Aliso Creek (Orange County)

Aliso Creek flows 19 miles (31 km) southwest from the Santa Ana Mountains to the Pacific Ocean. Its entire length runs through urban Orange County in the U.S. state of California; the mostly channelized creek collects seven main tributaries along its course. The name was given to the creek by Spanish conquistadors in the 1700s, although there are now many places in California that use the name. As of 2004, the 30.4-square-mile (78.7 km2) watershed had a population of 149,087 divided among eight major cities. The creek flows generally south-southwest through a narrow coastal watershed at the southern extreme of the arid Los Angeles Basin in a fairly straight course. Owing to the submersion of Southern California in the Pacific Ocean as recently as 10 million years ago, the creek flows over marine sedimentary rock that dates from the late Eocene to the Pliocene. The present-day form of the watershed, with its broad sediment-filled valleys and deeply eroded side canyons, was shaped by a climate change during the previous Ice Age that produced Aliso Canyon, the creek's final gorge.