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. . tt.) c:o tt.) 0,') . . . . . . . . . was 15, 9l9, 000 acre-feet per year, while for the 26-year dry cycle 1931- 56 the average discharge was 11, l83, 000 acre-feet per year. These are residual flows reflecting upstream depletions approximating l, 800, 000 J acre-feet in 19l4 and increasing to about 2,800,000 acre-feet in 1962. The average annual virgin flow at Lee Ferry, as unaffected by the activi- ties of man, is estimated at l4, 872 million acre-feet over the 52-year period 19l4- 65. This contribution would average about 2.5 inches in depth over the entire Upper Basin. The Upper Basin divides naturally into essentially three major drainage systems, referred to as subregions. These subregions are de- signated as the Upper Main Stem, the Green, and the San Juan (Figure 111-3). The Upper Main Stem subregion consists of the drainage area of the Colorado River above its junction with the Green River. The entire Green River drainage comprise s the Green subregion. The San Juan subregion is the drainage area of the Uppea Basin between the junction of the Green River with the Colorado River and Lee Ferry, Arizona. These three divisions follow the representations used by Iorns et al. (l964 and 1965) and facilitate concis~ presentation of information about an extensive and diverse area. When unaffected by the activities of man, runoff is referred to as "natural or virgin" flow. Except in the headwater reaches, few streams in the Upper Colorado River region now carry natural flows. Artificial di ver sions (inciuding out- of- basin exports) and regulatory action in lake s 17 , , .