In a February 2019 post, I discussed the importance of winter flows for fall-run salmon in the Central Valley. The peak fry emergence from gravel spawning beds is in winter. Millions of fry move to river margins to await flow pulses to carry them from upper main river and tributary spawning grounds to lower river floodplain, Delta, and Bay nurseries. Without such pulses, the fry stay in the cold rivers competing for limited food and habitat, which leads to poor overall survival and fewer smolts reaching the ocean.
Two January storms in 2020 show the importance of flow pulses for the emigration of fall-run salmon fry (Figures 1-3). Figure 1 shows fry moving downstream from spawning grounds above Red Bluff. Figure 2 shows fry reaching the lower river 100+ miles downstream of Red Bluff. Figure 3 shows fry reaching the north Delta near Sacramento.
What is missing is reservoir releases through tailwater spawning grounds during the storms that create pulses from tributary inflow further downstream. The tributary inflow moves fry downstream from the tributaries. It also moves fry from the mainstem rivers downstream once fry reach the river reaches downstream of the tributaries. But reservoirs capture almost all the flow on the mainstem rivers upstream of the tributaries. During early winter storms, fry aren’t stimulated to move out of the spawning reaches directly downstream of dams.
Figure 4 shows the complete lack of such storage releases in 2020, even after a wet water year when storage was well above average. Pulse flows are needed below all the main storage reservoirs: Shasta, Whiskeytown, Oroville, Folsom, Bullards Bar, Camanche, New Melones, etc. Fry movement from these prime tailwater spawning grounds would then take advantage of the natural rainfall in the main rivers moving through the Delta and on to the Bay nurseries.
Neither of the recent National Marine Fisheries Service’s (NMFS) consultations and the associated biological opinion with Reclamation on the Central Valley Project promotes such winter flow pulses.1 NMFS mandates spring pulses to help smolts (juveniles that are larger and older than fry) reach the Bay. Spring pulses are important, but they are not enough. While individual smolts are more likely to reach the Bay than individual fry, fry vastly outnumber smolts and should contribute substantially to the adult salmon populations. Winter flow pulses are needed because they will improve the survival to adulthood of wild salmon fry.
For more on the importance of increasing the survival rate of wild salmon fry in the Central Valley, see a recent paper by Sturrock et al. 2019. 2