Warming water in the Sacramento River in late April compromised salmon as rains diminished and Central Valley water managers captured snowmelt for storage and irrigation. The water warmed as wild juvenile salmon and 20 million or so hatchery smolts moved down the Sacramento River toward the ocean, and as adult winter-run and spring-run salmon migrated up the river.

Water temperatures increased despite a clear trajectory toward full reservoirs (Figure 1). Water temperatures at Wilkins Slough rose nearly 10^oF in late April to the low 60’s, while flows dropped from March flows of 25,000 cfs to only 12,000 cfs during the April hot spell (Figure 2). Though prescribed small flow pulses from Shasta Dam¹ (Figure 3) and cool weather (Figure 4) reduced thermal impacts in early May, that was not true for the end of April.

Allowing flows to drop and water temperatures to rise in the last half of April was irresponsible and easily avoidable. Releases from storage to keep the river cool would not have even come close to preventing Shasta from filling. The goal for the lower Sacramento River to minimize stress on migrating juvenile and adult salmon in a wet year like 2023 should be water temperatures below 60^oF (Figure 5).

Water managers need to manage for fish at the same time they manage for water supply, not as an afterthought. In a year of abundant water, active management for fish is not that hard and has no water cost. Even in less bountiful water years, the mindset and bias of filling before releasing requires adjustment.

Figure. 1. Shasta has dramatically filled this spring.

Figure 2. Streamflow and water temperature in the lower Sacramento River at Wilkins Slough in 2023.

Figure 3. End of April flow pulse from Keswick Dam near Redding.

Figure 4. Air temperatures at Red Bluff in 2023.

Figure 5. Interagency water temperature prescriptions for salmon recovery from the San Joaquin River Salmon Recovery Program.