The main stem is short at about 12.7 miles ( 20.4 km ) .
It drains an extensive basin of 218 square miles ( 560 km ) from the San Gabriel Mountains to the Santa Ana River near Corona .
Although the main stem is short at about 12.7 miles ( 20.4 km ) , it drains an extensive basin of 218 square miles ( 560 km ) from the San Gabriel Mountains to the Santa Ana River near Corona .