Prado Dam is an earth-fill dam across the Santa Ana River at the Chino Hills near Corona, California in Riverside County with the resulting impounded water creating Prado Flood Control Basin reservoir. The U.S. Army Corps of Engineers built the dam in Lower Santa Ana River Canyon. Its primary purpose is flood control and it is the downstream element of the Santa Ana River's flood control system, which is a natural constriction about 30.5 mi (49.1 km) upstream from the ocean. The area upstream from the dam contains 2,255 sq mi (5,840 km²) of the watershed's 2,650 sq mi (6,900 km²). The dam's construction was authorized in 1936 and the flood of 1938 demonstrated its necessity.