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<br />'eonditions can also be seen in Figure 3.1, which shows estimated natural <br />flows for the Colorado River at Lees Ferry, Arizona,l <br /> <br />) <br />) <br /> <br />Given this annual variability in runoff conditions, the period of <br />reeord which one chooses when performing hydrologic analyses beeomes <br />critical. For example, Figure 3,2 shows the different estimates of <br />natural flow at Lees Ferry which are arrived at by selecting different <br />periods. The most notable observation is that the long term annual <br />average is reduced by better than one million acre-feet when the <br />extremely wet period from 1906 through the mid-1920s is not taken <br />into account. Furthermore, there obviously have been shorter periods <br />when the average annual estimated flow was substantially below any <br />longer term average. <br /> <br />, <br />. <br /> <br />For the purposes of this report, the period from 1906 through 1974 <br />has been used as the primary basis of analysis. In selected instances, <br />the period from 1930 through 1974 (with average annual natural flows <br />estimated at 13,8 million acre-feet) has also been used for 'comparison <br />purposes (see chapter 7). In all events, it is imperative that the <br />reader remember that the period selected has a significant effect on <br />the outcome of the analysis, <br /> <br />. <br /> <br />The second salient characteristic of runoff conditions in the Upper <br />Basin is the monthly variation which occurs during anyone year, As can <br />be seen in Figure 3.3, runoff peaks during a four month period, with <br />about 65 to 75 percent of the annual runoff arising, on the average, <br />during those months.2 This distribution ,of monthly flows is, of course, <br />the result of snow melt, which constitutes the primary source of runoff <br />in .the Upper Basin. <br /> <br />1. "Lees Ferry" is the name of the USGS gauging station located on the <br />Colorado River upstream from the town of Lees Ferry and above the eon- <br />fluence of the Paria River with the Colorado, "Lee Ferry" is the point <br />on the Colorado River one mile below the mouth of the Paria which serves <br />as the point of demareation between the Upper and Lower Colorado River <br />Basins for the purposes of the 1922 Colorado River Compaet. As used in <br />this report, "Lees Ferry" means the gauging station, while the phrase, <br />Ilcompac.t point", will be used to refer to "Lee Ferry." <br /> <br />2. In making the natural flow estimates, the U.S, Bureau of Reclamation <br />did not, in all instances, take into account the lag effect of irrigation <br />return flows on gauged stream records, Thus, the monthly distribution <br />of natural flows shows peak month flows that are slightly lower than <br />would be the case if the lag effect had been taken into account, Like- <br />wise, late summer and early fall natural flows are slightly overestimated. <br />In both instances, the error is thought to be insignificant, <br /> <br />. <br /> <br />3-3 <br />