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<br />," <br /> <br />O",(':q.., <br />"-iJv"'i <br /> <br />PURPOSE OF RESERVOIRS <br /> <br />II <br /> <br />It may also be noted from Figure 2 that the water available <br />from the Fraser sources and fram Williams Fork sources is fairly <br />constant because this water is derived from the base flow of those <br />streams, The water available to Denver from the Blue River, haw- <br />ever, is erratic because it is that which remains for diversion at <br />Dillon after Green Mountain Reservoir is assured of one filling. <br />In other words, Green Mountain Reservoir has the advantage of <br />having available to it the base flow. <br /> <br />The reservoir capacity necessary to make available all of <br />the water from a fairly uniform source, such as the Fraser and <br />Williams Fork, is substantially less in relation to the average <br />annual supply than that required for regulation of the Blue River <br />source which will vary considerably in amount from year to year, <br />The South Platte storable water is illustrative of the latter type of <br />supply, the annual yield of South Platte storage rights being less <br />than 10 per cent of the operating storage capacity of the Board's <br />reservoirs after Eleven Mile Reservoir has been enlarged by <br />30,000 acre-feet, <br /> <br />The optimum storage required for regulotory purposes is <br />that which will make all of the water supplies usable over a rea- <br />sonable period of years, Reserve storage is desirable to insure <br />a water supply during a period when a portion af the source <br />might be unavailable because of accident to some part of the <br />water system. <br /> <br />Both an element of engineering judgment and a consider- <br />ation of economics are involved in the determination of the <br />amount of storage required to accomplish the above-mentioned <br />purposes, In some cases infrequent flood flows that are surplus <br />to a river system might be captured in a given reservoir potential <br />and withdrawn over a long period of time, This could be very <br />uneconomic because the annual yield would be only a fraction of <br />the reservoir capacity, It could be likened to the building of a <br />large hotel in a small municipality to take care of a convention <br />which might be held only once in some 20 years. <br /> <br />Because of the wide divergence of opinon as to the amount <br />of storage that should be provided by Denver incident to its pres- <br />ent program, the question was carefully studied, taking into con- <br />