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<br />water rights in the South Platte basin are inigation rights on the eastern plains dating back <br />to the 1860~s. The owners of those rights typically place their call on the river, forcing <br />Denver Water and Aurora to pass all natural water through their reservoirs. Denver <br />Water and Aurora are sometimes able to exchange or trade water with the senior water <br />rights holder and thereby store water. <br /> <br />2. NauveS"eamflows <br /> <br />The operation of water supply systems is affected by native (natural) runoff of individual <br />years and the cycles or groupings of years~ Native streamflows are the surface water <br />streamflows that would occur without the influence of humans. They reflect the <br />hydrology that existed prior to the development of water supply systems or the hydrology <br />that would exist if the effects of water supply systems were removed~ Most streams and <br />rivers in Colorado have their native streamflows altered by irrigation, municipal <br />diversions, and reservoirs. As a result, there is little or no measured data of native <br />streamflow for most streams in Colorado. However, a hydrologist can derive reasonably <br />accurate native streamflow data from historical diversion data. Conceptually this is done <br />by using historical non~native streamflow data and: <br />subtracting out historical water importations (i.e., transrnountain diversions) <br />adding back in historical reservoir evaporation <br />adding back in the historical diversions <br />subtracting out historical irrigation and municipal return streamflows <br />subtracting out storage releases from upstream reservoirs. <br /> <br />Figure 3 shows the annual native streamflows for the South Platte from 1916 through <br />1996. Using the same data, Figure 4 shows the ratio as compared to average annual <br />native streamflows for the same time period. As these two figures show, there is <br />substantial variation in native runoff. These variations occur from one year to the next <br />along with substantial periods of consecutive wet years and dry years. The operation of <br />water supply systems is not only dependent on these streamflows in the South Platte but <br />also on the streamflows that occur in the various other basins from which entities such as <br />Denver Water and Aurora obtain their water supply~ The Streamflow Management Plan <br />seeks to alter the naturally occurring streamflow fluctuations to benefit fisheries and <br />whitewater recreation~ <br /> <br />3. Historic Streamflows <br /> <br />In order to gain an understanding of historic streamflows, Table 3 shows seasonal historic <br />outflows from Eleven Mile and Cheesman reservoirs for the time periods 1947 through <br />1996 and 1985 through 1996. The later 1985 - 1996 time period is more reflective of <br />current operations, but does not include significant periods of successive dry years. The <br />two seasons~ April! - July 31 and August 1 ,. March 31, coincide with the periods of the <br />target goal streamflows in the Plan (Table 2)~ As expected, the longer time period (1947- <br />1996) contains more extreme events in terms of low and high streamflows. Also as <br />shown, the daily data has more extreme values than the average monthly values. <br /> <br />N ~\Supply\Waage\wildscen\attachb.doc <br /> <br />B...22 <br />