- Total 4,482 sq mi (11,610 km2)
- Land 4,177 sq mi (10,820 km2)
- Water 305 sq mi (790 km2)
Highest elevation 4,551 ft (1,387 m)
Population (April 1, 2010)
- Total 174,528
- Estimate (2017) 182,883
- Density 39/sq mi (15/km2)
Spanish explorer Melchor Díaz was one of the first Europeans to visit the area around Imperial Valley in 1540. The explorer Juan Bautista de Anza also explored the area in 1776. Years later, after the Mexican–American War, the northern half of the valley was annexed by the U.S., while the southern half remained under Mexican rule. Small scale settlement in natural aquifer areas occurred in the early 19th century (the present-day site of Mexicali), but most permanent settlement (Americans in the U.S. side, Mexicans in the other side) was after 1900.
In 1905, torrential rainfall in the American Southwest caused the Colorado River (the only drainage for the region) to flood, including canals that had been built to irrigate the Imperial Valley. Since the valley is partially below sea level, the waters never fully receded, but collected in the Salton Sink in what is now called the Salton Sea.
Imperial County was formed in 1907 from the eastern portion of San Diego County. The county took its name from Imperial Valley, itself named for the Imperial Land Company, a subsidiary of the California Development Company, which at the turn of the 20th century had claimed the southern portion of the Colorado Desert for agriculture. Much of the Imperial Land Company’s land also existed in Mexico (Baja California). The objective of the company was commercial crop farming development.
By 1910, the land company had managed to settle and develop thousands of farms on both sides of the border. The Mexican Revolution soon after severely disrupted the company’s plans. Nearly 10,000 farmers and their families in Mexico were ethnically cleansed by the rival Mexican armies. Not until the 1920s was the other side of California in America sufficiently peaceful and prosperous for the company to earn a return for a large percentage of Mexicans, but some chose to stay and lay down roots in newly sprouted communities in the valley.
The county experienced a period of migration of “Okies” from drought-trodden dust bowl farms by the need of migrant labor, and prosperous job-seekers alike from across the U.S. arrived in the 1930s and 1940s, especially in World War II and after the completion of the All American Canal from its source, the Colorado River, from 1948 to 1951. By the 1950 census, over 50,000 residents lived in Imperial County alone, about 40 times that of 1910. Most of the population was year-round but would increase every winter by migrant laborers from Mexico. Until the 1960s, the farms in Imperial County provided substantial economic returns to the company and the valley.
Currently, El Centro has one of the highest unemployment rates (above 30–34%) in the U.S. and ranks one of California’s poorest counties or have a lower than state and national average annual household income.