The Santa Ana River Watershed is the largest in Orange County, covering 153.2 square miles. The river begins almost 75 miles away in the San Bernardino Mountains, crossing central Orange County before emptying into the Pacific Ocean. The Orange County portion of the watershed includes segments of the cities of Anaheim, Brea, Huntington Beach, Orange, Placentia, Santa Ana, Villa Park, and Yorba Linda. Most of the area tributary to the river is above Prado Dam, northeast of Orange County. Santiago Creek is the largest tributary to the Santa Ana River within Orange County.
In addition to much of Orange County, the Santa Ana River Watershed includes the western corner of Riverside County, southwestern corner of San Bernardino County and a small portion of Los Angeles County. The San Jacinto watershed is a separate watershed, but for purposes of the Santa Ana Watershed Project Authority (SAWPA), of which OCWD is a member, the San Jacinto Watershed is considered to be part of the Santa Ana River Watershed. San Jacinto Watershed is bounded on the south by the Santa Margarita Watershed, and on the east by the Salton Sea. The combined watersheds are approximately 2,800 square miles in area.
Surface Water
The Santa Ana watershed is an arid region; therefore there is little natural perennial surface water in the watershed.
Most of the surface water bodies start in the upper portion 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 includes treated discharges from wastewater treatment plants. From the City of Riverside to the recharge basins below Imperial Highway, during non-storm conditions 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 width 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, which joins the river in the city of Santa Ana.
Lake Elsinore is the only natural freshwater lake of any substantial size. 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.
Groundwater
The occurrence of groundwater in the watershed is highly controlled by the geology of the area, both the configuration of bedrock and by the extensive geologic faults. Most groundwater basins are unconfined, much like a bowl full of sand that has water poured in. However, the variable depth to bedrock, variations in geologic layers, and the presence of faults cause pressure zones where water flows toward (or to) the ground surface. In general, groundwater flows the same direction 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-connected. Some of the largest groundwater basins include Orange County, Chino (Chino/Ontario/Fontana area), Bunker Hill (San Bernardino), San Timoteo (Yucaipa/Banning/Beaumont area) and the San Jacinto/Hemet basins.
Population
The Santa Ana River Watershed has one of the fastest growing populations in California. It is home 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 indicate that the region’s population 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 has 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 percent 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.