The San Gabriel River flows 75 miles (121 km) through southern Los Angeles County, California, USA. The river drains a long, narrow watershed extending from the mountains rimming the eastern Los Angeles Basin to the Pacific Ocean. It derives its name from the Spanish Mission San Gabriel Arcángel which was originally built in the Whittier Narrows (1771) before being moved to its present location in San Gabriel. Although once free flowing and lined with marshes and grasslands for much of its length, today most of the river is a concrete channel, and impounded in places by flood control dams.
In bygone times, the river ran freely across arid grasslands and through extensive marshes to the Pacific Ocean, flooding in the winter and spring then running nearly dry in the summer and fall. Once out of the mountains, the river's course would change frequently with every heavy inundation. Sometimes, the river would change course to run into the Los Angeles River in the west, and sometimes the Santa Ana River's floodwaters would travel westwards into the San Gabriel from Santa Ana Canyon. The River was historically in the tribal territory of the Tongva Native American group. Together with the Los Angeles and Santa Ana Rivers, the San Gabriel River provided sustenance for hundreds of members of this powerful coastal tribe, whose territory extended across the entire Los Angeles Basin.
In 1771, the Spanish founded Mission San Gabriel Arcángel near the present-day site of the city of San Gabriel, on the banks of the Rio Hondo, a distributary of the San Gabriel River. The river's modern name derives from the mission. The modern name of the Tongva, the Gabrielino, also was derived from the name of the mission. After California became part of the United States, Americans established the city of Los Angeles, and after the Los Angeles Aqueduct was constructed, the first development boom in the area began, creating many of the towns and cities that now line the San Gabriel River. Many, including Whittier, were named after their founders, and legend has it that Azusa's name was derived from "everything from A to Z in the USA", although it was actually obtained from a Native American word. In this period, agriculture was the primary economy of the region.
Devastating floods wreaked havoc along the San Gabriel River in the late 1800s and the early years of the 20th century. The most famous was the Los Angeles Flood of 1938, which produced the highest flows ever recorded in the river—some 27,000 cubic feet per second (760 m3/s) according to a U.S. Geological Survey river gauge near Los Alamitos. As a result, the U.S. Army Corps of Engineers began an ambitious effort to prevent flooding along the river in the lowlands. Much of the river downstream of Azusa was diked, channeled, lined with riprap or paved over with concrete. A cascade of 10 drop structures was constructed where the river empties out of San Gabriel Canyon to slow flood flows from the mountains. Check dams were constructed in upper canyons and the river itself was impounded in several artificial lakes.
The increased flood protection afforded by the dams and channels indirectly led to a housing boom from the 1960s to the 1980s. Most of the lowlands and agricultural areas in the watershed were paved over to construct residential districts. Except for the Angeles National Forest (San Gabriel Mountains) and the Puente Hills between the San Gabriel Valley and the Los Angeles Basin, the remaining flat land in the watershed was filled with development. The river also provides a small amount of municipal water.