{{Geobox|River}} The Sacramento River is the largest river of northern California in the United States. It begins in the Klamath Mountains, near Mount Shasta and flows south through the agricultural Sacramento Valley (the northern half of the Central Valley), to the Sacramento–San Joaquin River Delta where it empties into the Pacific Ocean via Suisun Bay. Sacramento, the state capital, is named for the river and sits at the confluence of the Sacramento and American rivers. The Sacramento River is 400-mile (640 km) long, draining a watershed of 26,500 square miles (69,000 km2) in nineteen California counties.
For most of its course, the river flows across a broad fertile floodplain consisting of alluvial sediments up to 5 miles (8.0 km) thick. Once home to enormous wetland and riparian habitats, and one of the largest Chinook salmon runs in North America, the Sacramento River basin has been populated by Native Americans for about 12,000 years. Spanish colonization of California starting in the 1700s largely avoided the Sacramento Valley due to the difficulty of navigating its swamps and seasonal channels. The river was named by Gabriel Moraga during an unsuccessful expedition to find suitable mission sites in the Central Valley. In the 1800s, fur trappers established trade routes through the Sacramento River watershed along old Native American trails.
Large-scale settlement by Europeans did not occur until after the Mexican-American War when California became part of the United States. In 1848 gold was discovered along a tributary of the Sacramento River, starting the Gold Rush which brought hundreds of thousands of people to California within a few years, establishing the Sacramento River as an important trade and travel route via steamboats, and accelerating the conversion of the Sacramento Valley to agriculture. High demand for fertile land in the valley led to conflicts with Native Americans and their eventual expulsion to reservations. Settlement along the river was not easy, due to its tendency of catastrophic flooding. The Great Flood of 1862, which destroyed Sacramento and turned the Central Valley into a 300-mile (480 km) long lake, was the impetus for construction of water works that would greatly transform the Sacramento River in the late 19th and 20th centuries.
The Sacramento River is the largest river by volume in California and is a key statewide water resource. Since the 1930s the watershed has been intensely developed by massive state and federal water projects, with the primary purpose of impounding floodwaters and delivering it to the drier parts of central and southern California. The Sacramento River irrigates 6 million acres (24,000 km2) of farmland and provides domestic water supply to over 20 million people, including parts of the San Francisco Bay Area and Los Angeles. As a consequence of this development, most of the original wetlands of the Sacramento Valley have been lost and the salmon run has massively declined. Many current water conservation efforts focus on environmental restoration, especially in the sensitive Delta region, but are controversial due to the potential impact on water supplies.