IMPERIAL VALLEY, a depressed area in Imperial county, California, and extending into the Mexican territory of Lower California. It is approximately no m. long by 4o m. wide, and constitutes a part of what is known as the Colorado desert. That the valley was formed by constructional depression rather than erosion is shown by the fact that the greater part of its area is below sea-level (surface of Salton Sink being 28o ft. below). The soil is chiefly silt deposited in the inland sea made by the Colorado delta, but in some places there is a large admixture of sand. The climate of the region is known for extremes, the daily range of temperature often exceeding 59° F. The extremes of tem perature range from 125° to a minimum of 24°, although the average for the year is 7 i °. The annual rainfall varies from 1.5 to 3 inches.
Prior to 1893, when the California Development company was organized for the purpose of securing and delivering water to irrigate and reclaim the Imperial valley, the region was waterless, barren and uninhabited. The first water was brought into the valley with the Imperial canal (45 m. long), which curved below the international boundary into Mexico, in March 1902. Diffi culties occasioned by the silting up of the California intake led to the opening of new intakes in Lower California in 1903 and 1904. Insufficient control gates allowed the Colorado river to widen the third intake during 1905-06 until its waters poured through the valley into Salton Sink. The situation became so alarming that President Roosevelt called on the Southern Pacific railway to assist in controlling the flood, and as a result the breach was closed on Nov. 7, 1906. High water soon caused another break and by Dec. 7, 1906 the entire flow of the river was again going back to Salton Sink, but this break was finally closed on Feb. I1, 1907. Levees were subsequently built to protect the region from further flood dangers. The main canal, over 5o m. in length, brought in a water-supply which, in 1919, irrigated 415,304 ac. and was capable of watering an additional 42,50o acres. The chief crops are alfalfa, cotton, cantaloupes, watermelons, asparagus, vegetables for winter markets, grapes and fruits. Cotton is the chief crop on the Mexican side of the border. The population of Imperial county increased from 13,59i in 191 o to 43,453 in 192o, or a gain of 219.7%. In 1930 it had grown to 60,903, a gain of 4o.2%. The chief centres of population are Calexico El Centro , Brawley , Imperial (1,943) and Holtville (1,758).