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Imperial Valley,fertile region in the Colorado Desert, SE Calif., extending S into NW Mexico. Once part of the Gulf of California, most of the region is below sea level; its lowest point is −232 ft (−71 m) at the southern shore of the Salton Sea. Receiving only c.3 in. (7.6 cm) of rain annually, the valley experiences extremely high temperatures (115°F;/46°C;) and has a great daily temperature range. Having one of the longest growing seasons in the United States (more than 300 days), the valley can, with irrigation, support two crops a year; it was first irrigated in 1901. Several disastrous floods on the Colorado River in 1905–6 inundated the area; not until 1936, with the completion of Hoover Dam, was the valley safe from floods. Approximately 1 million acres (404,700 hectares) have been irrigated, chiefly by the All-American Canal. The valley is an important source of winter fruits and vegetables for the northern areas of the United States; cotton, dates, grains, and dairy products are also important. Brawley, Calexico, and El Centro, Calif., are the main U.S. cities in the valley; Mexicali, Mexico, also in the valley, is the center of Mexico's important cotton-growing district.
(Lower California Valley), lowlands in the United States and Mexico occupying an intermontane tectonic depression that is a continuation of the basin of the Gulf of California. The valley is filled by a thick layer of alluvial sediments from the Colorado River, which flows through the southern part of the valley. In the past the valley was often flooded by the waters of the Colorado, resulting in the formation of salt lakes—the Salton Sea (81 m below sea level) in the north and Laguna Salada in the west. The climate is dry and subtropical; the valley receives about 100 mm of precipitation annually. The vegetation is of the “woody” desert type and includes creosote bushes, large cacti, and arborescent yuccas. Some of the land between the Colorado River and the Salton Sea is irrigated; the principal crop is cotton.