The Trinity River is the longest tributary to the Klamath River, and drains approximately 2,860 square miles, primarily in Trinity County, before joining the Klamath at Weitchpec in northern Humboldt County. The headwaters are deep within the remote and rugged Trinity Alps. The Lower Trinity passes through the Hoopah Valley Indian Reservation and was originally known as the Hoopah River. The Central Valley Project and construction of Trinity Dam and Lewiston Dam diverted much of the Upper Trinity’s water to the Sacramento Valley, but a minimum annual flow has since been established. Trinity Dam blocked salmon access to approximately half the spawning and rearing habitat in the Trinity River. The Trinity Hatchery was built and operates to mitigate for the loss of salmonid production due to the loss of habitat above the dam. The South Fork Trinity remains the largest undammed river in California. The dominant land uses are timber, agriculture and recreation. Trinity has a swift current and is a popular destination for whitewater rafting and kayaking.
The Trinity River is included on the federal Clean Water Act list of impaired water bodies due to excessive sediment, high temperatures and mercury. Coho salmon in the Trinity watershed — including the sub-watersheds of the Upper Trinity, Lower Trinity and South Fork Trinity — are state and federally listed as threatened; Chinook and steelhead are not currently listed.