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<br />Cl <br />(:;) <br />l\:) <br />(.) <br />u) <br />co <br /> <br />4 <br /> <br />1.2.2 Early Development of the Colorado River Basin <br /> <br />The Colorado River Basin, technically divided into an Upper and <br /> <br />Lower Basin (see Figure 1.1), has a total area of approximately 244,000 <br />2 <br />square miles (635,000 km ) and carries an average annual natural flow <br /> <br />of between 13 and 15 million acre-feet (15.6 to 18 km3/yr). Of this <br /> <br />amount, over 5 MAF/yr is exported from the basin to the Arkansas, Missouri <br /> <br />and Great Basins, and to the Southern California area. <br /> <br />By way of comparison, the flow per unit of basin area for four <br /> <br />U.S. rivers is shown below. <br /> <br />River Drainage Area Runoff per <br />Basin (in millions) Runoff unit area <br /> (acres) (km2) (MAF/yr) 3 (in/yr) (cm/yr) <br /> (km /yr) <br />Colorado 156.0 0.63 15.0 18.5 l.15 2.92 <br />Mississippi 790.0 3.2 440.0 543.0 6.7 17 .0 <br />Columbia 165.0 0.67 180.0 222.0 13.1 33.3 <br />Delaware 7.9 0.03 14.0 17.0 20.9 53.1 <br /> <br />It is interesting to note that the Columbia Basin, while of approxi- <br /> <br />mately the same size as the Colorado, carries an order of magnitude <br /> <br />larger flow. Because of the geographic and geologic conditions in the <br /> <br />basin, silting and salting problems in the Colorado Basin are more <br /> <br />pronounced than elsewhere. The same factors contribute to the irregu- <br /> <br />larities in flow. These qualities characterize the Colorado River as <br /> <br />an arid or semi-arid river basin. <br /> <br />Settlement of the Upper Basin by outsiders began in about 1860 <br />2 <br />and resulted in the irrigation of 800,000 acres (3200 km ) by 1905. <br />