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M 00 M N c' ,"" ~ SETTING Location The Lower Virgin River Unit includes the Lower Virgin River drainage from Littlefield, Arizona to the river's mouth at Lake /1ead (see Frontispiece Map). The Lower Virgin River Basin is bordered to the east and southeast by the Virgin Mountains, to the west and north- west by Mormon Mesa and the Mormon Mountains, and to the north by Tu1e Desert. The Lower Virgin River Basin drainage encompasses an area of about 1,740 square miles which includes parts of Nevada, Arizona, and Utah. The principal settlements in the area are Littlefield, Arizona, and Mesquite, Bunkerville, and Riverside, Nevada. Irrigated farming of alfalfa, pasture, small grains, and vegetables is the primary industry. The Virgin River is tributary to the Colorado River, originating in the Zion National Park region of Utah and terminating in the Overton Arm of Lake Mead. Prior to the damming of the Colorado River by the construction of Hoover Dam, the Muddy River in Nevada was a tributary to the Virgin River. The total relief of the Lower Basin varies from 1,150 feet mean sea level at Lake Mead to 8,350 feet in the Virgin /1ountains. The lower portion of Beaver Dam Wash, just north of Littlefield, Arizona, is the only perennial tributary to the Lower Virgin River, Climate The climate in the area is arid with an average annual precipita- tion of 3 inches in an average year to 6 inches in a wet year. Winter frontal storms produce the majority of precipitation. Isolated summer convective storms can cause flash flooding and erosion. The average annual free water surface evaporation is 80 inches. Temperatures associated with the valley area vary from an average minimum of 700F to an average maximum of 1050F in July and an average minimum of 300F to an average maximum of 600F in January. [2] 5