The Sacramento River salmon season opens on July 16 in the lower Sacramento River below Red Bluff, and in the lower Feather and American Rivers, as well as in the Bay-Delta. Will the Sacramento River remain cold enough to allow the fall-run salmon to leave the Bay and Delta for the rivers? Are the lower rivers warming into the 72-75°F range that blocks migration and stresses the adult salmon?

Salmon will be headed for the upper river reaches where water temperatures are cool, near 60°F. The Bay and west Delta remain below 70°F. At Rio Vista and Freeport on the Sacramento River in the north Delta water temperature have reached 70°F and are slowly rising. Upstream of the Delta, downstream of the points of relatively cool inflow from the Feather and American rivers, the Sacramento River remains just below 70°F.

Upstream from the mouth of the Feather, water temperatures in the Sacramento River are increasingly problematic. The hundred miles of river upstream from the mouth of the Feather to Hamilton City (RM 200) have a Central Valley Basin Plan upper temperature limit of 68°F in summer to protect the salmon during their run up the river in summer and fall. The only water temperature recorder is at Wilkins Slough, about mid-way in the reach, about 20 miles below Colusa (RM 143). Water temperature at Wilkins Slough has risen from 65°F to 72°F in the past week.

Summer water temperatures at Wilkins Slough vary with air temperatures, but are also determined in part by river flow. In the critically dry year last summer, water temperatures in July approached the lethal level for salmon of 80°F (Figure 1) under very low river flows (Figure 2). In below-normal year 2010, water temperatures were cooler (Figure 3) under higher river flows (Figure 4).

The flow at Wilkins Slough has increased over the past week from 3500 cfs to 5000 cfs, but the water temperature has continued to rise to 72°F with higher air temperatures. With even warmer air temperatures forecasted in the coming weeks, river flow should be increased to 6000 cfs or higher as in 2010 to maintain water temperature below 72°F. Temperatures above this level impede migration and stress adult salmon. A flow of 8000 cfs, as occurred in the summer of dry year 2012, would be more protective (Figures 5 and 6).

Figure 1. Water temperature at Wilkins Slough on the Sacramento River (RM 125) in June-July 2015, a critically dry year.

Figure 2. Sacramento River flow at Wilkins Slough in June-July 2015.

Figure 3. Water temperature at Wilkins Slough on the Sacramento River (RM 125) in June-July 2010, a below normal year.

Figure 4. Sacramento River flow at Wilkins Slough in June-July 2010.

Figure 5. Water temperature at Wilkins Slough on the Sacramento River (RM 125) in June-July 2012, a dry year.

Figure 6. Sacramento River flow at Wilkins Slough in June-July 2012.