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Show IMPOUNDMENTS The Colorado River is almost completely controlled by the Upper Colorado River Basin Storage Projects and Lake Mead, having combined storage capacity of about 60 million acre-feet. Within the Lower Colorado Region, aggregate usable capacity of 15 significant impound- ments on the Colorado River and tributaries is 28.6 million acre-feet and 3.2 million acre-feet, respectively. CLIMATE The Lower Colorado Region*s climate varies widely because of a large difference in elevation, a considerable range in latitude, and the distribution of mountain ranges and highlands. See Figure 1 for climatic data. Generally, the climate of the more densely populated areas of the Region, such as Las Vegas, Nevada, and Phoenix, Tucson, and Yuma, Arizona, is characterized by mild to warm annual temperatures, low to moderate humidity, and low annual precipitation. Summer temperatures often reach 100 degrees and above. Rainfall is predominantly in the form of thunder- showers which are sometimes very intense and produce flash flooding locally. Except in the mountains, snow is rare and short-lived. Annual frost-free days in the lower desert range from over 270 days to yearlong. Yuma, Arizona, has experienced several years in succession without a killing frost. These climatic factors are very favorable to the irrigated agri- culture concentrated in the lower elevations. In contrast, such cities as St. George, Utah; Winslow and Holbrook, Arizona; and Gallup, New Mexico, have more of a four-season climate with shorter frost-free periods and colder winter months. Flagstaff, Arizona, and other cities in mountainous, rimrock, and plateau locations have sub- stantially shorter frost-free periods, mild summers, cold winters, and. considerably more rain and snow. Most of the recharge to ground water and the surface runoff in the Region results from precipitation in the high mountainous areas. Up to 30 inches of precipitation a year are recorded, much of which is in the form of snow. SOCIO-ECONOMIC ENVIRONMENT In 1965j the Lower Colorado Region had a population of nearly 1.9 million people and was increasing at the rate of about k to 5 percent per year. Nevada and Arizona were No. 1 and No. 3 in national standing for rates of increase. 11 |