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 in eastern Ventura County 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 th ...

, of southern

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the ...

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 U.S. state, states, a Washington, D.C., federal district, five ma ...

Geography

The Simi Hills are aligned east-west and run for , and average around north-south. The Simi Hills are part of the central Transverse Ranges System. They lie almost entirely within southeastern Ventura County, with some southern and eastern foothills within western

. The Simi Hills are on the western edge of the San Fernando Valley. 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. T ...

lies to the north, and the Conejo Valley 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 Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the largest city in the state of California and the second most populous city in the United States after New York City, as well as one of the world' ...

and

county A county is a geographic region of a country used for administrative or other purposes Chambers 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 and Westlake Village are also located in Los Angeles County, generally southwest of the Simi Hills. The cities of Thousand Oaks (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 adjacent on the northeast across Santa Susana Pass; 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

watershed Watershed is a hydrological term, which has been adopted in other fields in a more or less figurative sense. It may refer to: Hydrology * Drainage divide, the line that separates neighbouring drainage basins * Drainage basin, called a "watershe ...

s for several year-round creeks and numerous seasonal streams. They include Las Virgenes Creek (tributary of Malibu Creek), 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); ...

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 the ...

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

Peaks

Peaks in this region include Simi Peak, 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 ...

y, mild winters.

Snow Snow comprises individual ice crystals that grow while suspended in the atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet ...

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 combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...

s do occur here. Cool winds from the Pacific Ocean come from the Oxnard Plain and blow into the inland areas through the Santa Clara River Valley and the Conejo Valley, though some low hills, such as Conejo Mountain, 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 San Fernando Valley.

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 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 Mediterrane ...

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 ...

and oak woodlands. The Simi Hills are part of the

California chaparral and woodlands The California chaparral and woodlands is a terrestrial ecoregion of southwestern Oregon, northern, central, and southern California ( United States) and northwestern Baja California ( Mexico), located on the west coast of North America. It ...

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 o ...

. The oaks (''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, a ...

valley oak ('' Quercus lobata''), and the scrub oak ('' 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 ...

plants include California sycamores (''Platanus racemosa'') and

arroyo willow ''Salix lasiolepis'' (arroyo willow) is a species of willow native to western North America. Distribution The core range of the arroyo willow includes most of California, including the California Coast Ranges, Arizona, Klamath Mountains, Peninsu ...

s (''Salix lasiolepis''). Spring wildflowers include the redbush monkey flower (''Mimulus aurantiacus''), Plummer's mariposa lily (''Calochortus plummerae''), and canyon sunflower (''Encelia californica''). poison oak (''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 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 ...

linking the coastal

with the inland Santa Susana Mountains, Topatopa Mountains, and San Gabriel Mountains, 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 is a proposed vegetated overpass spanning the Ventura Freeway 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 ...

. Large sections of the Simi Hills are protected by parks and open space preserves. The Santa Susana Field Laboratory 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 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 historica ...

, and Tataviam peoples traded and lived beside Bell Creek below Escorpión Peak, at the present day Bell Canyon Park. 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, an example of the Rock art of the Chumash people, 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 List of federal agencies in the United States, agency of the Federal government of the United States, United States federal government within the United States Department of the Interior, U.S. Department of ...

administers the Juan Bautista de Anza National Historic Trail 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 Cast ...

and crosses across the southern Hills through Upper Las Virgenes Canyon Open Space Preserve and Cheeseboro/Palo Comado Canyon Park to the Conejo Valley. 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 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 fi ...

es in the Simi Hills for filming motion pictures on location. The gated community of

Bell Canyon Bell Canyon is a major drainage of the Santa Ana Mountains in Orange County, California in the United States. Bell Creek (also known as Bell Canyon Creek or Arroyo de las Campanas) flows about in a southerly direction to its confluence with San ...

began development of geographic Bell Canyon in the 1968. To the north of U.S. 101, east of Thousand Oaks, 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 Ahmanson may refer to: People with the surname *Caroline Leonetti Ahmanson (1918–2005), American fashion consultant, businesswoman and philanthropist *Howard Ahmanson, Jr. (born 1950), American philanthropist and writer *Howard F. Ahmanson, Sr. ...

were eventually stopped by local citizens and the Santa Monica Mountains Conservancy 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, California, Canoga Park, in the western San Fernando Valley of suburban Los Angeles, California, Los Angeles, in southern California. The Rocke ...

Santa Susana Field Laboratory (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 from nu ...

experiments with four nuclear accidents 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

* Bell Canyon Park with Bell Creek; West Hills and

* Challenger Park *Cheeseboro/Palo Comado Canyon Park - in the Santa Monica Mountains National Recreation Area, western Simi Hills in Ventura County. *

and the Moore's Canyon Juan Bautista de Anza National Historic Trail segment; West Hills, Los Angeles * Rocky Peak Park - ''adjacent and above Santa Susana Pass'' *

Ronald Reagan Presidential Library The Ronald Reagan Presidential Library is the repository of presidential records from the administration of Ronald Reagan, the 40th president of the United States, and the burial place of the president and first lady, Nancy Reagan. It is the la ...

complex with museum * Sage Ranch Park - near the Santa Susana Field Laboratory with trails and camping. * Santa Susana Pass State Historic Park - with the Old Santa Susana Stage Road in the rocky northern Simi Hills. * Upper Las Virgenes Canyon Open Space Preserve - (previously the Ahmanson Ranch Park), 3,000 acres (12 km²) Park in the southern Simi Hills, managed by the Santa Monica Mountains Conservancy.

Nearby ranges

* Chalk Hills * Santa Susana Mountains *

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