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.