This page provides a brief overview of the environment and history of the watershed, in particular as it relates to water.

WHAT IS A WATERSHED?

Click here for a conceptual graphic of a watershed

A watershed is a region drained by a stream, lake, or other body of water.  In other words, it is a bowl or basin-shaped area in which all water within the area (rain, snow, etc.) will flow to the same outlet point. 

OVERVIEW

Click on the link below to jump to the section of interest:

Location

The Santa Ana River watershed is located in southern California, south and east of the city of Los Angeles.  The watershed includes much of Orange County, the northwestern corner of Riverside County, the southwestern corner of San Bernardino County, and a small portion of Los Angeles County. The EPA identifies the San Jacinto watershed as a separate watershed; for SAWPA's purposes, the San Jacinto watershed is considered to be part of the Santa Ana river watershed. The watershed is bounded on the south by the Santa Margarita watershed, on the east by the Salton Sea and Southern Mojave watersheds, and on the north/west by the Mojave and San Gabriel watersheds.  The watershed is approximately 2,800 square miles in area.

Physiography

The watershed is located in the Peninsular Ranges and Transverse Ranges Geomorphic Provinces of Southern California (California Geological Survey Note 36).  The highest elevations (upper reaches) of the watershed occur in the San Bernardino (San Gorgonio Peak  -- 11,485 feet in elevation) and eastern San Gabriel Mountains (Transverse Ranges Province; Mt. Baldy  -- 10,080 feet in elevation) and in the San Jacinto Mountains (Peninsular Ranges Province, Mt. San Jacinto – 10,804 ft) .  Further downstream, the Santa Ana Mountains and the Chino Hills form a topographic high before the river flows into the Coastal Plain (in Orange County) and into the Pacific Ocean.  Primary slope direction is northeast to southwest, with secondary slopes controlled by local topography.

Geology

As is true for much of California, the geology of the Santa Ana River watershed is defined and created by seismic activity.  The dominant structural feature is the San Andreas fault zone, which trends in a southeast-northwest direction at the base of the San Bernardino Mountains; motion along this fault has caused the uplift of the San Bernardino and San Gabriel mountain ranges.  Additional major fault structures include the San Jacinto fault zone and the Elsinore Fault Zone; the San Jacinto mountains are caused by motion from both the San Andreas and San Jacinto zones.  Fault zones/lines are shown in red on the figure below. The area between the San Jacinto zone and the Elsinore Zones is a down-dropped block which is partly in-filled with sediments from the surrounding mountains.

There are too many geologic units in the watershed to describe separately, but the predominant features are intrusive rocks of the southern California batholith (granitic and andesitic rocks) which have been uplifted/eroded to form the mountain ranges (shown in green shades below), alluvial/fluvial sediments (materials eroded from the mountains and deposited in the basins, shown in tan/light tones), and semi-consolidated sedimentary units (maroon/brown color).

History

Native American period (approximately 10,000 BC to 1768 AD).  Native Americans have inhabited the Santa Ana Watershed for at least 12,000 years (perhaps longer) based on artifacts in the Calico area (San Bernardino County).  Native Americans used the Santa Ana River as a source of food and water, and did not raise crops or practice agriculture or irrigation.  Within the past 3,000 years, the Serranos occupied the foothills of the San Bernardino Mountains, the Gabrielenos lived in the West Valley, and the Luisenos occupied an area south of Mt. San Jacinto.

Mission period (approximately 1769-1833).  European settlement of the watershed began, centered around conversion of Native Americans to Christianity. The Spanish brought knowledge of aqueducts to California, introducing the practice of irrigation to the area.

Rancho period (approximately 1833-1850). Large portions of land were settled by private ranchers.  Floodplain development began in the Santa Ana watershed.  Settlers began diverting water from the Santa Ana River for irrigation of gardens.  The City of Santa Ana was founded.

Agricultural Boom period (approximately 1850-early 1900's).  Mormon colonists settled near Mill Creek, establishing the City of San Bernardino in 1854.  Settlers diverted water from Mill, Lytle, and Warm Creeks. By the 1880's, large tracts of land were dedicated to citrus and other agricultural activities.  Water diversions from the Santa Ana River and other sources were common.

Industrial/Urbanization period (from the early 1900's to today).  Entertainment, tourism, military, and aircraft industries (among others) lead to a boom in in development and population growth in Southern California.  Originally based in the Los Angeles area, this development began overtaking the agricultural uses in the Santa Ana watershed during the second half of the 20th century.

Climate

The climate of the watershed is Mediterranean with hot, dry summers and cooler, wetter winters. The extent of this climate is limited worldwide. Other than the central and south coast of California, it only occurs in coastal zones along the Mediterranean Sea, western and southern Australia, the Chilean coast, and the Cape Town portion of South Africa. Average annual precipitation ranges from 12 inches per year in the coastal plain to 18 inches per year in the inland alluvial valleys, reaching 40 inches or more in the San Bernardino Mountains.

Most of the precipitation occurs between November and March in the form of rain with variable amounts of snow in the higher elevations.  The climatological cycle of the region results in high surface water flows in the spring and early summer, followed by low flows during the dry season. Winter and spring floods generated by storms are not uncommon in wet years. Similarly, during the dry season, infrequent summer storms can cause torrential floods in local streams.

Surface Water

The Santa Ana watershed is an arid region, and therefore there is little natural perennial surface water in the watershed.

Surface waters start in the upper erosion zone of the watershed- primarily the San Bernardino and San Gabriel Mountains. This upper zone has the highest gradient and soils/geology that do not allow large quantities of percolation of surface water into the ground. Flows consist mainly of snowmelt and storm runoff from the lightly developed San Bernardino National Forest; this water is generally high quality at this point. In this zone, the Santa Ana River is generally confined in its lateral movement, contained by the slope in the mountainous regions.

In the upper valley, flows from the Seven Oaks dam to the city of San Bernardino consist mainly of storm flows, flows from the San Timoteo Creek, and groundwater that is rising due to local geological conditions. From the City of San Bernardino to the City of Riverside, the river flows perennially, and it includes treated discharges from wastewater treatment plants.  From the City of Riverside to the recharge basins below Imperial Highway, river flow consists of highly treated wastewater discharges, urban runoff, irrigation runoff, and groundwater forced to the surface by shallow/rising bedrock. Near Corona, the river cuts through the Santa Ana Mountains and the Puente-Chino Hills.

The river then flows into the Orange County Coastal Plain; the channel lessens and the gradient decreases.  In a natural environment, a river in this area would have a much wider channel, increase in meandering, and sediment would build up.  However, much of the Santa Ana river channel in this area has been contained in concrete-lined channels, which modifies the flow regime and sediment deposition environment.  The only major tributary of the Santa Ana River in Orange County is Santiago Creek, joins the river in the city of Santa Ana.

There is only one natural freshwater lake of any size- Lake Elsinore.  A variety of water storage reservoirs (Lake Perris, Lake Mathews, Big Bear Lake) and Flood Control areas (Prado Dam area, Seven Oaks Dam area) have been created to hold surface water.

Make a Surface Water Map!

Ground Water

Groundwater in the watershed is highly controlled by the geology of the area, both the configuration of bedrock and by the extensive faulting.  Most groundwater basins are unconfined, much like a bowl full of sand that has water poured in halfway.  However, the variable depth to bedrock, and the presence of faults cause pressure zones where water flows towards (or to) the ground surface. In general, groundwater flows the same directions as surface water- from the mountains in the east/north to the Pacific Ocean in the west.  There are about 40 groundwater basins in the watershed (depending on how they are defined and boundaries are drawn); many are inter-related.

Some of the largest groundwater basins include the Chino Basin (Chino/Ontario/Fontana area), the Orange County basin, the Bunker Hill Basin (San Bernardino) the San Timoteo Basin (Yucaipa/Banning/Beamont area) and the San Jacinto/Hemet Basins.

Make a Groundwater Map!

Population

The Santa Ana River watershed has one of the fastest growing populations in California.  It is home the to the most developed portion of Orange County, much of the built-up portions of Riverside and San Bernardino Counties, and a small portion of Los Angeles County. 

The 2000 census indicated that the watershed is home to 4.8 million people; the inland areas include some of the fastest growing areas in the country.  Studies by a SAWPA consultant project that populations will grow to about 7 million by 2025 and almost 10 million by 2050.

Water Demand

There are numerous demands on water in the watershed.  The area is rich in agricultural history, and the area still contains concentrations of citrus, dairy, and other agricultural areas that demand significant quantities of water.  The area had a large industrial/commercial base, and the rapidly expanding population demands a large quantity of water. In the year 2000, the watershed required 1.4 million acre-feet of water (467 billion gallons) to meet demand.  Projections are that this demand will increase 47% in the next 50 years, so that, in 2050, the watershed will require 2.1 million acre-feet (687 billion gallons) of water to meet demands.

Copyright: Santa Ana Watershed Project Authority, 2002
Last modified 07/23/2004