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.

Etymology

The first recorded use of the name "Aliso" was for the Rancho Cañada de los Alisos Mexican land grant in 1841. The rancho area was renamed El Toro sometime before 1900, but the name "Aliso Creek" persisted.^[7] The word aliso means "alder" in Spanish, and refers to the riparian forests that historically occurred along the creek. The California sycamore, Platanus racemosa, is also known as aliso in Spanish, and is common in the area around the creek.

According to the U.S. Geological Survey's Geographic Names Information System, the creek has also been historically called "Los Alisos Creek" and "Alisos Creek".^[1] Several nearby geographical features also share the name, including the city of Aliso Viejo, Aliso Beach, Aliso Peak (a headland near the creek's mouth), Los Alisos Intermediate School in Mission Viejo, and Aliso Creek Road.

Course

Aliso Creek rises along the Loma Ridge in the foothills of the Santa Ana Mountains, near the community of Portola Hills, Lake Forest. The creek's headwaters are at an elevation of 1,700 feet (520 m) in the Cleveland National Forest near Whiting Ranch Wilderness Park. The creek flows south along Country Home Road then begins to parallel Santiago Canyon Road, which becomes El Toro Road south of Cook's Corner.^[8] The creek enters Mission Viejo, flowing freely in a natural streambed along the bottom of a brushy, shaded ravine.^[8] It turns southwest, crossing under the 241 toll road and passing Saddleback Church, then receives an unnamed tributary from the right. The creek then enters Lake Forest, where it receives Munger Creek from the right and English Canyon Creek from the left.^[8][9][10]

Below English Canyon, Aliso Creek flows in a concrete channel through Heroes Park and then reverts to a natural channel once again in El Toro Park. Past Muirlands Boulevard the creek flows in a concrete channel, making a sharp turn to the southeast before veering back south towards Interstate 5. Downstream of the freeway Aliso Creek flows through Laguna Hills then through Aliso Park in the retirement community of Laguna Woods Village.^[8] It enters Aliso Viejo at the Moulton Parkway bridge near Sheep Hills Park. Below this point the valley widens at the northernmost tip of Aliso and Wood Canyons Wilderness Park, the beginning of a greenbelt that stretches from here nearly to the Pacific Ocean.^[8]

Soon after entering the park, Aliso Creek is joined by the Dairy Fork from the right then the Aliso Hills Channel from the left, before passing under the 73 toll road. Below this point, the creek flows past Journey School, Aliso Niguel High School and Wood Canyon Elementary School as well as the Laguna Niguel Skate and Soccer Park, where it skirts the northwestern part of Laguna Niguel. Below Aliso Creek Road it is joined from the east by its largest tributary, Sulphur Creek. Sulphur Creek drains much of northern Laguna Niguel and is dammed to form Laguna Niguel Lake, the main feature of Laguna Niguel Regional Park.^[8][11]

From there, Aliso Creek turns west and enters Aliso Canyon, a nearly 1,000-foot (300 m) deep gorge which cuts through the San Joaquin Hills approaching the Pacific.^[8] About a mile (1.6 km) below Sulphur Creek, Aliso Creek is impounded at a small concrete dam. It then receives its second largest tributary, Wood Canyon Creek, to the south of Soka University of America.^[8] Below Wood Canyon it turns south, winding through the wilderness park then turning west at the South Orange County Wastewater Agency (SOCWA)'s Coastal Treatment Plant. It then flows through the former Aliso Creek Inn and Golf Course (now The Ranch at Laguna Beach), under Pacific Coast Highway, and empties into the sea at Aliso Beach in Laguna Beach.^[8][11][12]

Aliso Creek forms a narrow tidal lagoon just above its mouth at Aliso Beach. The lagoon originally covered a large area at the outlet of Aliso Canyon, dammed by a sandbar that only breached during the rainy season. Due to development of the golf course and parking lots at the mouth of the creek, the lagoon has been significantly reduced from its original size. The increased freshwater inflow caused by urban runoff has further disrupted the hydrologic regime of the lagoon, which occasionally breaches causing large surge flows into the Pacific Ocean before the sandbar re-forms.^[13][14]

Hydrology

Aliso Creek was historically a seasonal stream with a few sections containing water year round, including the headwaters and lower Aliso Canyon. Significant flow only occurred in the rainy months of November through March.^[15] As recently as 1982, the creek was observed to be dry in the summer.^[3] As of 2012, urban runoff contributed a dry season flow of 5 million gallons (20,000 m³) per day, or approximately 8 cubic feet per second (0.23 m³/s), at the creek's mouth.^[3] Urban runoff accounts for at least 80 percent of the creek's dry season flow.^[13][14]

The United States Geological Survey operated a stream gage on the creek at the El Toro Road bridge in Mission Viejo from 1930 to 1980. This gage measured runoff from 7.91 square miles (20.49 km²), or 26 percent of the watershed area.^[16] There was also a gauge in Laguna Beach which measured runoff from the entire watershed, but it operated only from 1982 to 1987.^[17]

The average annual flow at El Toro was 0.92 cubic feet per second (0.026 m³/s), ranging from 4.7 cubic feet per second (0.13 m³/s) in February to 0.05 cubic feet per second (0.0014 m³/s) in July.^[18] The highest peak flow was 2,500 cubic feet per second (71 m³/s) on February 24, 1969.^[16][19] At the Laguna Beach gage, the average annual flow was 19.2 cubic feet per second (0.54 m³/s), with a high of 50 cubic feet per second (1.4 m³/s) in March and a low of 4.5 cubic feet per second (0.13 m³/s) in June.^[20] The largest flow recorded at the Laguna Beach gauge was 5,400 cubic feet per second (150 m³/s) during the El Niño event on March 1, 1983.^[17]

Urbanization is the main cause of increased winter flooding, due to the covering of land with impervious surfaces. From 1931 to 1960, the average annual peak flow at the El Toro gauge was 511 cubic feet per second (14.5 m³/s), and between 1960 and 1980, the average peak flow was 1,178 cubic feet per second (33.4 m³/s).^[16]

Watershed

The Aliso Creek drainage basin lies in the south central part of Orange County, roughly halfway between the Santa Ana River and the border of San Diego County.^[21][22] It is an elongated area of 34.9 square miles (90 km²), ranging from about a mile (1.6 km) in width in the north to 5 miles (8.0 km) wide in the south. The watershed is characterized by rolling hills, with very little flat land except in the alluvial valleys along Aliso Creek. The portion of the Santa Ana Mountains in the Aliso Creek watershed top out at approximately 2,300 feet (700 m), while the San Joaquin Hills rise to 1,020 feet (310 m) at Temple Hill, locally called "Top of the World", in Laguna Beach west of Aliso Canyon.^[23] Aside from 44-acre (18 ha) Laguna Niguel Lake, an impoundment of the Sulphur Creek tributary, there are no major bodies of fresh water.^[24]

The watershed experiences a dry Mediterranean climate. As of 2001, the average annual precipitation in the San Juan Hydrological Unit, which Aliso Creek is part of, was 16.42 inches (417 mm).^[25] The watershed borders five major Orange County watersheds: Santiago Creek to the north, San Diego Creek to the west, Laguna Canyon to the southwest, Salt Creek to the southeast, and San Juan Creek to the east.^[9][26]

As of 2018, the Aliso Creek watershed had a population of 144,000 divided among seven incorporated cities, or a population density of 4,100 persons per square mile (1,600 persons per km²).^[4] Nine communities were established in the creek's watershed as it was developed in the 20th century. By 2001 seven of them had become cities (from mouth to source, Laguna Beach, Laguna Niguel, Aliso Viejo, Laguna Hills, Laguna Woods, Lake Forest (formerly El Toro), and Mission Viejo), and the last two, Foothill Ranch and Portola Hills, were incorporated into the city of Lake Forest in 2000.

The largest land use in the watershed is residential, which as of 2009 accounted for 39.6 percent of the total area. Other urban land uses are commercial (10.7 percent), miscellaneous (4.9 percent), agriculture (3.7 percent) and industrial (1.6 percent). Public lands, including national forest and county parks, comprised 26.4 percent of the watershed, and another 13.1 percent was unincorporated.^[24] Much of the terrain in the watershed has been regraded to build homes and roads, and a number of smaller tributaries such as Munger Creek have been completely filled in.

Crossings

v t e Aliso Creek
Legend
Source Priority Way El Toro Road Glenn Ranch Road Unnamed northern fork California State Route 241 Munger Creek Saddleback Parkway Portola Parkway Normandale Road El Toro Road El Toro stream gauge Entidad Creekside Hollow Trabuco Road English Canyon Creek Jeronimo Road Muirlands Boulevard Interstate 5 Paseo de Valencia Aliso Hills Channel Laguna Hills Drive Moulton Parkway California State Route 73 Dairy Fork Pacific Park Drive Concrete drop structure Aliso Creek Road Concrete drop structure Sulphur Creek Sulphur Creek Reservoir Aliso Canyon Dam Wood Canyon Creek Stream gauge and pump Pacific Ocean

Crossings of the creek are listed from mouth to source (year built in parentheses).^[27][28] The creek is crossed by roughly 30 major bridges.