Aliso Creek (Spanish for "Alder Creek"; also called Alisos Creek) is a 19-mile (31 km)-long urban stream that runs through Orange County in the U. 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 30.4-square-mile (78.7 km 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.
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.
Pollution, floods and development of the watershed and the surrounding county have blighted the water quality and wildlife of the creek since the 1960s, The word aliso means "alder tree" in Spanish, and likely refers to the riparian vegetation that lines the creek especially near its mouth.
The California sycamore, Platanus racemosa, is also known as aliso in Spanish, and is common in the area around the creek.
According to the Geographic Names Information System of the United States Geological Survey, there are now 46 places in California that use the name, Several nearby geographical features also are named for the creek, including Aliso Peak, a 683-foot (208 m) headland.
A middle school in the Saddleback Valley Unified School District, Los Alisos Intermediate School, borders the creek.
The creek is also the namesake of Aliso Creek Road, which crosses the creek once and only parallels it for a short length.
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 (Acjachemem) and Gabrieleño (Tongva) 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.