The Trinity River flows for 700 twisting, winding miles from the rugged Trinity Alps in northwestern California to the Pacific Ocean. In the upper reach of the Trinity River are two dams – Trinity Dam and Lewiston Dam – that store and divert water to the Sacramento Valley as part of the Central Valley Project (CVP).

The Trinity River is the largest tributary of the Klamath River in the Klamath watershed that straddles Northern California and southern Oregon. The Trinity River once supported steelhead and several different runs of salmon that were important to the lives and subsistence of the Yurok and Hupa Native American tribes. But the Trinity and Lewiston dams have blocked spawning grounds for steelhead and salmon, causing fish populations to decline.

BACKGROUND

Though seemingly a long way from California’s Central Valley, the Trinity Dam, a 538-foot-tall earthfill structure, helps supply irrigation water for Valley farmers and produces hydropower.

Constructed in the 1950s, Trinity Dam and the 91-foot-tall Lewiston Dam, just downstream, increased the storage capacity of the federal CVP by more than 2.5 million acre-feet.

To get the water to the Central Valley, the 11-mile Clear Creek Tunnel transports water from Lewiston Reservoir through the Trinity Mountains to Whiskeytown Reservoir, 10 miles southwest of Shasta Dam. From Whiskeytown, Trinity River water is transported through a 3-mile tunnel to Clear Creek, which flows into the Keswick Reservoir, passing through five power plants before reaching the Sacramento River.

The Trinity River was first considered by the federal government as a possible location for a hydroelectric power plant in the 1920s. But it wasn’t until the 1950s that Congress authorized the Trinity River Division as a part of the CVP.

CHALLENGES AND CONTROVERSIES

The 1955 Trinity River Division Authorization Act required minimum water releases of 120,000 acre-feet to maintain Trinity River salmon and steelhead populations, and a fish hatchery was built downstream from Lewiston Dam. With up to 90 percent of the river’s average annual flow diverted to the Sacramento River, however, the diversion contributed to the declines in Trinity River chinook and coho salmon populations. (In 2003, the Trinity River was also diverted to boost flows in the lower Klamath River after a large number died in the Klamath the previous year.)

Consequently, the Bureau of Reclamation established the Trinity River Fish Hatchery to boost declining fish populations. As part of the establishment of the Central Valley Project Improvement Act in 1992, minimal annual releases from Lewiston Dam into the Trinity River are set at 340,000 acre-feet.

In 2000, the Trinity River Restoration Program was established to restore more naturally spawning salmon and steelhead populations in the river to pre-dam levels. The recovery plan for the Trinity River and its fish and wildlife populations increased flows based on water-year type and a combination of river channel rehabilitation, watershed restoration and sediment management. The plan guides restoration projects that recreate at a smaller scale the environment that existed prior to construction of the Trinity and Lewiston dams.

Flow releases in some years have been supplemented to protect the health of fish in the river, actions that have been controversial among some water users.

LOOKING AHEAD

Climate change is expected to bring more extreme weather, which could result in wetter winters with precipitation falling more as rain than snow or more extended dry spells. The dry spells could reduce water available to the CVP for Central Valley farms and cities as well as the Trinity River fishery. Meanwhile, the Trinity River Restoration Program continues to improve river habitat to increase survival of salmon and steelhead and their return for spawning. 

Updated May 2026