The Simi Hills are a low rocky

mountain range A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...

of the

Transverse Ranges The Transverse Ranges are a group of mountain ranges of southern California, in the Pacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of the California Coast Ranges and lie within Santa ...

in eastern

Ventura County Ventura County () is a county in the southern part of the U.S. state of California. As of the 2020 census, the population was 843,843. The largest city is Oxnard, and the county seat is the city of Ventura. Ventura County comprises the Oxnar ...

and western

Los Angeles County Los Angeles County, officially the County of Los Angeles, and sometimes abbreviated as L.A. County, is the most populous county in the United States and in the U.S. state of California, with 9,861,224 residents estimated as of 2022. It is the ...

, of southern

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 ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

Geography

The Simi Hills are aligned east-west and run for , and average around north-south. The Simi Hills are part of the central

System. They lie almost entirely within southeastern

, with some southern and eastern foothills within western

. The Simi Hills are on the western edge of the

San Fernando Valley The San Fernando Valley, known locally as the Valley, is an urbanized valley in Los Angeles County, California. Located to the north of the Los Angeles Basin, it contains a large portion of the City of Los Angeles, as well as unincorporated ar ...

. The

Simi Valley Simi Valley (; Chumash: ''Shimiyi'') is a city in the valley of the same name in the southeast region of Ventura County, California, United States. Simi Valley is from Downtown Los Angeles, making it part of the Greater Los Angeles Area. The ...

lies to the north, and the

Conejo Valley The Conejo Valley is a region spanning both southeastern Ventura County and northwestern Los Angeles County in Southern California, United States. It is located in the northwestern part of the Greater Los Angeles Area. Communities in the Conejo ...

lies to the southwest. The San Fernando Valley communities of Chatsworth, West Hills, and Woodland Hills are in the eastern hills and adjacent valley floor in Los Angeles city and

county A county is a geographic region of a country used for administrative or other purposesChambers Dictionary, L. Brookes (ed.), 2005, Chambers Harrap Publishers Ltd, Edinburgh in certain modern nations. The term is derived from the Old French ...

. The cities of

Agoura Hills Agoura Hills () is a city in the Santa Monica Mountains region of Los Angeles County, California, United States. Its population was 20,330 at the 2010 census, which decreased to 20,299 in 2020. It is in the eastern Conejo Valley between the S ...

and

Westlake Village Westlake Village is a city in Los Angeles County on its western border with Ventura County. The City of Westlake Village incorporated in 1981 becoming the 82nd municipality of Los Angeles County.Baker, Pam (2002). ''Thousand Oaks Westlake Vil ...

are also located in Los Angeles County, generally southwest of the Simi Hills. The cities of

Thousand Oaks Thousand Oaks is the second-largest city in Ventura County, California, United States. It is in the northwestern part of Greater Los Angeles, approximately from the city of Los Angeles and from Downtown. It is named after the many oak tree ...

(to the west) and Simi Valley city (to the north) are in the hills and adjacent valleys within Ventura County. The two nearby mountain ranges are: the higher

Santa Susana Mountains The Santa Susana Mountains are a transverse range of mountains in Southern California, north of the city of Los Angeles, in the United States. The range runs east-west, separating the San Fernando and Simi valleys on its south from the Santa C ...

adjacent on the northeast across

Santa Susana Pass The Santa Susana Pass, originally Simi Pass, is a low mountain pass in the Simi Hills of Southern California, connecting the San Fernando Valley and Los Angeles neighborhood of Chatsworth, to the city of Simi Valley and eponymous valley. It h ...

; and the

Santa Monica Mountains The Santa Monica Mountains is a coastal mountain range in Southern California, next to the Pacific Ocean. It is part of the Transverse Ranges. Because of its proximity to densely populated regions, it is one of the most visited natural areas in ...

running nearby along the south.

Watersheds

The hills provide the complete or partial watersheds for several year-round creeks and numerous seasonal streams. They include Las Virgenes Creek (tributary of

Malibu Creek Malibu Creek is a year-round stream in western Los Angeles County, California. It drains the southern Conejo Valley and Simi Hills, flowing south through the Santa Monica Mountains, and enters Santa Monica Bay in Malibu, California. The Malibu C ...

), Moore's Canyon Creek, Bell Creek, Dayton Creek, Woolsey Canyon Creek, Brandeis Creek, Runkle Canyon Creek, Arroyo Simi, Palo Comado Creek, Cheeseboro Creek, and

Arroyo Calabasas Arroyo Calabasas (also known as Calabasas Creek) is a U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map, accessed March 16, 2011 tributary of the Los Angeles River, in the southwestern San Fernando ...

(northern fork). Bell Creek and Arroyo Calabasas are the headwaters of the

Los Angeles River , name_etymology = , image = File:Los Angeles River from Fletcher Drive Bridge 2019.jpg , image_caption = L.A. River from Fletcher Drive Bridge , image_size = 300 , map = LARmap.jpg , map_size ...

, by name its beginning with their

confluence In geography, a confluence (also: ''conflux'') occurs where two or more flowing bodies of water join to form a single channel. A confluence can occur in several configurations: at the point where a tributary joins a larger river (main stem); o ...

in nearby

Canoga Park Canoga Park is a neighborhood in the San Fernando Valley region of the City of Los Angeles, California. Before the Mexican–American War, the district was part of a rancho, and after the American victory it was converted into wheat farms and t ...

. 90% of the Santa Susana Field Lab property drains into the Los Angeles River via tributaries.

Peaks

Peaks in this region include

Simi Peak Simi Peak is the highest peak in the Simi Hills of Southern California, at . Geography Simi Peak is located in eastern Ventura County, California, near the cities of Simi Valley to the north, and Thousand Oaks to the west. Nearby peaks Chatswor ...

, 2,403 ft (732 m), Chatsworth Peak, 2,314 ft (700 m), and Escorpión Peak (aka: Castle Peak), 1,475 ft (450 m).

Climate

Because of its low elevation, the Simi Hills typically experience

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

y, mild winters.

Snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

is rare in the Simi Hills, even in the highest areas. Summers are warm and dry and

wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...

s do occur here. Cool winds from the Pacific Ocean come from the

Oxnard Plain The Oxnard Plain is a large coastal plain in southwest Ventura County, California, United States surrounded by the mountains of the Transverse ranges. The cities of Oxnard, Camarillo, Port Hueneme and much of Ventura as well as the unincorpora ...

and blow into the inland areas through the

Santa Clara River Valley The Santa Clara River Valley is a rural, mainly agricultural, valley in Ventura County, California that has been given the moniker Heritage Valley by the namesake tourism bureau. The valley includes the communities of Santa Paula, Fillmore, Piru ...

and the

, though some low hills, such as

Conejo Mountain Conejo Mountain is a in Ventura County, California near Camarillo on the eastern boundary of the Oxnard Plain. At the western edge of the Conejo Valley, it is adjacent to the Santa Monica Mountains. Crossing what was once a formidable barrier fo ...

, block these winds from the Conejo Valley. The Simi Hills further block these winds, which bring cool weather in both summer and winter from the

Flora and fauna

Flora

The southern lower hills are mostly covered in

grassland A grassland is an area where the vegetation is dominated by grasses (Poaceae). However, sedge (Cyperaceae) and rush (Juncaceae) can also be found along with variable proportions of legumes, like clover, and other herbs. Grasslands occur natur ...

s and

oak An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...

savanna. The northern rocky hills area is primarily

chaparral Chaparral ( ) is a shrubland plant community and geographical feature found primarily in the U.S. state of California, in southern Oregon, and in the northern portion of the Baja California Peninsula in Mexico. It is shaped by a Mediterranean c ...

shrubland Shrubland, scrubland, scrub, brush, or bush is a plant community characterized by vegetation dominated by shrubs, often also including grasses, herbs, and geophytes. Shrubland may either occur naturally or be the result of human activity. It m ...

and oak woodlands. The Simi Hills are part of the California chaparral and woodlands

ecoregion An ecoregion (ecological region) or ecozone (ecological zone) is an ecologically and geographically defined area that is smaller than a bioregion, which in turn is smaller than a biogeographic realm. Ecoregions cover relatively large areas of l ...

. The

s (''Quercus spp.'') include: the

evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which ...

coastal live oak (''

Quercus agrifolia ''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 ...

''), the

deciduous In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, aft ...

valley oak (''

Quercus lobata ''Quercus lobata'', commonly called the valley oak or roble, grows into the largest of California oaks. It is endemic to California, growing in interior valleys and foothills from Siskiyou County to San Diego County. Mature specimens may attain ...

''), and the scrub oak (''

Quercus dumosa ''Quercus dumosa'' is a species of plant in the family Fagaceae, belonging to the white oak section of the oak genus (''Quercus''). This tree goes by the common names coastal sage scrub oak and Nuttall's scrub oak. Description ''Quercus dumosa ...

'').

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 ...

plants include

California sycamore ''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 C ...

s (''Platanus racemosa'') and arroyo willows (''Salix lasiolepis''). Spring

wildflower A wildflower (or wild flower) is a flower that grows in the wild, meaning it was not intentionally seeded or planted. The term implies that the plant probably is neither a hybrid nor a selected cultivar that is in any way different from the ...

s include the redbush monkey flower (''Mimulus aurantiacus''), Plummer's mariposa lily (''Calochortus plummerae''), and canyon sunflower (''Encelia californica'').

poison oak Poison oak refers to two plant species in the genus ''Toxicodendron,'' both of which can cause skin irritation: *''Toxicodendron diversilobum'' or Western poison oak, found in western North America *''Toxicodendron pubescens ''Toxicodendron pub ...

(''Toxicodendron diversilobum'') is also an important member of the native plant habitat community here.

Fauna

The Simi Hills is the principal, and much wider, of only two terrestrial wildlife corridors linking the coastal

with the inland

Topatopa Mountains The Topatopa Mountains are a mountain range in Ventura County, California, north of Ojai, Santa Paula, and Fillmore. They are part of the Transverse Ranges of Southern California. Etymology A name for the mountains was first inscribed within the ...

, and

San Gabriel Mountains The San Gabriel Mountains ( es, Sierra de San Gabriel) are a mountain range located in northern Los Angeles County, California, Los Angeles County and western San Bernardino County, California, United States. The mountain range is part of the Tr ...

, all of the transverse ranges fauna community. The Simi Hills are a critical wildlife corridor linkage for the Santa Monica Mountains to these and other Transverse Ranges further east. The undeveloped native

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

provides routes that protect larger land wildlife of the Santa Monicas from genetic isolation. The

Wallis Annenberg Wildlife Crossing The Wallis Annenberg Wildlife Crossing is a vegetated overpass spanning the Ventura Freeway and Agoura Road under construction at Liberty Canyon in Agoura Hills, California. The bridge will be one of the largest urban wildlife crossings in the ...

is a proposed vegetated overpass spanning the

Ventura Freeway The Ventura Freeway is a freeway in southern California, United States, running from the Santa Barbara/ Ventura county line to Pasadena in Los Angeles County. It is the principal east-west route (designated north-south) through Ventura Count ...

at Liberty Canyon in

Agoura Hills, California Agoura Hills () is a city in the Santa Monica Mountains region of Los Angeles County, California, United States. Its population was 20,330 at the 2010 census, which decreased to 20,299 in 2020. It is in the eastern Conejo Valley between the S ...

. Large sections of the Simi Hills are protected by parks and open space preserves. The

Santa Susana Field Laboratory The Santa Susana Field Laboratory (SSFL), formerly known as Rocketdyne, is a complex of industrial research and development facilities located on a portion of Southern California in an unincorporated area of Ventura County in the Simi Hills betw ...

property, a crucial wildlife corridor to the Santa Susanas, has been proposed for public open space parkland after the closed site's cleanup completion. The population of red-legged frogs is small and isolated, and was impacted by the

Woolsey Fire The Woolsey Fire was a wildfire that burned in Los Angeles and Ventura Counties of the U.S. state of California. The fire ignited on November 8, 2018, and burned of land. The fire destroyed 1,643 structures, killed three people, and prompted th ...

swept through the area in November 2018.

History

The Simi Hills were inhabited for over 8,000 years by Paleo-indians and Chumash-Venturaño Native Americans for settlements and hunting grounds. The Chumash had the established village of ''Hu'wam'' in ''Cañon del Escorpión'' (Bell Canyon). It was a multicultural 'crossroads' destination, where Chumash,

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 ...

, and

Tataviam The Tataviam (Kitanemuk: ''people on the south slope'') are a Native American group in Southern California. The ancestral land of the Tataviam people includes northwest present-day Los Angeles County and southern Ventura County, primarily in th ...

peoples traded and lived beside Bell Creek below Escorpión Peak, at the present day

Bell Canyon Park Bell Canyon Park is a large open-space regional park located in the Simi Hills at the western end of the San Fernando Valley in West Hills, Los Angeles and Bell Canyon, California. Bell Creek, a primary tributary to the Los Angeles River, flows ...

. This peak in the Simi Hills (aka: Castle Peak) is one of nine alignment points in Chumash territory and is essential to maintaining the balance of the natural world. Upstream were healing springs and are rock outcrop 'grinding stones.' The

Burro Flats Painted Cave The Burro Flats site is a painted cave site located near Burro Flats, in the Simi Hills of eastern Ventura County, California, United States. The Chumash-style "main panel" and the surrounding 25-acres were listed on the National Register of Hi ...

, an example of the

Rock art of the Chumash people Chumash rock art is a genre of paintings on caves, mountains, cliffs, or other living rock surfaces, created by the Chumash people of Southern California. Pictographs and petroglyphs are common through interior California, the rock painting tradi ...

, is nearby. The Juan Bautista de Anza expedition passed through the area in 1769, being the first European sighting of the Simi Hills. The U.S.

National Park Service The National Park Service (NPS) is an agency of the United States federal government within the U.S. Department of the Interior that manages all national parks, most national monuments, and other natural, historical, and recreational propertie ...

administers the

Juan Bautista de Anza National Historic Trail The Juan Bautista de Anza National Historic Trail is a trail extending from Nogales on the U.S.-Mexico border in Arizona, through the California desert and coastal areas in Southern California and the Central Coast region to San Francisco. ...

which enters at Moore Canyon in

El Escorpión Park El Escorpión Park is a three-acre (12,000 m²) park located in the Simi Hills of the western San Fernando Valley, in the West Hills district of Los Angeles, California. The park contains the geographic landmark known as Escorpión Peak or Castl ...

and crosses across the southern Hills through

Upper Las Virgenes Canyon Open Space Preserve The Upper Las Virgenes Canyon Open Space Preserve is a large open space nature preserve owned and operated by the Santa Monica Mountains Conservancy spanning nearly in the Simi Hills of western Los Angeles County and eastern Ventura County. Or ...

and Cheeseboro/Palo Comado Canyon Park to the

. In 1845 the

land grant A land grant is a gift of real estate—land or its use privileges—made by a government or other authority as an incentive, means of enabling works, or as a reward for services to an individual, especially in return for military service. Grants ...

for

Rancho El Escorpión Rancho El Escorpión was a Mexican land grant in present day Los Angeles County, California given in 1845 by Governor Pío Pico to three Chumash Native Americans - Odón Chijulla, Urbano, and Mañuel.

, beside the Peak and named for it, was issued to one Chumash and two Tongva people and a rare instance of Native Americans being grantees, by Mexican Governor Pío Pico. The

Rancho El Conejo Rancho El Conejo was a Spanish land grant in California given in 1803 to Jose Polanco and Ygnacio Rodriguez that encompassed the area now known as the Conejo Valley in southeastern Ventura and northwestern Los Angeles Counties. ''El Conejo'' is ...

was to the west, and included that end of the Simi Hills. In the first half of the 20th century, there were four large

movie ranch A movie ranch is a ranch that is at least partially dedicated for use as a set in the creation and production of motion pictures and television shows. These were developed in the United States in southern California, because of the climate. The fir ...

es in the Simi Hills for filming motion pictures on location. The gated community of Bell Canyon began development of geographic Bell Canyon in the 1968. To the north of

U.S. 101 U.S. Route 101, or U.S. Highway 101 (US 101), is a north–south United States Numbered Highway that runs through the states of California, Oregon, and Washington, on the West Coast of the United States. It is also known as (The Royal Roa ...

, east of

, and west of

the early 1960s suburban expansion of metropolitan Los Angeles brought the development of small to significantly sized parcels of land in the Simi Hills. Hillside subdivisions regraded natural contours into terraced lots, changing the Hills viewshed, drainage patterns, and habitats in those areas. The extensive planned new community projects at Jordan Ranch and Ahmanson Ranch were eventually stopped by local citizens and the

Santa Monica Mountains Conservancy The Santa Monica Mountains Conservancy is an agency of the state of California in the United States founded in 1980 and dedicated to the acquisition of land for preservation as open space, for wildlife and California native plants habitat Natu ...

keeping extensive Hills acreage natural in open space parks. The Simi Hills were home of the

Rocketdyne Rocketdyne was an American rocket engine design and production company headquartered in Canoga Park, in the western San Fernando Valley of suburban Los Angeles, in southern California. The Rocketdyne Division was founded by North American Avia ...

(SSFL) for open-air testing of rocket engines from 1947 to 1990, and the site of experimental

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

experiments with four

nuclear accident A nuclear and radiation accident is defined by the International Atomic Energy Agency (IAEA) as "an event that has led to significant consequences to people, the environment or the facility. Examples include lethal effects to individuals, lar ...

s between 1959 and 1969; these projects and accidents were declassified in 1989. The first accident, in 1959, was the most serious with a full meltdown. The extent of the accident is unknown due to instrument limitations, other than that it released extensive radiation. More radiation was released in the 1959 event than in the

Three Mile Island accident The Three Mile Island accident was a partial meltdown of the Three Mile Island, Unit 2 (TMI-2) reactor in Pennsylvania, United States. It began at 4 a.m. on March 28, 1979. It is the most significant accident in U.S. commercial nuclea ...

20 years later. The groundwater under portions of the Simi Hills, contaminated with toxins and radionuclides that were also historically used at SSFL, has been and is a key concern in new development decisions and the SSFL property's future limited to parkland use after an impending cleanup, initiating preservation of more open space in the range. The NP
Rim of the Valley Trail Corridor
is in the planning stages and includes the Simi Hills open space parklands and proposed new areas.

Parks

with Bell Creek; West Hills and Bell Canyon *

Challenger Park Challenger Park is a located in southern portion of Simi Valley, California in the Simi Hills.Murphy, Kelly (2012). ''Local Multi-Use Trails''. Multi-Use Trails. Page 152. . The park is owned and maintained by the Rancho Simi Recreation and Park ...

*Cheeseboro/Palo Comado Canyon Park - in the

Santa Monica Mountains National Recreation Area The Santa Monica Mountains National Recreation Area is a United States national recreation area containing many individual parks and open space preserves, located primarily in the Santa Monica Mountains of Southern California. The SMMNRA is in t ...

, western Simi Hills in

. *

and the Moore's Canyon

segment;

West Hills, Los Angeles West Hills is a suburban / residential community in the western San Fernando Valley region of the City of Los Angeles, California. The percentage of residents aged 35 and older is among the highest in Los Angeles County. The neighborhood was for ...

* Rocky Peak Park - ''adjacent and above

'' * Ronald Reagan Presidential Library complex with museum *

Sage Ranch Park Sage Ranch Park is a and wildlife corridor located at a height in the northwestern Simi Hills on the northwestern plateau of the Simi Valley, bordering Los Angeles County and its San Fernando Valley.Stone, Robert (1998). ''Day Hikes in Ventur ...

- near the

with trails and camping. *

Santa Susana Pass State Historic Park Santa Susana Pass State Historic Park is a California State Park of approximately located on the boundary between Ventura and Los Angeles counties, between the communities of Chatsworth and Simi Valley. Geologically, the park is located where ...

- with the

Old Santa Susana Stage Road The Old Santa Susana Stage Road, or Santa Susana Wagon Road, is a route taken by early travelers between the San Fernando Valley and Simi Valley near Chatsworth, California, via the Santa Susana Pass. The main route climbs through what is now the ...

in the rocky northern Simi Hills. *

- (previously the Ahmanson Ranch Park), 3,000 acres (12 km²) Park in the southern Simi Hills, managed by the

Nearby ranges

Chalk Hills The Chalk Hills are a north–south-running low 'mountain' range in the San Fernando Valley perpendicular to and adjoining the Santa Monica Mountains. They are located in the Woodland Hills District of the City of Los Angeles in Southern Calif ...

Verdugo Mountains The Verdugo Mountains, also known as the Verdugo Hills or simply The Verdugos, are a small, rugged mountain range of the Transverse Ranges system in Los Angeles County, California. Located just south of the western San Gabriel Mountains, the Ver ...

References

External links

;Parks
SMMC: Upper Las Virgenes Canyon Open Space Preserve
(formerly Ahmanson Ranch)
SMMC: Sage Ranch Park
* ttp://venturacountytrails.org/TrailMaps/Cheeseboro/CheeseboroTrails.htm Ventura County Trails.org: Cheeseboro-Palo Commado Parks
Ventura County Trails.org: Wood + Lang Ranches Regional Parks
;Park finders
Santa Monica Mountains Conservancy (SMMC): Simi Hills Park-Finder
— ''interactive website''.

— ''interactive website''.
Santa Monica Mountains Conservancy (SMMC): All local ranges Park-Finder
— ''interactive website''. {{Authority control Hills of California Transverse Ranges Mountain ranges of Los Angeles County, California Mountain ranges of Ventura County, California Geography of the San Fernando Valley Geography of Simi Valley, California San Fernando Valley Agoura Hills, California Hidden Hills, California Thousand Oaks, California West Hills, Los Angeles Woodland Hills, Los Angeles