California Department of Fish and Wildlife has announced that it opened the American River Nimbus Hatchery ladder on Nov 2. At the same time, the feds have managed to cool down the river to allow salmon in the river to also begin spawning (Figure 1). The Bureau of Reclamation cooled the river by opening lower level outlets of Folsom Dam for a portion of the day beginning in late October (Figures 2 and 3). This allowed the release of colder water from deep in the reservoir into the Nimbus regulating reservoir directly downstream of Folsom Reservoir. There, the cold water mixed with warmer reservoir waters before discharge to the lower American River.
The real story here is that the feds had to wait until November to cool water both in the river, and in the Nimbus Fish Hatchery just downstream of Nimbus Dam. The supply of cold water in Folsom Reservoir is limited this fall because of excessive releases of stored water to the Delta this past summer (see prior post). The summer shrinkage of Folsom’s cold-water pool subjected the salmon that entered the lower American River in September and October to a month or more of stress from warm water. That stress will likely reduce survival of pre-spawn and spawning salmon, diminish their success in spawning, and make many of eggs and embryos spawned in the river unviable.