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;>:{.> ~.~,~~ .~~:;. can also be seen in figure 3.1, which shows estimated natural flows for the Colorado River at Lees Ferry, Arizona. 1 f\~ ~ Given this annual variability in runoff conditions, the period of ~ record which one chooses when performing hydrologic analyses becomes ~ critical. For example, figure 3.2 shows the different estimates of natu- ral flow at Lees Ferry which are arrived at by selecting different periods. The most notable observation is that the long-term annual average is re- duced by better than one million acre-feet when the extremely wet period from 1906 through the 1920s is not taken into account. Furthermore, there obviously have been shorter periods when the estimated average annual natural flow was substantially below any longer term average. For the purposes of this report, the period 1906-1974 has been used as the primary basis of analysis. In selected instances, the period 1930- 1974 (with average annual natural flows estimated at 13.8 million acre- feet) has also been used for comparison purposes (see chapter 9). In all events, it is imperative that the reader remember that the period selected has a significant effect on the outcome of the analysis. Furthermore, of course, regulation of inflows has a profound effect on any hydrologic re- gime, and this fact must also be taken into account. .'... :~,':-:.: <";:.: The second salient characteristic of runoff conditions in the Upper Basin is the monthly variation which occurs during anyone year. As can be seen in figure 3.3, runoff peaks during a four-month period, ..with about 65 to 75 percent of the annual runoff arising, on the average, during those months.2 This distribution of monthly flows is, of course, the result of snowmelt, which constitutes the primary source of runoff in the Basin. 1. "Lees Ferry" is the name of the U. S. Geological Survey gauging station located on the Colorado River upstream from the town of Lees Ferry and above the confluence of the Paria River with the Colorado. "Lee Ferry" is the point on the Colorado River one mile below the mouth of the Paria which serves as the point of demarcation between the Upper and Lower Colorado River Basins for the purposes of the 1922 Colorado River Compact. As used in this report, "Lees Ferry" means the gauging station, while the phrase "compact point" will be used to refer to "Lee Ferry." 2. In making the natural flow estimates, the U.S. Bureau of Reclamation did not, in all instances, take into account the lag effect of irrigation return flows on gauged stream records. Thus, the monthly distribution of natural flows shows peak month flows that are slightly lower than would be the case if the lag effect had been taken into account. Like- wise, late summer and early fall natural flows are slightly overestimated. In both instances, the error is thought to be insignificant. .>:, ~. :.: 3-3