions specify priority be given to the fish under these specific circumstances, export restrictions are the appropriate prescription.

Figure 1. Delta inflow from the Sacramento River at Freeport (FPT) and San Joaquin River at Mossdale (MSD) November-December 2021.

Figure 2. Internal Delta tidally-filtered flows in Old and Middle River en route to south Delta export pumps in December 2021.

Figure 3. Exports and salvage of winter-run salmon at export pumps from December 1, 2002 to February 1, 2003.

Figure 4. Exports and salvage of Delta smelt at export pumps from December 1, 2002 to February 1, 2003.