The Stanislaus River is a tributary of the San Joaquin River in north-central California in the United States. The main stem of the river is long, and measured to its furthest headwaters it is about long. Originating as three forks in the high Sierra Nevada, the river flows generally southwest through the agricultural San Joaquin Valley to join the San Joaquin south of Manteca, draining parts of five California counties. The Stanislaus is known for its swift rapids and scenic canyons in the upper reaches, and is heavily used for irrigation, hydroelectricity and domestic water supply.