San Diego County (), officially the County of San Diego, is a southern county in the U.S. state of California, north to its border with Mexico. As of the 2020 census, the population was 3,298,634; it is the second-most populous county in California and the fifth-most populous in the United States. Its county seat is San Diego, the second-most populous city in California and the eighth-most populous in the United States.

San Diego County is the southwesternmost county in the 48 contiguous United States, and is a border county. It is home to 18 Indian reservations, the most of any county in the U.S. There are 16 military installations of the United States Navy, Marine Corps, and Coast Guard in the county.

San Diego County comprises the San Diego–Chula Vista–Carlsbad, CA Metropolitan Statistical Area, which is the 18th most populous metropolitan statistical area and the 18th most populous primary statistical area in the U.S. San Diego County is also part of the San Diego–Tijuana region, the largest metropolitan area shared between the U.S. and Mexico. From north to south, San Diego County extends from the southern borders of Orange and Riverside counties to the Mexico–U.S. border and the municipalities of Tijuana and Tecate in Baja California. From west to east, San Diego County stretches from the Pacific Ocean to its boundary with Imperial County, which separated from it in 1907.

San Diego County has more than of coastline. This forms the most densely populated region of the county, which has a mild Mediterranean to semiarid climate and extensive chaparral vegetation, similar to the rest of the western portion of Southern California. Precipitation and temperature extremes increase to the east, with mountains that receive frost and snow in the winter. These lushly forested mountains receive more rainfall than the average in Southern California, while the desert region of the county lies in a rain shadow to the east, which extends into the Desert Southwest region of North America.

History

The area which is now San Diego County has been inhabited for more than 12,000 years by the Kumeyaay (also known as 'Iipai-Tiipai or by the historical Spanish name Diegueño), Luiseño (also known as Payómkawichum or "People of the West"), Cupeño, Cahuilla, and the Acjachemen people and their local predecessors.

In 1542, the explorer Juan Rodríguez Cabrillo, who may have been born in Portugal but sailed under the flag of Castile, claimed San Diego Bay for the Spanish Empire, and he named the site San Miguel. In November 1602, Sebastián Vizcaíno surveyed the harbor and what are now Mission Bay and Point Loma and named the area for Saint Didacus, a Spaniard more commonly known as San Diego. European settlement in what is now San Diego County began with the founding of the San Diego Presidio and Mission San Diego de Alcalá by Spanish soldiers and clerics in 1769. This county was part of Alta California under the Viceroyalty of New Spain until the Mexican declaration of independence. From 1821 through 1848, this area was part of Mexico.

San Diego County became part of the United States as a result of the Treaty of Guadalupe Hidalgo in 1848, ending the Mexican–American War. This treaty designated the border to pass through a point one marine league south of the southernmost point of the port of San Diego, ensuring that the United States received all of the natural harbor of San Diego Bay.

San Diego County was one of the original counties of California, created at the time of California statehood in 1850.

At the time of its establishment in 1850, San Diego County was relatively large, and included all of southernmost California south and east of Los Angeles County. It included areas of what are now Inyo and San Bernardino Counties, as well as all of what are now Riverside and Imperial Counties.

During the later part of the 19th century, there were numerous changes in the boundaries of San Diego County, when various areas were separated to make up the counties mentioned above. The most recent changes were the establishments of Riverside County in 1893 and Imperial County in 1907. Imperial County was also the last county to be established in California, and after this division, San Diego no longer extended from the Pacific Ocean to the Colorado River, and it no longer covered the entire border between California and Mexico.

Geography

According to the U.S. Census Bureau, the county has an area of , of which is land and (7.0%) is water. The county is larger in area than the combined states of Rhode Island and Delaware.

San Diego County has a varied topography. On its western side is more than of coastline.Gerber, James ed. Economic Profile of the San Diego-Tijuana Region: Characteristics for Investment and Governance Decisions . Institute for Regional Studies of the Californias. 1995. p.11 Most of San Diego between the coast and the Laguna Mountains consists of hills, mesas, and small canyons. Snow-capped (in winter) mountains rise to the east, with the Sonoran Desert farther to the east. The Cleveland National Forest is spread across the central portion of the county, while Anza-Borrego Desert State Park occupies most of the northeast.

Although the county's western third is primarily urban, the mountains and deserts in the eastern two-thirds are primarily undeveloped backcountry. Most of these backcountry areas are home to a native plant community known as chaparral. San Diego County contains more than of chaparral, twice as much as any other California county.

Periodically the area has been subject to wildfires that force thousands to evacuate. The most recent are the December 2017 Lilac Fire, the May 2014 San Diego County wildfires, the Witch Creek Fire in 2007, and the Cedar Fire in 2003. California defines a fire season in which fires are most likely to occur, usually between late July and late October (which are the driest months of the area). Signs posted in numerous spots of the county provide information on the level of threats from fires based on weather conditions.

Regions Northern San Diego County is known as North County; the eastern suburbs are collectively known as East County; the rural areas located further east and extending to the Imperial County line are known as the Mountain Empire; and the southern suburbs, extending to the Mexican border, are collectively known as the South Bay or South County, including South San Diego, an exclave of the city of San Diego which has no land connection to the rest of the city.

Location San Diego County is bordered on the northwest by Orange County, on the north by Riverside County; on the east by Imperial County; on the south by Mexico; and on the west by the Pacific Ocean.

Climate

Under the Köppen climate classification system, the urban and suburban San Diego area straddles areas of Mediterranean climate (Csa) to the north and hot semi-arid climate (BSh) to the south and east. As a result, it is often described as "arid Mediterranean" and "semi-arid steppe." Farther east, arid desert conditions prevail. Western San Diego's climate is characterized by warm, dry summers and mild winters with most of the annual precipitation falling between November and March. The city has mild, mostly dry weather, with an average of 201 days above and low rainfall ( annually). Summer temperatures are generally warm, with average highs of and lows of . Temperatures exceed only four days a year. Most rainfall occurs from November to April. Winter temperatures are mild, with average high temperatures of and lows of .

The climate in the San Diego area, like much of California, often varies significantly over short geographical distances resulting in microclimates. In San Diego's case this is mainly due to the city's topography (the Bay, and the numerous hills, mountains, and canyons). Frequently, particularly during the "May gray/June gloom" period, a thick marine layer will keep the air cool and damp within a few miles of the coast, but will yield to bright cloudless sunshine approximately inland. This happens every year in May and June. Even in the absence of June gloom, inland areas tend to experience much more significant temperature variations than coastal areas, where the ocean serves as a moderating influence. Thus, for example, downtown San Diego averages January lows of and August highs of . The city of El Cajon, just northeast of downtown San Diego, averages January lows of and August highs of . Julian, in the mountains, has an average January low of and August high of . Borrego Springs, in the Colorado Desert, has an average January low of and August high of .

Rainfall along the coast averages about of precipitation annually, which occurs mainly during the cooler months of December through April. Though there are few wet days per month during the rainy period, rainfall can be heavy when it does occur. However, the rainfall is greater in the higher elevations of San Diego. Some of the higher areas of San Diego, such as Palomar Mountain and the Laguna Mountains, receive of rain per year, supporting lush forests similar to the Sierra Nevada and California Coast Range. The Colorado Desert portion of the county lies to the east of the mountains, which receives the least amount of precipitation; Borrego Springs, the largest population center in the desert, averages only , with a high evaporation rate.

Adjacent counties and municipalities

National protected areas Cabrillo National Monument Cleveland National Forest (part) San Diego National Wildlife Refuge Complex, which includes San Diego National Wildlife Refuge San Diego Bay National Wildlife Refuge Tijuana Slough National Wildlife Refuge Seal Beach National Wildlife Refuge (located in Orange County)

There are seven official wilderness areas in San Diego County that are part of the National Wilderness Preservation System (NWPS). Four of these are integral parts of Cleveland National Forest, whereas three are managed by the Bureau of Land Management (BLM). Some of these extend into neighboring counties (as indicated below):

Otay Mountain Wilderness (BLM) Pine Creek Wilderness (Cleveland National Forest) Hauser Wilderness (Cleveland National Forest) Carrizo Gorge Wilderness (BLM) Sawtooth Mountains Wilderness (BLM) Agua Tibia Wilderness (Cleveland National Forest) partly in Riverside County San Mateo Canyon Wilderness (Cleveland National Forest) mostly in Riverside County

State parks and protected areas

Anza-Borrego Desert State Park (majority) Torrey Pines State Natural Reserve Cuyamaca Rancho State Park Palomar Mountain State Park San Pasqual Battlefield State Historic Park Old Town San Diego State Historic Park Border Field State Park Tijuana River National Estuarine Research Reserve San Onofre State Beach Moonlight State Beach Carlsbad State Beach South Carlsbad State Beach Leucadia State Beach San Elijo State Beach Cardiff State Beach Torrey Pines State Beach Silver Strand State Beach

Mountains

Cuyamaca Mountains In-Ko-Pah Mountains Jacumba Mountains Laguna Mountains Palomar Mountain Peninsular Ranges San Ysidro Mountains Santa Ana Mountains Volcan Mountains

There are 236 mountain summits and peaks in San Diego County including:

Black Mountain Cuyamaca Peak (second highest point in San Diego County) Cowles Mountain (highest point in the city of San Diego) Mount Helix Hot Springs Mountain (highest point in San Diego County) Margarita Peak Mount Soledad Stonewall Mountain El Cajon Mountain Woodson Mountain (known for Potato Chip Rock)

Bays and lagoons

Buena Vista Lagoon Agua Hedionda Lagoon Batiquitos Lagoon San Elijo Lagoon Los Peñasquitos Lagoon Mission Bay San Diego Bay

Lakes

Lake Cuyamaca Lake Hodges Santee Lakes Sweetwater Reservoir Upper Otay Reservoir Lower Otay Reservoir Lake Wohlford El Capitan Reservoir Sutherland Reservoir Lake Henshaw Lake Murray San Vicente Reservoir Lake Jennings Barrett Dam