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Show CHAPTER E - WATER AND RELATED LAND RESOURCES AVAILABILITY WATER Surface Water Colorado River Flows originating in the Upper Colorado Region and released through Glen Canyon Dam, some 17 miles upstream from the Lower Colorado Region's hydrologic boundary at Lee Ferry, constitute a major source of water supply to the Region, Portions of the flow released from Glen Canyon Dam, together with tributary inflow in the Lower Colorado Region, are consumed by native vegetation, lost by evaporation from water surfaces, and are delivered to Mexico pursuant to treaty. The remaining portion of the flow is then available for use within the Lower Basin States of Arizona, Nevada, and California, The Supreme Court decreed that, of the first 7.5 million acre-feet of Colorado River water annually available for use within the States, Arizona, Nevada, and California are apportioned 2.8 million acre-feet, 0,3 million acre-feet, and k.k million acre-feet, respectively. The Mexican Treaty of 19^-4 provides for delivery of 1,5 million acre-feet of water annually to Mexico. The Colorado River Compact provides, among other things, that the river at Lee Ferry will not be depleted below an aggregate of 75 million acre-feet for any period of 10 consecutive water* years, and for the sharing between the Upper and Lower Basins of any burden which might arise because of the water treaty with Mexico. The average annual undepleted flow of the Colorado River as it enters the Lower Colorado Region is estimated at about 15.09 million acre-feet for the 60-year period 1906 to 1965. In its undepleted state9 with the contribution of the Gila River near the Mexican border, the Colorado River's average annual flow into Mexico would be about 15.9 million acre-feet. The annual virgin undepleted flow of the Colorado River at Lee Ferry, Arizona, and the progressive 10-year average are shown on Figure 8. Local Runoff Approximately 100 million acre-feet of precipitation fall each year upon the Region, of which approximately 3 million acre-feet reach the streams or ground-water reservoirs for downstream use. The distri- bution, by subregion, of average annual runoff (undepleted flow) 53 |