The Trinity River is a major river in northwestern California in the United States and is the principal tributary of the Klamath River. The Trinity flows for 165 miles (266 km) through the Klamath Mountains and Coast Ranges, with a watershed area of nearly 3,000 square miles (7,800 km2) in Trinity and Humboldt Counties. Designated a National Wild and Scenic River, along most of its course the Trinity flows swiftly through tight canyons and mountain meadows.

The Klamath River flows 257 miles (414 km) through Oregon and northern California in the United States, emptying into the Pacific Ocean. By average discharge, the Klamath is the second largest river in California after the Sacramento River. Its nearly 16,000-square-mile (41,000 km2) watershed stretches from the high desert of south-central Oregon to the temperate rainforest of the North Coast. Unlike most rivers, the Klamath begins in a desert region and flows through the rugged Cascade Range and Klamath Mountains before reaching the ocean; National Geographic magazine has called the Klamath "a river upside down".