By the mid-2010s, California agencies had placed more than 200,000 tons of gravel below Keswick Dam, restoring egg-laying habitat for endangered salmon

One of California's more unusual salmon conservation efforts involves trucking thousands of tons of gravel to carefully engineered sites along the Sacramento River. The work has continued for decades below Keswick Dam in an effort to restore spawning habitat for endangered winter-run Chinook salmon.

Where the gravel went missing

It all began with Shasta Dam and Keswick Dam, two very large concrete structures that were put up across the Sacramento River in the 1940s and 1950s. These dams provide huge amounts of water and power for California, and the consequence that they had on the fish species that live there, in particular the winter-run Chinook salmon, was a very bad one.


Salmon need much more than water to survive. They need gravel, or, more precisely, loose, clean, and mobile gravel within the riverbed for female Chinook salmon to burrow into and lay their eggs. This nest is called a redd. In the past, when floodwaters rushed downstream, they carried gravel with them and deposited it further down the river, constantly replenishing the shifting gravel beds.

Then they put in dams and largely shut that process down. All that upstream gravel stopped behind the dam and was trapped there. While water that passed over the dam did carry away remaining gravel downstream, tributaries only contributed so much gravel locally, cutting off the primary upstream sources.

Years later, sections of the river below Keswick had lost much of the suitable spawning gravel that salmon had historically used.

And the winter-run Chinook salmon that had historically used upstream river sections had few alternatives. Most of their historic access to the colder, spring-fed waters of the Pit, McCloud, and Sacramento rivers had already been blocked by dams. As a result, they were confined to a relatively short stretch of the upper Sacramento River below Keswick Dam, where suitable gravel was essential for spawning. After years of population decline, the Sacramento River winter-run Chinook salmon were listed as an endangered species in 1994.

When dams stopped the gravel, trucks took over.

The agencies developed a relatively straightforward intervention: trucking gravel to carefully engineered sites in the river where it could provide suitable spawning habitat. Teams of the U.S. Bureau of Reclamation, California Department of Fish and Wildlife, and local water districts have placed truckload after truckload of clean, perfectly-sized gravel into the places on the river that salmon need since the 1990s.

It has helped, but it hasn’t solved the entire problem. Since the flow of materials upstream has been cut off, gravel can’t naturally be replaced at anything like the rate it used to, so human crews have to keep coming back and add more at specific points known to be good spots for spawning in some river sections every year.

Reports from the mid-2010s showed more than 200,000 tons of gravel dumped in the upper Sacramento River since the 1990s. One NOAA report estimated 213,000 tons dumped since 1997, and a 2024 California review says that together, water districts and agencies have dumped roughly 358,200 tons. Together, those estimates represent hundreds of millions of pounds of gravel placed in the river over several decades.

Does dumping rocks into a river actually work?

The approach is relatively straightforward, but projects continue because monitoring has documented salmon spawning in areas where gravel has been added. Salmon spawning has been documented in areas with added gravel, providing direct evidence that the intervention is producing functional spawning habitat.

Nevertheless, there is no silver bullet. Winter-run Chinooks are still endangered, and their populations fluctuate dramatically year-to-year based on factors such as drought, water temperature, and the volume of cold water in storage behind Shasta Dam. The drought of 2020-2022 yielded particularly low juvenile survival; NOAA reported that juvenile production and survival in 2022 were the lowest on record at that time, gravel or not. Spawning habitat is just one component of a much larger picture that includes water temperature, flow timing, and predators, all factors that gravel supplementation does not ameliorate. It simply assures that when appropriate conditions are otherwise met, there will be a place for salmon to spawn.

Why this keeps happening

The need to supplement gravel on an ongoing basis results from the changes the dams have made to the river's sediment system. During high flows, water moving downstream can mobilize and transport gravel along the riverbed. Under natural conditions, downstream transport of gravel would be balanced by an influx of new gravel from upstream over time. Shasta and Keswick Dams block much of the upstream sediment supply, leaving the river below them with limited capacity to replace gravel that continues to move downstream.

Due to this, new gravel can then be dispersed or lost in the next high flows. Thus, agencies track potential spawning beds and supplement the gravel as needed to maintain healthy habitat. This is not a single restoration but a management strategy in response to the dramatically modified supply of gravel in the system.