satellite view showing 4 major dams in san gabriel river system. san gabriel (above right) built in conjunction cogswell (top left) in 1930s.
in 1920s, los angeles county flood control district proposed impound san gabriel river below confluence of east , west forks 512-foot (156 m) high, 2,500-foot (760 m) long concrete arch dam capture floods , provide water conservation. named san gabriel forks dam, project canceled state engineer after having convened inquiry investigate problems occurring @ san gabriel dam site, including landslide destroyed large portion of construction site, in november 1929. inquiry panel concluded , issued report state engineer stating proposed dam cannot constructed without creating menace life , property.
subsequently, design of san gabriel river flood control project changed 1 large dam 2 smaller dams: san gabriel no. 1 (san gabriel dam), 2 miles (3.2 km) below original forks site, , san gabriel no. 2 (cogswell dam), on west fork 15 miles (24 km) above confluence. cost of san gabriel no. 1 understated, creating scandal; in context great depression, los angeles times observed not necessary gamble $26 million on [troubled , wasteful agency] . bond measure necessary financing of project defeated narrow margin of 52 48 percent. result, city turned federal public works administration money complete project.
the san gabriel no. 1 dam made of rock-fill, provides greater seismic stability. construction of 315-foot (96 m) high san gabriel dam @ site began in 1932 , completed in 1939.
in 1938, before dam finished, southern california hit record floods. heaviest rains fell in san gabriel mountains, swelling san gabriel river; on night of march 2–3, 1938 flood of 150,000 cu ft/s (4,200 m/s) poured out of mountains , san gabriel reservoir. san gabriel dam able knock 40,000 cu ft/s (1,100 m/s) off peak of flood. further downstream, morris reservoir able absorb 30,000 cu ft/s (850 m/s), reducing flood less half of have been if not dams.
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