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0031u6 Page 5 Below the San Juan, the Colorado enters the Lower Basin, winding through the deep gorges of Marble and Grand Canyons, and eventually emerging into more open country along the borders of Nevada, Arizona and California. A lesser amount of inflow occurs in the Lower Basin, principally via the Little Colorado, Virgin, Bill Williams and Gila Rivers. The natural flows in the Basin are highly irregular in occurrence. While the virgin flow at Lee Ferry has averaged 14.8 million acre-feet annually over its measured period of record, flows in excess of 23 million acre-feet and less than 7 million acre-feet annually have been recorded. Over 70% of the annual virgin flow occurs in the months of May, June, and July. Flows have been recorded for less than 100 years at most points on the river. I A formidable array of reservoirs and diversion facilities alter the flow of the river. The river's two principal reservoirs, Lake Powell and Lake Mead, formed by Glen Canyon and Hoover Dams, respectively, provide over 50 million acre-feet of storage. Other major reservoirs include Lake Granby, Dillon and Green Mountain Reservoirs on the mains tern Colorado, Fontenelle and Flaming Gorge Reservoirs on the Green, the Curecanti unit on the Gunnison, and Navajo Reservoir on the San Juan. J Water is diverted from the river at hundreds of relatively small diversion points in the Upper Basin; in the Lower Basin diversions tend to be larger and considerably fewer in number. Major diversion structures in the Lower Basin include the Colorado River Aqueduct which delivers water to the Los Angeles Basin, the Granite Reef Aqueduct supplying the Central Arizona Project, and the All American Canal System bringing water to the Imperial Valley of California. Below the international boundary, Morelos Dam diverts water for irrigation in Sonora, Mexico.