San Diego County encompasses an area of over 4000 square miles in the southwest corner of California. The County is bounded on the north by Orange and Riverside counties, on the east by Imperial County, on the south by Mexico, and on the west by the Pacific Ocean (map). The northwest to southeast trending Peninsular Range, the most prominent topographic feature in the region divides the County into the San Diego and Colorado hydrologic regions. The Peninsular Range includes the Santa Ana, Agua Tibia, Palomar, Hot Springs, Aguanga, Volcan, Cuyamaca, and Laguna Mountain systems and reaches a maximum elevation of over 6,500 feet above mean sea level. 

The climate in coastal San Diego County is generally mild with temperatures averaging about 65 degrees Fahrenheit and annual rainfall totaling 10-14 inches. Proceeding inland from the coast, diurnal and seasonal temperatures fluctuate to a greater degree and annual rainfall amounts typically increase. The foothills west of the Peninsular Range average 16 – 20 inches of annual rainfall and the western slope of the Peninsular Range receives as much as 45 inches of annual precipitation. The region has a distinct rainy season with the vast majority of precipitation occurring during the period October – April.

There are a total of 11 hydrologic units (HUs) in the San Diego Hydrologic Region (SDHR), encompassing a land area of nearly 3,000 square miles. Eight major stream systems originate on the western slope of the Peninsular Range and discharge into the Pacific Ocean. From north to south they are San Juan Creek, and the Santa Margarita, San Luis Rey, San Dieguito, San Diego, Sweetwater, Otay, and Tijuana Rivers. In addition, there are three HUs whose headwaters are located between the Peninsular Range and the Pacific Ocean. These include the Carlsbad, Los Penasquitos, and Pueblo San Diego units.

Spanish missionaries constructed the first dam in San Diego County across the San Diego River in the 18th century and by 1923 every major drainage system in the County included at least one reservoir. Today, numerous water reservoirs capture and store surface flows throughout San Diego County however; the County is unable to meet its water supply requirements solely through the storage of local runoff. In 1947 the newly constructed San Vicente Reservoir was the first facility to store water from the Colorado River. Today, the State Water Project also brings water from Northern California rivers to San Diego County via a network of large-diameter pipelines.

The population of San Diego County grew slowly until the later part of the 19th century when several regional water development projects and a railroad were completed. The County population, which was less than 50,000 at the turn of the 20th century, reached 200,000 by 1940 and passed 1 million in the late 1950’s. The present population of San Diego County is approximately 2.7 million residents. The San Diego HU is the County’s most populous with over 500,000 residents. The most densely populated HUs in the County are the Pueblo San Diego, Penasquitos, and Carlsbad units with 13.09, 4.27, and 3.49 persons per acre, respectively.

Residential, agricultural and undeveloped land uses are generally the most important in terms of area in the SDHR with substantial variations occurring between the 11 hydrologic units (Table). Approximately 50% of the land area west of the Peninsular Range is currently undeveloped. Of this vacant land, 40% is planned for future development, mainly for residential use. The current land use distribution in the SDHR also includes residential (15%), commercial/ industrial (5%), parks/ open space (10%), freeways/ roads (6%), and agriculture (13%). The activities associated with these land uses generate a variety of water pollutants.

The surface water, coastal, and groundwater resources of the SDHR support numerous economic, water supply, recreational, and habitat-related beneficial uses. San Diego County is also host to a diverse array of natural habitats including montane forests, chaparral covered hillsides, coastal sage scrub, riparian woodlands, freshwater wetlands, coastal salt and brackish marshes, vernal pools, lagoons, enclosed bays, tide pools, and open ocean. These areas support numerous unique and sensitive biological habitats and are home to rare, threatened, and endangered animal and plant species like the California gnatcatcher, the arroyo toad, the southwestern pond turtle, the salt marsh daisy, and the Otay Mesa mint.

The region’s rapid economic development and urbanization has resulted in many of the water bodies in the SDHR being identified as impaired on the California 303(d) list for a host of conditions including elevated coliform bacteria and trace metals, aquatic and sediment toxicity, nutrient enrichment, and sedimentation. Several water bodies are impaired for multiple stressors including Mission Bay, Chollas Creek, and the Tijuana River. (Graph of the 45 San Diego regional impaired waters grouped by water body type, and the graph of the total number of San Diego regional water bodies affected by pollutant / stressor.) Mitigating the widespread impairments to beneficial uses and protecting and restoring the ecological integrity of the County’s diverse habitats will be among the region’s most important future challenges.