Aliso Creek is a -long,
mostly
urban stream
An urban stream is a formerly natural waterway that flows through a heavily populated area. Urban streams are often polluted by urban runoff and combined sewer outflows. Water scarcity makes flow management in the rehabilitation of urban stream ...
in south
Orange County
Orange County most commonly refers to:
*Orange County, California, part of the Los Angeles metropolitan area
Orange County may also refer to:
U.S. counties
*Orange County, Florida, containing Orlando
*Orange County, Indiana
*Orange County, New ...
,
California
California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
. Originating in the
Cleveland National Forest
Cleveland National Forest encompasses 460,000 acres (), mostly of chaparral, with a few riparian areas. A warm dry mediterranean climate prevails over the forest. It is the southernmost U.S. National Forest of California. It is administered by ...
in the
Santa Ana Mountains
The Santa Ana Mountains are a short peninsular mountain range along the coast of Southern California in the United States. They extend for approximately southeast of the Los Angeles Basin largely along the border between Orange and Riverside co ...
, it flows generally southwest and empties into the
Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
at
Laguna Beach
Laguna Beach (; ''Laguna'', Spanish for "Lagoon") is a seaside resort city located in southern Orange County, California, in the United States. It is known for its mild year-round climate, scenic coves, environmental preservation efforts, and a ...
. The creek's
watershed drains , and it is joined by seven main
tributaries
A tributary, or affluent, is a stream or river that flows into a larger stream or main stem (or parent) river or a lake. A tributary does not flow directly into a sea or ocean. Tributaries and the main stem river drain the surrounding drainag ...
. As of 2018, the watershed had a population of 144,000 divided among seven incorporated cities.
Aliso Creek flows over highly erosive marine
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
of late
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
to
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 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
The San Joaquin Hills are a low mountain range of the Peninsular Ranges System, located in coastal Orange County, California.
They extend in a northwest–southeast direction, starting in the northwest in Newport Beach at the southern edge of t ...
began to uplift in the path of Aliso Creek. Occasionally swollen by wetter climates during glacial period
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betwe ...
s, the creek carved the deep water gap
A water gap is a gap that flowing water has carved through a mountain range or mountain ridge and that still carries water today. Such gaps that no longer carry water currents are called wind gaps. Water gaps and wind gaps often offer a prac ...
known today as Aliso Canyon
Aliso Canyon is a canyon located in Orange County, California in the United States. The canyon is a water gap across the San Joaquin Hills carved out by Aliso Creek, possibly as recently as the last ice age. Located in a semi-arid climate, it ...
, the main feature of Aliso and Wood Canyons Wilderness Park
Aliso and Wood Canyons Wilderness Park is a major regional park in the San Joaquin Hills of Orange County, California in the United States. Comprising of rugged coastal canyons, open grassland, and riparian woodland, the park borders the sub ...
.
Historically, Aliso Creek served as the boundary between the Acjachemem
The Acjachemen (, alternate spelling: Acagchemem) are an Indigenous people of California. They historically lived south of what is known as Aliso Creek and north of the Las Pulgas Canyon in what are now the southern areas of Orange County and t ...
(Juaneño) and Tongva
The Tongva ( ) are an Indigenous people of California from the Los Angeles Basin and the Southern Channel Islands, an area covering approximately . Some descendants of the people prefer Kizh as an endonym that, they argue, is more historically ...
(Gabrieleño) Native Americans. Spanish explorers and missionaries reached the area in the 1700s and established Mission San Juan Capistrano
Mission San Juan Capistrano ( es, Misión San Juan Capistrano) is a Spanish mission in San Juan Capistrano, Orange County, California. Founded November 1, 1776 in colonial ''Las Californias'' by Spanish Catholic missionaries of the Franciscan O ...
, whose lands included part of the Aliso Creek watershed. In the 1840s the watershed was divided between several Mexican land grant
The Spanish and Mexican governments made many concessions and land grants in Alta California (now known as California) and Baja California from 1775 to 1846. The Spanish Concessions of land were made to retired soldiers as an inducement for ...
s. 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
Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain , storms and other precipit ...
have become chronic issues. 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
Rancho Cañada de los Alisos was a Mexican land grant in present-day Orange County, California given by Governor Juan Bautista Alvarado to Jose Antonio Fernando Serrano in 1842, and enlarged by a second grant by Pio Pico in 1846. The name mean ...
Mexican land grant in 1841. The rancho area was renamed El Toro sometime before 1900, but the name "Aliso Creek" persisted. The word ''aliso'' means "alder
Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...
" in Spanish, and refers to the riparian forests that historically occurred along the creek. The California sycamore, ''Platanus racemosa
''Platanus racemosa'' is a species of plane tree known by several common names, including California sycamore, western sycamore, California plane tree, and in North American Spanish aliso. ''Platanus racemosa'' is native to California and Baja ...
'', is also known as ''aliso'' in Spanish, and is common in the area around the creek.
According to the U.S. Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and ...
's Geographic Names Information System
The Geographic Names Information System (GNIS) is a database of name and locative information about more than two million physical and cultural features throughout the United States and its territories, Antarctica, and the associated states of ...
, the creek has also been historically called "Los Alisos Creek" and "Alisos Creek".[ Several nearby geographical features also share the name, including the city of ]Aliso Viejo
Aliso Viejo (Spanish for "Old Sycamore") is a city in the San Joaquin Hills of southern Orange County, California. It had a population of 47,823 as of the 2010 census, up from 40,166 as of the 2000 census. It became Orange County's 34th city on ...
, Aliso Beach, Aliso Peak (a headland
A headland, also known as a head, is a coastal landform, a point of land usually high and often with a sheer drop, that extends into a body of water. It is a type of promontory. A headland of considerable size often is called a cape.Whittow, John ...
near the creek's mouth), Los Alisos Intermediate School in Mission Viejo
Mission Viejo ( ; corruption of ''Misión Vieja'', Spanish for "Old Mission") is a commuter city in the Saddleback Valley in Orange County, California, United States. Mission Viejo is considered one of the largest master-planned communities eve ...
, and Aliso Creek Road.
Course
Aliso Creek rises along the Loma Ridge in the foothills of the Santa Ana Mountains
The Santa Ana Mountains are a short peninsular mountain range along the coast of Southern California in the United States. They extend for approximately southeast of the Los Angeles Basin largely along the border between Orange and Riverside co ...
, near the community of Portola Hills
Portola Hills is a district in the city of Lake Forest, California in Orange County, United States. It was formerly an unincorporated community and census-designated place before it was annexed into Lake Forest in 2000. The population was 6,391 ...
, Lake Forest. The creek's headwaters are at an elevation of in the Cleveland National Forest
Cleveland National Forest encompasses 460,000 acres (), mostly of chaparral, with a few riparian areas. A warm dry mediterranean climate prevails over the forest. It is the southernmost U.S. National Forest of California. It is administered by ...
near Whiting Ranch Wilderness Park
Whiting Ranch Wilderness Park is a public regional park in southern Orange County, California. Whiting Ranch features riparian, oak woodland, grassland, chaparral, and coastal sage scrub environments throughout various canyons and hillscapes. T ...
. The creek flows south along Country Home Road then begins to parallel Santiago Canyon Road, which becomes El Toro Road
There are 34 routes assigned to the "S" zone of the California Route Marker Program, which designates county routes in California. The "S" zone includes county highways in Imperial, Orange, Riverside, San Diego, and Santa Barbara counties.
S ...
south of Cook's Corner
Cook's Corner is an Orange County, California bar built in 1884 that is popular with motorcyclists. It is located between the city of Lake Forest and the unincorporated community of Silverado. Cook's Corner is situated at the intersection of ...
.[ The creek enters ]Mission Viejo
Mission Viejo ( ; corruption of ''Misión Vieja'', Spanish for "Old Mission") is a commuter city in the Saddleback Valley in Orange County, California, United States. Mission Viejo is considered one of the largest master-planned communities eve ...
, flowing freely in a natural streambed along the bottom of a brushy, shaded ravine. It turns southwest, crossing under the 241 toll road and passing Saddleback Church
Saddleback Church is a Baptist Evangelical multi-site megachurch, affiliated with the Southern Baptist Convention, located in Lake Forest, California. It is the largest church in California, and one of the largest in the United States of America ...
, 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.
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
Interstate 5 (I-5) is the main north–south Interstate Highway on the West Coast of the United States, running largely parallel to the Pacific coast of the contiguous U.S. from Mexico to Canada. It travels through the states of Californi ...
. Downstream of the freeway Aliso Creek flows through Laguna Hills
Laguna Hills (; ''Laguna'', Spanish for "Lagoon") is a city in south Orange County, California, United States. Its name refers to its proximity to Laguna Canyon and the much older Laguna Beach. Other newer cities nearby—Laguna Niguel and Lagun ...
then through Aliso Park in the retirement community of Laguna Woods Village
Laguna Woods Village is an age-restricted community for persons aged 55 and over in Laguna Woods, California. The development, formerly known as Leisure World, was developed by Ross Cortese, a former fruit stand owner turned retirement community de ...
.[ It enters ]Aliso Viejo
Aliso Viejo (Spanish for "Old Sycamore") is a city in the San Joaquin Hills of southern Orange County, California. It had a population of 47,823 as of the 2010 census, up from 40,166 as of the 2000 census. It became Orange County's 34th city on ...
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
Aliso and Wood Canyons Wilderness Park is a major regional park in the San Joaquin Hills of Orange County, California in the United States. Comprising of rugged coastal canyons, open grassland, and riparian woodland, the park borders the sub ...
, the beginning of a greenbelt
A green belt is a policy and land-use zone designation used in land-use planning to retain areas of largely undeveloped, wild, or agricultural land surrounding or neighboring urban areas. Similar concepts are greenways or green wedges, which h ...
that stretches from here nearly to the Pacific Ocean.[
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
Laguna Niguel () is a city in Orange County, California, United States. The name Laguna Niguel is derived from the words "Laguna" (Spanish for "lagoon") and "Niguili" (the name of a Native American village once located near Aliso Creek). As of ...
. 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.
From there, Aliso Creek turns west and enters Aliso Canyon, a nearly deep gorge which cuts through the San Joaquin Hills
The San Joaquin Hills are a low mountain range of the Peninsular Ranges System, located in coastal Orange County, California.
They extend in a northwest–southeast direction, starting in the northwest in Newport Beach at the southern edge of t ...
approaching the Pacific.[ 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
Soka University of America (SUA) is a private liberal arts college in Aliso Viejo, California. Originally founded in 1987, it was established on its current campus in 2001 by Daisaku Ikeda, the founder of the Soka Gakkai International Buddhist ...
.[ 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
Laguna Beach (; ''Laguna'', Spanish for "Lagoon") is a seaside resort city located in southern Orange County, California, in the United States. It is known for its mild year-round climate, scenic coves, environmental preservation efforts, and a ...
.
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.
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. As recently as 1982, the creek was observed to be dry in the summer. As of 2012, urban runoff contributed a dry season flow of 5 million gallons (20,000 m3) per day, or approximately , at the creek's mouth. Urban runoff accounts for at least 80 percent of the creek's dry season flow.
The United States Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...
operated a stream gage
A stream gauge, streamgage or stream gauging station is a location used by hydrologists or environmental scientists to monitor and test terrestrial bodies of water. Hydrometric measurements of water level surface elevation ("stage") and/or volu ...
on the creek at the El Toro Road bridge in Mission Viejo
Mission Viejo ( ; corruption of ''Misión Vieja'', Spanish for "Old Mission") is a commuter city in the Saddleback Valley in Orange County, California, United States. Mission Viejo is considered one of the largest master-planned communities eve ...
from 1930 to 1980. This gage measured runoff from , or 26 percent of the watershed area.[ There was also a gauge in Laguna Beach which measured runoff from the entire watershed, but it operated only from 1982 to 1987.][
The average annual flow at El Toro was , ranging from in February to in July. The highest peak flow was on February 24, 1969.][
] At the Laguna Beach gage, the average annual flow was , with a high of in March and a low of in June. The largest flow recorded at the Laguna Beach gauge was during the El Niño
El Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date L ...
event on March 1, 1983.
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 , and between 1960 and 1980, the average peak flow was .[
]
Watershed
The Aliso Creek drainage basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, t ...
lies in the south central part of Orange County, roughly halfway between the Santa Ana River
The Santa Ana River is the largest river entirely within Southern California in the United States. It rises in the San Bernardino Mountains and flows for most of its length through San Bernardino and Riverside Counties, before cutting through ...
and the border of San Diego County
San Diego County (), officially the County of San Diego, is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county and the f ...
. It is an elongated area of , ranging from about a mile (1.6 km) in width in the north to 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 , while the San Joaquin Hills rise to at Temple Hill, locally called "Top of the World", in Laguna Beach west of Aliso Canyon. Aside from Laguna Niguel Lake, an impoundment of the Sulphur Creek tributary, there are no major bodies of fresh water.
The watershed experiences a dry Mediterranean climate
A Mediterranean climate (also called a dry summer temperate climate ''Cs'') is a temperate climate sub-type, generally characterized by warm, dry summers and mild, fairly wet winters; these weather conditions are typically experienced in the ...
. As of 2001, the average annual precipitation in the San Juan Hydrological Unit, which Aliso Creek is part of, was . The watershed borders five major Orange County watersheds: Santiago Creek
Santiago Creek is a major watercourse in Orange County in the U.S. state of California. About long, it drains most of the northern Santa Ana Mountains and is a tributary to the Santa Ana River. It is one of the longest watercourses entirely with ...
to the north, San Diego Creek
San Diego Creek is a urban waterway flowing into Upper Newport Bay in Orange County, California in the United States. Its watershed covers in parts of eight cities, including Irvine, Tustin, and Costa Mesa. From its headwaters in Laguna Woo ...
to the west, Laguna Canyon
Laguna Canyon (also called Cañada de las Lagunas, meaning "Lake Canyon" in Spanish) is a canyon that cuts through the San Joaquin Hills in southern Orange County, California, in the United States, directly south of the city of Irvine. The ca ...
to the southwest, Salt Creek to the southeast, and San Juan Creek
San Juan Creek, also called the San Juan River, is a long stream in Orange and Riverside Counties, draining a watershed of .7.5 Minute Quadrangle Map, U.S. Geological Survey, San Juan Capistrano, 1968, photorevised 1981 Its mainstem begins in ...
to the east.
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). 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 Beach (; ''Laguna'', Spanish for "Lagoon") is a seaside resort city located in southern Orange County, California, in the United States. It is known for its mild year-round climate, scenic coves, environmental preservation efforts, and a ...
, Laguna Niguel
Laguna Niguel () is a city in Orange County, California, United States. The name Laguna Niguel is derived from the words "Laguna" (Spanish for "lagoon") and "Niguili" (the name of a Native American village once located near Aliso Creek). As of ...
, Aliso Viejo
Aliso Viejo (Spanish for "Old Sycamore") is a city in the San Joaquin Hills of southern Orange County, California. It had a population of 47,823 as of the 2010 census, up from 40,166 as of the 2000 census. It became Orange County's 34th city on ...
, Laguna Hills
Laguna Hills (; ''Laguna'', Spanish for "Lagoon") is a city in south Orange County, California, United States. Its name refers to its proximity to Laguna Canyon and the much older Laguna Beach. Other newer cities nearby—Laguna Niguel and Lagun ...
, Laguna Woods
Laguna Woods (''Laguna'', Spanish for "Lagoon") is a city in Orange County, California, United States. The population was 16,192 at the 2010 census, down from 16,507 at the 2000 census, with a median age of 78.
Laguna Woods became Orange Count ...
, Lake Forest (formerly El Toro), and Mission Viejo
Mission Viejo ( ; corruption of ''Misión Vieja'', Spanish for "Old Mission") is a commuter city in the Saddleback Valley in Orange County, California, United States. Mission Viejo is considered one of the largest master-planned communities eve ...
), and the last two, Foothill Ranch
Foothill Ranch is a master planned community in Orange County, California, United States. The population was 10,899 at the 2000 census. Foothill Ranch was incorporated into the city of Lake Forest, California, in 2000. Prior to that, it was a c ...
and Portola Hills
Portola Hills is a district in the city of Lake Forest, California in Orange County, United States. It was formerly an unincorporated community and census-designated place before it was annexed into Lake Forest in 2000. The population was 6,391 ...
, 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. 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
Crossings of the creek are listed from mouth to source (year built in parentheses).["South Orange Street Map- San Clemente/Mission Viejo". ''American Maps''. Langenscheidt Publishing Group, 2001.] The creek is crossed by roughly 30 major bridges.
*
* Six footbridges in the Aliso Creek Golf Course
* SOCWA Coastal Treatment Plant access road
* Service Road for ACWHEP Dam
* AWMA (Aliso Water Management Agency) Road
* Aliso Creek Road (1988)
* Pacific Park Drive
*
* Aliso Creek Trail
* Moulton Parkway (northbound 1969, southbound 1987)
* Laguna Hills Drive—twin bridges (1985)
* Avenida Sevilla
* Two footbridges in Aliso Park
* Paseo de Valencia (1966)
* Aliso Creek Trail
*
* Aliso Creek Trail
* Los Alisos Boulevard (1973)
* Muirlands Boulevard (1973)
* Private road in Lake Forest Golf Center
* Surf Line
The Surf Line is a railroad line that runs from San Diego north to Orange County along California's Pacific Coast. It was so named because much of the line is near the Pacific Ocean, within less than in some places. The tracks are now owned by ...
* Jeronimo Road (1974)
* Aliso Creek Trail/2nd Street pedestrian bridge
* Aliso Creek Trail
* Trabuco Road—twin bridges (1975)
* Creekside Drive (1980)
* El Toro Road (1975)
* Normandale Drive (1987)
* Portola Parkway
* Saddleback Parkway
*
* Aliso Creek Trail
* Aliso Creek Trail
* Glenn Ranch Road
* Aliso Creek Trail
* El Toro Road
* El Toro Road
* Santiago Canyon Road
* Crystal Canyon Road
* Country Home Road
Tributaries
From mouth to source, Aliso Creek is joined by seven major tributaries. Another forty-six minor streams and drains flow into the creek.
Geology
Most of Southern California
Southern California (commonly shortened to SoCal) is a geographic and Cultural area, cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most po ...
, including all of Orange County, was part of the Pacific Ocean until about 10 million years ago (MYA) when regional uplift began. The Santa Ana Mountains, where the creek originates, began to rise about 5.5 million years ago along the Elsinore Fault. Most of the Aliso Creek watershed sits on several layers of marine sedimentary strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
, the oldest dating from the Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
(55.8–33.9 MYA) and the most recent, the Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[alluvial
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...](_blank)
sediments range from in depth. The watershed includes outcrops of the Topanga Formation, Monterey Formation, San Onofre Breccia, Capistrano Formation and Niguel Formation. Generally throughout the watershed, there are five major soil and rock outcrop types—Capistrano sandy loam, Cieneba sandy loam, Marina loamy sand, Myford sandy loam, and Cieneba-rock outcrop. The water table
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
T ...
ranges from deep.[
About 1.22 million years ago, the ]San Joaquin Hills
The San Joaquin Hills are a low mountain range of the Peninsular Ranges System, located in coastal Orange County, California.
They extend in a northwest–southeast direction, starting in the northwest in Newport Beach at the southern edge of t ...
along the Orange County coast began their uplift along the San Joaquin Hills blind thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
Thrust geometry and nomenclature
Reverse faults
A thrust fault is a type of reverse fault that has a dip of 45 degrees or less.
If ...
which extends south from the Los Angeles Basin
The Los Angeles Basin is a sedimentary Structural basin, basin located in Southern California, in a region known as the Peninsular Ranges. The basin is also connected to an wikt:anomalous, anomalous group of east-west trending chains of mountai ...
. The hills rose directly in the path of Aliso Creek, which cut a water gap
A water gap is a gap that flowing water has carved through a mountain range or mountain ridge and that still carries water today. Such gaps that no longer carry water currents are called wind gaps. Water gaps and wind gaps often offer a prac ...
through the range, forming Aliso Canyon
Aliso Canyon is a canyon located in Orange County, California in the United States. The canyon is a water gap across the San Joaquin Hills carved out by Aliso Creek, possibly as recently as the last ice age. Located in a semi-arid climate, it ...
. The uplift also diverted Sulphur Creek, which originally flowed south into Salt Creek, to turn north and join Aliso Creek.
During the last glacial period (110,000 to 10,000 years ago), especially in the Wisconsinian glaciation
The Wisconsin Glacial Episode, also called the Wisconsin glaciation, was the most recent glacial period of the North American ice sheet complex. This advance included the Cordilleran Ice Sheet, which nucleated in the northern North American Cord ...
(31,000 to 10,000 years ago), the climate of Southern California became periodically much wetter, with a climate similar to the present-day Pacific Northwest
The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...
. At these times Aliso Creek was a river carrying much more water than it does today. During glacial periods sea level was as much as lower, increasing the stream gradient and thus its erosive force. These factors led to Aliso Creek carving out a much larger series of valleys than would appear possible with its present-day volume.[ As sea levels rose after the Wisconsinian glaciation, Aliso Canyon became a long narrow bay. Over thousands of years Aliso Creek filled in the bay with sediment, creating the flat alluvial valley floor seen today, while the creek itself remains as an ]underfit stream
A misfit stream is a river that is either too large or too small to have eroded the valley or cave passage in which it flows. This term is also used for a stream or river with meanders that obviously are not proportional in size to the meanders ...
.[
]
Ecology
Plants
Before urbanization of the watershed, Aliso Creek and some of its tributaries supported a significant riparian zone
A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks a ...
dominated by native hardwoods such as coast live oak
''Quercus agrifolia'', the California live oak, or coast live oak, is a highly variable, often evergreen oak tree, a type of live oak, native to the California Floristic Province. It may be shrubby, depending on age and growing location, but is ...
, sycamore
Sycamore is a name which has been applied to several types of trees, but with somewhat similar leaf forms. The name derives from the ancient Greek ' (''sūkomoros'') meaning "fig-mulberry".
Species of trees known as sycamore:
* ''Acer pseudoplata ...
, alder
Alders are trees comprising the genus ''Alnus'' in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few sp ...
, cottonwood and arroyo willow.[ Many of the trees in Aliso Creek's riparian zone, especially near the mouth of the creek, were cut down in the Spanish Mission period to construct colonial settlements.][ During the early 20th century, groundwater withdrawal for agriculture killed off many of the remaining trees along the creek.][ The most significant remaining riparian habitat today occurs in Aliso and Wood Canyons, and along the uppermost headwaters of Aliso Creek.
Increased erosion and pollution caused by urban runoff have had adverse impacts on the riparian zone. Invasive plants, including tobacco tree, ]castor bean
''Ricinus communis'', the castor bean or castor oil plant, is a species of perennial flowering plant in the spurge family, Euphorbiaceae. It is the sole species in the monotypic genus, ''Ricinus'', and subtribe, Ricininae. The evolution of ...
, pampas grass Pampas grass or pampas-grass is a common name which may refer to any of several similar-looking, tall-growing species of grass:
* Species of ''Cortaderia'' including:
:* ''Cortaderia selloana'' and its selected cultivars
:* '' Cortaderia jubata'' ( ...
, periwinkle, and artichoke thistle, but most notably the giant reed
''Arundo donax'' is a tall perennial cane. It is one of several so-called reed species. It has several common names including giant cane, elephant grass, carrizo, arundo, Spanish cane, Colorado river reed, wild cane, and giant reed. ''Arundo'' an ...
, have in many places replaced native trees.[ Giant reed was originally planted in the 1970s to control erosion. These invasive species are most prevalent along upper Sulphur Creek, the lower half of Aliso Creek, and Wood Canyon Creek.][
The hilly terrain of the watershed supports mostly grassland and coastal scrub vegetation. Native shrub species present in the watershed include California brittlebush, ]California buckwheat
''Eriogonum fasciculatum'' is a species of wild buckwheat known by the common names California buckwheat and flat-topped buckwheat. Characterized by small, white and pink flower clusters that give off a cottony effect, this species grows vari ...
, California sagebrush
''Artemisia californica'', also known as California sagebrush, is a species of western North American shrub in the sunflower family.
Description
''Artemisia californica'' branches from the base and grows out from there, becoming rounded; it gro ...
, California goldenbush, coyote brush
''Baccharis pilularis'', called coyote brush (or bush), chaparral broom, and bush baccharis, is a shrub in the family Asteraceae native to California, Oregon, Washington, and Baja California. There are reports of isolated populations in New Mexi ...
and mule fat
''Baccharis salicifolia'' is a blooming shrub native to the sage scrub community and desert southwest of the United States and northern Mexico, as well as parts of South America. Its usual common name is mule fat;Mojave Desert Wildflowers, Pam M ...
. Like the riparian zones, native grassland and shrublands have been heavily impacted by invasive species. A 2009 survey conducted in the Aliso Creek watershed found non-native grass coverage of between 66 and 100 percent, and non-native shrub coverage of between 0 and 50 percent, across fourteen sample sites. Although most of the creek channel is tightly bound by urban development, it is considered a potential wildlife corridor
A wildlife corridor, habitat corridor, or green corridor is an area of habitat connecting wildlife populations separated by human activities or structures (such as roads, development, or logging). This allows an exchange of individuals between ...
between these two areas. The U.S. Army Corps of Engineers describes the corridor as "meager at best with several places where Aliso Creek is very narrow, concrete, or incorporates golf courses parks and school grounds."
The watershed supports multiple native bird species, including California least tern
The California least tern, ''Sternula antillarum browni'', is a subspecies of least tern that breeds primarily in bays of the Pacific Ocean within a very limited range of Southern California, in San Francisco Bay and in northern regions of Mexico ...
, least Bell's vireo
Bell's vireo (''Vireo bellii'') is a songbird that migrates between a breeding range in Western North America and a winter range in Central America. It is dull olive-gray above and whitish below. It has a faint white eye ring and faint wing bar ...
, southwestern willow flycatcher
The willow flycatcher (''Empidonax traillii'') is a small insect-eating, neotropical migrant bird of the tyrant flycatcher family. There are four subspecies of the willow flycatcher currently recognized, all of which breed in North America (inc ...
, California gnatcatcher
The California gnatcatcher (''Polioptila californica'') is a small long insectivorous bird which frequents dense coastal sage scrub growth. This species was recently split from the similar black-tailed gnatcatcher of the Sonoran and Chihuahuan ...
, and western snowy plover
The western snowy plover (''Charadrius nivosus nivosus'') is a small wader in the plover bird family. It breeds in the southern and western United States and the Caribbean.
On March 5, 1993, the western snowy plover was listed as a threatened s ...
. The loss of riparian zones has reduced bird habitat in the watershed. These species are mostly found in undeveloped areas of Aliso and Wood Canyons, the upper reaches of Aliso Creek, and some parts of English Canyon Creek.[ Aliso Canyon is one of the most diverse bird habitats in Orange County, with some 122 nesting and migratory species found there.] The canyon also has raptors including northern harrier
The northern harrier (''Circus hudsonius''), or ring-tailed hawk, is a bird of prey. It breeds throughout the northern parts of the northern hemisphere in Canada and the northernmost USA.
The northern harrier migrates to more southerly areas ...
, Cooper's hawk
Cooper's hawk (''Accipiter cooperii'') is a medium-sized hawk native to the North American continent and found from southern Canada to Mexico. This species is a member of the genus ''Accipiter'', sometimes referred to as true hawks, which are f ...
, golden eagle
The golden eagle (''Aquila chrysaetos'') is a bird of prey living in the Northern Hemisphere. It is the most widely distributed species of eagle. Like all eagles, it belongs to the family Accipitridae. They are one of the best-known bird of p ...
and peregrine falcon. ,
California legless lizard
''Anniella pulchra'', the California legless lizard, is a limbless, burrowing lizard often mistaken for a snake.
Description
These lizards are around long from snout to vent (not including tail). They have small, smooth scales typically color ...
, and
two-striped garter snake.
once inhabited the creek, but they were extirpated by damage to the channel following floods in 1983.