Due to its mostly dry climate, San Diego County depends on outside sources for much of its water. In fact, only 20 percent of the county’s water originates from rainfall and local sources and supplies.
Fifty percent of the county’s water comes from the Colorado River. In 1922, the Colorado River Compact was signed by representatives of seven states through which the Colorado River and its dis-tributaries pass through on their way to Mexico. The compact divides the usable flow of the river between the “Upper Basin” (Colorado, New Mexico, Utah, and Wyoming) and the “Lower Basin” (Arizona, California, and Nevada). Each basin, in turn, divides its allocation among its member states. Water from the Colorado River reaches San Diego County through an aqueduct system called the San Diego Project (popularly referred to as the San Diego Aqueduct).
The remaining 30 percent of San Diego County’s water is delivered through the California State Water Project, under the management of the California Department of Water Resources. The State Water Project, which made its first water deliveries to Southern California in 1973, collects water from rivers in Northern California and redistributes it to other parts of the state, including San Diego County. In order to reach Southern California, the water must be pumped over the Tehachapi Mountains, reaching elevations as high as 2,000 feet — the highest single water lift in the world. Originally, a tunnel through the Tehachapis was proposed in order to bring the water south, but due to concerns over the faults riddling this mountain range, that plan was scrapped in favor of the overland pumping system.