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The Upper Main Stem (formerly the Grand Division) is the drainage area of the Colorado River above its junction with the Green River (26,192 mi2).. Annual Discharge Large yearly variations in flow occur in the Colorado River be- cause of annual variations in precipitation and longer-term climatic changes. Man-induced depletions are having an increasing effect on flow variation. The average annual discharge (basin outflow) of the upper basin recorded at Lee's Ferry is 12,500,000 acre-feet for the 79-year period, 1896-1975 (Figure 1.2). A maximum of 22,000,000 acre-feet was estimated in water year 1907 and a minimum of 2,414,000 acre-feet was recorded in 1964. Discharge for 1975 was 11,288,300 acre-feet. Of the three hydrologic subregions, the largest volume of water comes from the upper Main Stem (about 50 percent in 1975). The Green River Subregion provides approximately 41 percent, and the San Juan- Colorado Subregion contributes the remainder. The major source of water is runoff from high, snow-laden moun- tains in the upper zone of the main stem. These high mountain areas comprise only about 13 percent of the basin area but produce about 75 percent of the runoff (Upper Colorado Region State-Federal Inter- Agency Group 1971). Depletions Annual discharge represents residual river flow, or the net volume of water remaining in the river after man-induced depletions (con- sumptive use and loss). River flow without these depletions has been computed since 1896 by the Bureau of Reclamation (Figure 1.2). This average annual undepleted (virgin) flow at Lee's Ferry for the 79- year period is estimated at 14,900,000 acre-feet. Depletions have increased from 250,000 acre-feet in 1896 to 1,800,000 acre-feet in 1914; 3,450,700 acre-feet in 1965; and 3,823,900 acre-feet in 1975. Water depletion can best be summarized by use of a water budget 9