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<br />C1': <br />C.J <br />~ <br />C'J <br /> <br />c <br /> <br />Juan, Little Colorado, and Virgin Rivers. Elevations range from <br />about 650 feet at the dam to approximately 14,000 feet in the high <br />mountainous peaks at the headwaters of the river. <br /> <br />f.-,~-::',~~~} <br />-;;.-::;;::~.. <br /> <br />4. Hydrometeorological characteristics. --The climate of the <br />basin general.ly is arid or semiarid. However, wide variations in <br />climate exist because of differences in latitude, topography, and <br />elevation. In the northern part of the basin, winters are long and <br />cold and s\lllllllers are short and warm. In the southern part, s\lllllllers <br />are longer and winters are shorter and more mild. A summary of <br />annual climatological data at representative stations in and near <br />the basin is given in table 1 and a summary of mean monthly precipi- <br />tation at these stations is given in table 2. Locations are shown <br />on plate 2. <br /> <br />5. Most of the precipitation in the drainage area upstream <br />from Hoover Dam occurs during the fall and winter aa snow, although <br />general ra inatorms and thunderstorms occur during the summar months. <br />The average annual precipitation varies from a min1lllum of about 4 <br />inches in valJ.ey areas to a maximwn of more than 30 inchea in the <br />highest mountains of the headwater region. Isohyets of mean seasonal <br />precipitation are shown on plate 2. <br /> <br />6. Runoff. --Floods resulting from snowmelt constitute about <br />70 percent of the annual runoff of the Colorado River at take Mead. <br />These floods can be forecast months ahead. Accumulation of snow <br />begins around 1 October, and melting starts about 1 April.. The re- <br />sulting floods occur during the months of April through July; and <br />the volumes range from about 2,000,000 acre-feet to 17,000,000 acre- <br />feet, averaging approximately 8,820,000 acre-feet. For the water <br />years 1913-14 through 1965-66, incluaive, at Grand Canyon( Ariz. <br />(where flow is approximately equal to inflow at take Mead), the <br />annual water-year runoff volumes, the April through July runoff <br />volumes, and the maximum mean daily discharges are given in table 3. <br />It should be noted that values have been estimated through water <br />year 1921-22. For this reason, the remaining record is considered <br />more reliable for use in deriving forecast1ng procedures. <br /> <br />" <br /> <br />7. While most of the annual runoff comes from snowmelt, floods <br />from rsinfall occur sporadical.ly, psrticularly during the months of <br />August through October. These floods start within a few days of the <br />rainfall and are not subject to forecasting far in sdvance. Sizable <br />floods from rainfsll (with discharges over 40,000 cubic feet per <br />second) can be expected about lout of 3 years. Peaks somet1mes <br />exceed 100,000 cubic feet per second, and volumes occasionally <br />approach 2,000,000 acre-feet. A tabulation of maximum 15-day-runoff <br />volumes snd maximum mean daily diScharges for the largest rainfall <br />floods thst occurred at the gaging station at Grand Canyon, Ariz., <br />during the period 1909 through 1966 is given in table 4. <br /> <br />2 <br />