By Gary Pitzer, Bureau of Reclamation
In addition to sufficient flows of cold water, chinook salmon migrating in the Sacramento River depend on having sufficient gravel in the riverbed to support spawning.
In response to that need, Reclamation and its partners — U.S. Fish and Wildlife Service, California Department of Fish and Wildlife, Sacramento River Settlement Contractors, Reclamation District 108, and the Glenn-Colusa Irrigation District — recently completed placing 20,000 tons of spawning gravel on the west bank of the Sacramento River below Keswick Dam. With the passage of time and the arrival of high flow, the gravel will wash downstream into the riverbed, creating the vital habitat that helps ensure the species’ survival.
Gravel placement began April 4 with 1,200 tons distributed a few hundred yards downstream of Keswick Dam, in front of the Keswick office building.
“Placing gravel at this location at the dam is critical for maintaining as much habitat in the coolest water as we can,” said John Hannon, fish biologist with Reclamation’s Bay-Delta Office. “The coolest water in the summer winter-run chinook spawning period is that coming out of the dam and in dry years like this the water temperature is a big limiting factor to winter-run successfully reproducing. If they can’t reproduce then the habitat for juveniles in the rest of the river does not matter.”
The Central Valley Project Improvement Act authorizes a continuing program to restore and replenish spawning gravel lost due to the construction and operation of Central Valley Project dams and other actions that have reduced the availability of spawning gravel and rearing habitat in the Sacramento River from Keswick Dam to Red Bluff Diversion Dam. Gravel was last placed at Keswick in 2019.
The endangered Sacramento River winter-run chinook salmon consists of one population confined to the Sacramento River. At present, the freshwater life stages of winter-run chinook salmon are found only in the Sacramento River below Keswick Dam, thus all winter-run production occurs in the Sacramento River. The fish require river gravel with a mix of sizes as large as 4 inches. This size of gravel not only provides protection for the eggs, but also allows water to flow through and deliver oxygen that the eggs need to develop and is small enough for the adult female to move. Access to approximately 58 percent of the original winter-run chinook salmon habitat has been blocked by dam construction.
“Dams not only block water but they also block sediment, so we need to replace the sediment downstream of our dams to build habitat for anadromous fishes,” said Derek Rupert, fish biologist with Reclamation’s Northern California Area Office.
Salmon typically choose their nesting sites in riffles where the water is shallow, and the flow is fast and turbulent. Their nests are called redds, which are small pockets in the gravel riverbed, created by the female salmon using her tail. Female salmon typically die within a week after laying their eggs. For example, the Pacific salmon live in the ocean for many years before migrating up rivers to spawn. However, they die after laying their eggs, so they only spawn once in their lifetime.
As gravel moves downstream, it builds river bars and riffles. “The salmon will key on these areas,” Rupert said. “They will spawn on riffle crests and as their juveniles hatch, they’ll find refugia in areas behind river bars in slow and slack water where they can feed and grow before they start their migration out to the ocean.”
