Eagle River Inventory and Assessment

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Gore Creek between the Black Gore confluence and the Eagle River is estimated at 7% and 35%, respectively. Similar patterns exist in the Eagle main stem, particularly between Minturn and Edwards where basin topography, development practicality, and human desires to be near the water place disproportionate development pressures in alluvial valleys, Results of an analysis of the imperviousness of developed areas (an often-cited indicator of stream health) revealed comparable trends. Future development will continue increasing imperviousness as the population of Eagle County approaches anticipated growth of 75% (Colorado Division of Local Government, 2003) over the next 25 years. These patterns are perhaps not surprising but nonetheless underscore the added challenges surrounding management of impervious surfaces, stormwater, and riparian zones in mountainous watersheds with rapidly growing population centers focused in and near ecologically critical zones. ES.4.2 Water Use C~~su~nptiv~ us~ accounts for a~p~oximat~ly 10 ~o ~ 5% of t~~ E~~~~ ~v~r's av~~a~~ an~ual yi~~~ of 410,000 ~c~~ ~~~t a~ t~e Gypsum ~~~e~ 7C~~ ~~eat~st co~su~n~tive use i~ t~e bas~m ~esu~ts ~rorn tr~ns~ou~tain exp~r~s which account for GO% of con~~mp~iv~ ~s~~ Irrigation is the second largest consumptive use at approximately 32%. Domestic/municipal and reservoir evaporation uses are estimated at 3% and 4%, respectively. From 1985 to 1995, a shift from irrigation consumptive use to domestic consumptive use was apparent, as was a 54% decrease in irrigation consumptive use. ES.4.3 Hydrology Although the flow regime of the Eagle main stem is more intact than some other rivers of comparable size on the western slope of Colorado, reservoirs and diversions have influenced the flow regime of several segments in the basin. We performed a detailed analysis of current and historical flow regime characteristics at 351ocations in the Eagle River watershed. We have included numerous metrics describing the magnitude, frequency, duration, timing, and rate of change of streamflows at these gaging sites in the main report. Changes in flow regime are most pronounced in Homestake Creek below Homestake Reservoir and in the upper Eagle River below the Wurtz and Columbine Ditches. The combined effects of these flow modifications are transmitted below the confluence of Homestake Creek and the Eagle River through the Eagle Mine site and Minturn, The relative hydrologic effects of these modifications diminish in a step-like fashion as the Eagle main stem accumulates tributary inflows. Reductions in peak flows have both geomorphic and ecological implications discussed in the Edgle River Inventory dnd Assessment. In general, the analysis and description of several ecologically relevant flow attributes performed in this study provide a framework for managing flows with a better understanding of historical variability. Despite distinct shifts in peak flow magnitudes in the upper Eagle and Homestake Creek, differences in pre- and post-alteration low flow regimes are much more subtle, Although there are detectable changes in low flows in the upper Eagle and Homestake Creek, it appears that the most detectable changes in the low flow regime of the Eagle River are associated with a reduction in irrigation return flows and the timing of low flow minima. Thus, it appears that there has been a historical sequence in which baseflows were originally supported by greater overbank flooding during snowmelt, then altered by irrigation diversions, return flows, and peak flow reductions, and most recently reduced by a shift from irrigation to domestic/municipal uses. The net effect of these changes over the last half century is a reduction in low flows in the main stem Eagle in the late summer and fall months when greater irrigation returns formerly supported higher baseflows, ES.4.4 Instream Flows Instream flows rights have been established by Colorado law to "preserve the natural environment to a reasonable degree," In the Eagle River watershed, instream flow rights have priority dates ranging from 1975 Eagle River Inventory and Assessment v