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<br />4:l <br />00 <br />~ <br />CO <br /> <br />. <br /> <br />all flows leaving the Upper Basin is provided by the Glen Canyon reservoir. <br /> <br />Although natural river flows fLlctuate widely, these variations are significantly . <br /> <br />reduced by the reservoirs. <br /> <br />Lower Colorado River Region. The total area of the Lower Colorado <br /> <br />. <br /> <br />River region is about l4l, 000 square miles. Almost the entire streamflow <br /> <br />input to this region is the Colorado River itself at Lee Ferry in northern <br /> <br />Arizona. Two major tributaries are the Little Colorado River and the Gila, . <br /> <br />but the flows which reach the main stem of the Colorado River from these <br /> <br />two sources is negligible. Average annual runoff varies widely, averaging <br /> <br />0.05 inches or less in the de.serts to 8 inches or more in the mountains <br /> <br />. <br /> <br />(Framework Study, Appendix V, 1971.) In its natural state the Colol"ado <br /> <br />River would gain about 1 million acre-feet betwElen Lee Ferry and Hoover <br /> <br />. <br /> <br />Dam. Below this point there is a streambed loss of about this same quantity. <br /> <br />Like the Upper Region, the Lower Region subdivides naturally into <br /> <br />three subregions which are designated as the Lower Main Stem, the Little <br /> <br />. <br /> <br />Colorado River drainage, and the Gila River basin (Figure UI.4). The <br /> <br />hydrology of each of these subregions is described briefly by Appendix V <br /> <br />. <br /> <br />of the Framework Study (l971), and is summarized here. The water supply <br /> <br />available within the Lower Main Stem subregion consists of (l) natural runoff <br /> <br />originating within the subregion; (2) releases from Lake Powell; and <br /> <br />. <br /> <br />(3) groundwater. <br /> <br />Many of the natural tributaries within the subregion flow only during <br /> <br />periods of heavy rainfall, and surface .water supplies originating within the <br /> <br />. <br /> <br />zo <br /> <br />. <br />