Aliso Creek (Orange County)

Aliso Creek Alisos Creek, Los Alisos Creek[1]
Aliso Creek flows underneath the Aliso Creek Road bridge before it enters Aliso and Wood Canyons Wilderness Park.
Map of the Aliso Creek watershed showing major tributaries and cities
Location of the mouth of Aliso Creek in California
Etymology	Rancho Cañada de los Alisos land grant; "Aliso" means alder or sycamore in Spanish.
Location
Country	United States
State	California
Counties	Orange County
Cities	Laguna Beach, Laguna Niguel, Aliso Viejo, Laguna Woods, Laguna Hills, Lake Forest, Mission Viejo
Physical characteristics
Source	Loma Ridge in the Santa Ana Mountains
• location	Cleveland National Forest north of Portola Hills
• coordinates	33°42′10″N 117°37′20″W / 33.70278°N 117.62222°W / 33.70278; -117.62222[1]
• elevation	1,704 ft (519 m)[1]
Mouth	Pacific Ocean
• location	Aliso Beach Park, Laguna Beach
• coordinates	33°30′38″N 117°45′9″W / 33.51056°N 117.75250°W / 33.51056; -117.75250[1]
• elevation	0 ft (0 m)[1]
Length	19.8 mi (31.9 km)[2]
Basin size	34.9 sq mi (90 km2)
Discharge
• location	Laguna Beach[3]
• average	8 cu ft/s (0.23 m3/s)[3]
• minimum	0 cu ft/s (0 m3/s)
• maximum	5,400 cu ft/s (150 m3/s)
Basin features
Tributaries
• left	English Canyon Creek, Aliso Hills Channel, Sulphur Creek
• right	Munger Creek, Dairy Fork, Wood Canyon Creek

Aliso Creek is a 19.8-mile (31.9 km)-long,^[2] mostly urban stream in south Orange County, California. Originating in the Cleveland National Forest in the Santa Ana Mountains, it flows generally southwest and empties into the Pacific Ocean at Laguna Beach. The creek's watershed drains 34.9 square miles (90 km²), and it is joined by seven main tributaries. As of 2018, the watershed had a population of 144,000 divided among seven incorporated cities.^[4]

Aliso Creek flows over highly erosive marine sedimentary rock of late Eocene to Pliocene age.^[5] What would become the Aliso Creek watershed originally lay at the bottom of the Pacific Ocean, before being uplifted as recently as 10 million years ago. About 1.2 million years ago, the San Joaquin Hills began to uplift in the path of Aliso Creek. Occasionally swollen by wetter climates during glacial periods, the creek carved the deep water gap known today as Aliso Canyon, the main feature of Aliso and Wood Canyons Wilderness Park.

Historically, Aliso Creek served as the boundary between the Acjachemem (Juaneño) and Tongva (Gabrieleño) Native Americans. Spanish explorers and missionaries reached the area in the 1700s and established Mission San Juan Capistrano, whose lands included part of the Aliso Creek watershed. In the 1840s the watershed was divided between several Mexican land grants. After California became part of the United States, the ranchos were gradually partitioned and sold off to farmers and settlers; starting in the 1950s, real estate companies acquired most of the land for development.

By the 21st century, more than 70 percent of the Aliso Creek watershed was urbanized. Most of the creek's course has been channelized or otherwise impacted by development. Pollution and erosion from urban runoff have become chronic issues.^[6][5] However, parts of the creek remain free flowing and provide important regional wildlife habitat, especially in the Aliso Canyon section. The creek has recently been the focus of projects to restore the stream channel and improve water quality.