In Above Normal water year 2024, an increase in summer freshwater flows released from reservoirs to the Bay-Delta estuary has proven a viable action to reduce threats to Central Valley fishes.  Water management actions in early July heat waves alleviated extreme water temperatures that threaten the native fishes in rivers and the Bay-Delta.  These actions can be described as adaptive management experiments to test their potential performance for the update of the State Board’s Bay-Delta Plan. 

Actions

• Increasing lower Sacramento River flow from 5000 cfs to near 8000 cfs has helped lower Wilkins Slough gage (WLK) water temperatures from 70-72^oF to the water quality standard of 68^oF (Figure 1), despite record-high air temperatures (Figure 2).
• Increasing Delta inflow at Freeport (FPT) from 14,000 cfs to 20,000-22,000 cfs (a combination of increased Wilkins Slough, Feather River, and American River flows) has lowered Freeport water temperatures in the north Delta from 72^oF to 70^oF (Figure 1).
• A rise in Delta outflow from 8,000 cfs to 12,000 cfs has helped reduce Rio Vista (RVB) water temperatures at the Delta’s exit to the Bay from 75-76^oF during the early July heat wave to 72^oF after the heat wave (Figure 2).

Benefits

The actions may not seem that dramatic, but they are very important to the river, Delta, and Bay environments and to the salmon, smelt, steelhead, sturgeon, and other native Central Valley fishes that depend on these habitats.  Water temperatures in the 72-75^oF range are highly stressful or lethal to these native fishes.  Such temperatures favor non-native predatory and competing fishes.  Water temperatures of 68-72^oF are at the upper favorable limits for the native fish and are necessary to maintain viable growth, survival, and reproduction.  These temperatures also help ensure that dissolved oxygen is adequate and that algae blooms do not reach excessive levels.  In other words, they promote a healthier ecosystem.  Water temperatures near or above 75^oF, which occurred in the Delta of drought years 2021 and 2022 (Figure 3) under extreme low flows (Figure 4), are deadly to native Delta fishes.

Conclusion

Although water managers in 2024 probably did not have these bold actions in mind to save fish (flows were increased to allow maximum summer water diversions from the Delta), their fortuitous implementation clearly highlights early summer flow measures that should be included in the update of the Bay-Delta Plan.  Two additional actions I would recommend are higher flows in the San Joaquin River to provide some minimal benefit to the San Joaquin’s native fish community, and reductions in water exports.

Figure 1.   DTO = Delta Outflow to the Bay, FPT = Sacramento River Freeport gage, WLK = gage below Wilkins Slough on lower Sacramento River upstream of the Delta, RVB = Rio Vista Bridge in Sacramento River channel northwest Delta, near entrance to eastern Bay.

Figure 2.  Average daily air temperatures at Red Bluff (KRDD) in Sacramento Valley, Modesto (KMOD) in San Joaquin Valley, and Rio Vista Bridge (RVB) in west Delta May-July 2024.  Note Delta air temperatures are generally 5-15^oF lower in the Delta than the valleys.  Also note the record or near-record air temperatures in early July.

Figure 3.  Average daily water temperatures in the north Delta channel of the Sacramento River at Freeport May-July 2021-2024 and average of years for decade 2001-2010.

Figure 4.  Average daily (tidally filtered) streamflow in the north Delta channel of the Sacramento River at Freeport, May-July 2021-2024 and average of years for decade 2001-2010.