Siltcoos Lake (silt’-koos), at 3,164 acres (1,280 ha), is the largest lake on the Oregon Coast of the United States. Fed by runoff from a basin of about 68 square miles (180 km2) in Douglas and Lane counties, it is about 7 miles (11 km) south of Florence and 0.5 miles (1 km) east of U.S. Route 101. Its name comes from a Lower Umpqua (Siuslaw) placename, a variant of which is Tsiltcoos.
The lake, with a mean elevation of 13 feet (4 m) above sea level, empties into the Siltcoos River, which meanders generally west for about 3 miles (4.8 km) to its mouth on the Pacific Ocean. A dam about a half-mile downriver regulates the lake's outflow.