The San Gabriel River flows 60.6 miles (97.5 km) through southern Los Angeles County, California in the United States. Its main stem is about 43 miles (69 km) long, while its farthest tributaries extend almost 18 miles (29 km) altogether. It drains a long, narrow watershed basin extending from high in the San Gabriel Mountains above the eastern Los Angeles Basin, across the San Gabriel Valley, to the Pacific Ocean.
The river derives its name from the Spanish Mission San Gabriel Arcángel founded in 1771, now in the present Day city of San Gabriel. It was free flowing with natural banks and a riparian zone habitat lined with riparian forests, marshes, and grasslands for much of its length and a large estuary at its mouth until the last century.