Aliso Creek is a 19mi-long urban stream that runs through Orange County in the U.S. state of California from the Santa Ana Mountains to the Pacific Ocean, collecting seven main tributaries. The creek is mostly channelized, and as of 2004, the 30.4sqmi watershed had a population of 149,000 divided among seven incorporated cities.The creek flows generally south-southwest through a narrow coastal watershed at the southern extreme end 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.The name was given to the creek by Spanish explorers in the 18th century, although there are now many places in California that use the name. Historically, the creek served as the boundary between the Juaneño and Gabrieleño Indians. The creek's watershed then became a major portion of the 1842 Rancho Niguel Mexican Land Grant to Juan Avila, later purchased by two American ranchers. Although attempts to use the creek and its watershed as a municipal water source date to the early 20th century, the water it provided was of poor quality and erratic occurrence. As a result, the creek became neglected throughout the late part of the century, eventually becoming little more than an open wastewater drain. Despite this general decline, the Aliso Creek watershed still supports some biodiversity, and it remains a popular recreational area.
