Sorry to say this is not in California – it’s the Columbia River system with its eight major mainstem dams.1 Summer “spills” have been the heart of the Columbia salmon recovery because they have helped smolts reach the ocean. The cost of “spills” is primarily lost hydropower to federal and state utilities. That was the price for keeping all the dams.
In the Central Valley the dams were built for hydropower, flood control, and water supply. Here we have Settlement Contractors with water rights that preceded the dams, who agreed to contracts that allowed the dams to be built. These folks come first in line when it comes to federal and state water rights to stored water. After these folks come the big water districts and urban water contractors of the Central Valley Project and the State Water Project.
The Columbia dams also have fish ladders that allow adult salmon to reach tributaries and headwaters and require smolts to pass the dams to reach the ocean. In the Central Valley we have wild and hatchery salmon populations below the large dams, and there are no fish ladders. (Note there are also no ladders on the big Grand Coulee and Hells Canyon Columbia system dams.)
The equivalent action of “summer spills” on the Columbia would be “spring spills” from Central Valley reservoirs. However, with stored water over-appropriated (even in most flood years when too little storage is carried over for the following year), there really is no water for “spring spills” without taking water away from people, mostly irrigators, who expect to get that water. (Note there are some higher flow requirements on the Sacramento and San Joaquin rivers and from the Delta in wetter years, but these are not near the amount of water in “spills”.)
Would Central Valley salmon benefit from “spring spills”? Yes, substantially, especially in non-flood wet and normal years. Obviously, there would be insufficient water for “spills” in some drier years and even in some normal years.
What do they do on the Columbia River in dry years? They have a smolt collection and transport program that collects wild and hatchery smolts and transports them with trucks and barges around the dams and to the estuary.
The federal recovery plan for Central Valley salmon does not include either spills or transport, but instead requires trap-and-haul above the dams. This is important and likely essential to prevent extinction of some salmon races (e.g., winter and spring run), but we also need spill and transport programs downstream of the dams. If we want to retain these fisheries, we must invest in spill and transport programs now. These programs, like those on the Columbia, should be paid for by those benefitting from the flood control, electricity, water supply and recreation provided by the dams.