The river is formed at the west base of the
San Jacinto Mountains by the confluence of its North and South forks. The South Fork flows from near Santa Rosa Summit, through
Pine Meadow and
Garner Valley to
Lake Hemet, which holds of water. Hemet Dam was built in 1895 to supply water to the city of
Hemet. Downstream of the
dam, the South Fork joins the North Fork east of the town of
Valle Vista near
Highway 74, and the main stem of the San Jacinto River continues northwest until it discharges into
Mystic Lake, a couple of miles east of
Lake Perris. Overflow from the river then flows southwest, passing under Ramona Expressway and
Interstate 215, and through
Railroad Canyon to
Railroad Canyon Reservoir, also called Canyon Lake, which has a capacity of . Downstream of Railroad Canyon Dam, the river continues flowing roughly west southwest through the canyon through the
Temescal Mountains for about until it drains into
Lake Elsinore. The lake usually has no outflow other than evaporation, but in years of heavy rainfall it overflows into
Temescal Creek, which flows northwest to the
Santa Ana River in
Corona, California. ==List of tributaries of the San Jacinto River==