Water years 2016 and 2020 were below-normal water years in the Central Valley. Water year 2016 followed three critically dry, drought years, whereas 2020 followed two wet years (2017 and 2019) and one normal (2018) year. So one might assume that 2020 would have been better for Sacramento River salmon than 2016. But it ain’t so – because two different federal administrations were managing Shasta operations. The Trump administration’s policy to “maximize deliveries” of water that began in 2020 had consequences that turned deadly for salmon in critically dry 2021.
First and foremost, Shasta Reservoir storage in 2016 was surprisingly about 500,000 acre-feet or more higher than it was in 2020 after the first of April (Figure 1). Although water year 2020 started out nearly 2 million acre-feet (MAF) higher after a wet year, Shasta storage rose sharply in 2016, nearly filling (4.6 MAF capacity) with winter rain. But the real question is why reservoir storage did not recover in spring 2020. The reason is simply that in 2020, high spring and early summer reservoir releases for water deliveries released water from Shasta almost as fast as it was coming in (Figure 2). If in mid-March, when the reservoir storage was at 3.5 MAF in both years, similar storage-release constraints were in place in 2020 as in 2016, then 2020 would have ended the summer about 500,000 acre-feet higher than it did, near the 2016 storage level (Figure 3).
As a consequence of the storage difference and summer reservoir management, water temperatures downstream of Shasta Reservoir were significantly higher in 2020 than they were in 2016 (Figures 4 and 5). One reason for this was a much reduced volume of the Shasta Reservoir cold-water pool in 2020 compared to 2016 (Figure 6).
In conclusion, the Bureau of Reclamation managed water for winter-run salmon in normal water year 2016 much better than it did in normal water year 2020. Knowing the reservoir would likely not fill in 2020, Reclamation should have deployed a more conservative spring and summer release pattern, similar to what it did in 2016, to sustain cold-water releases from the reservoir through the summer and fall.