San Gabriel River (California) - Floods

Floods

Devastating floods wreaked havoc along the San Gabriel River in the late 19th century and the early years of the 20th century. The most famous were the Great Flood of 1862 and the Los Angeles Flood of 1938, which produced some of the highest flows ever recorded in the river. The 1938 flood raised the river to some 65,700 cu ft/s (1,860 m3/s) according to a U.S. Geological Survey river gauge near Azusa; although the 1862 flood probably produced an even higher flow, its discharge was not recorded. The flood of 1938 would have been far worse if it were not for the dams already on the San Gabriel River, which knocked nearly 85,000 cu ft/s (2,400 m3/s) off the crest of the flood. 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. (See #River modifications.)

The increased flood protection afforded by the dams, stormwater management ponds, and flood control channels led to a housing boom from the 1950s to the 1980s. Most of the lowlands and agricultural areas in the watershed were built and paved over to construct residential, commercial, and industrial districts. Except for the Angeles National Forest protected San Gabriel Mountains and the Puente Hills between the San Gabriel Valley and the Los Angeles Basin, the remaining former flood plain land in the watershed was filled with urban sprawl developments. The river's reservoirs also provide a small amount of municipal water.