The Santa Ana River begins in San Bernardino County, California in the San Bernardino National Forest . After traveling nearly 100 miles, it eventually reaches its terminus between Newport Beach and Huntington Beach where is spills into the Pacific Ocean. Despite much of the river being channelized in recent times, it still represents one of the largest river systems in Southern California.

Due to Southern California's dry climate, very little water can be found in the river channel throughout the year. However, the Santa Ana River is still prone to flooding in wet seasons. The Army Corps of Engineers considers it to be the greatest flood hazard west of the Mississippi River. It produced devastating floods in 1862, 1938, and 1969. As a result of the 1938 flood, the Prado Dam was built near Corona in 1941. Officials in Orange County felt further protection was needed. The Seven Oaks Dam was completed in 1999, located a few miles north of Redlands.

In early 2005, Southern California experienced one of its wettest periods in recent history, which placed unusual stress on on the Santa Ana River system. Prado Dam, which was under construction for expansion at the time, began to crack, and residents of the local communities of Corona and Yorba Linda were evacuated as a precaution. The dam was damaged, but there was no major flooding as a result, and the evacuated residents were soon allowed to return home.

See also: List of California rivers