The South Coast Basin of southern California includes the second largest urban area in the United States as well as the two largest cities in California. In addition to the major centers of Los Angeles and San Diego, there are numerous other urban and suburban communities. The 1980 population of the entire region was 12.01 million, compared with a prewar (1940) total of only 2.9 million. Over the past 40 years, the dramatic growth in population, which has averaged 10 percent annually, has been fueled by a variety of factors, including a favorable climate and the rise of defense and aerospace-related industry. This growth was achieved despite the severe limitations of local water supplies.

Mean annual precipitation in the region averages only 14 inches. Over the period of record, annual precipitation has been quite variable, ranging between 5 and 38 inches annually. In addition, the area has a typically Mediterranean climate in which rainfall occurs predominantly between November and April. As a consequence, there exists not only a dearth of locally generated water supplies but an incongruity between the winter period, when those supplies are more readily available, and the summer period, when water demands are at a peak.

The modern history of the region has been characterized by the development of supplemental water supplies and the storage facilities necessary to regulate water flows so as to redress the natural imbalance between periods of peak supply and peak demands. The physical manifestations of this development include three major aqueducts that, with their associated storage facilities, permit the South Coast Basin to import water from the Colorado River, the Central Valley of California, and the Owens Valley to the northeast.