Eagle River Inventory and Assessment (2)

Home Browse Search

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 35 locations in the Eagle River watershed. We have included numerous metrics describing the magnitude, frequency, duration, tlmmg, 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 8~ , X _ ~~ ~~ ~"' "'~,,--~.° Wit= .~,: ~ ~,~ ~., ~ ~ti ~~_- ~.~, ,~ ~ ~r,~ r -~ ~ ,• s q. F, _~ :. - ,_ z ~ ~~ River through the Eagle Mlne 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 Eagle River Inventory and 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 domesticlmunicipal 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 to 1986. We analyzed data from USGS flow gages in stream and river segments with decreed Instream flow rights to (1) examine the frequency of flows below the Instream flow rights, and (2) compare the Instream flow rights to the average and deviation of actual flows at different times of year. ®f 6~ segments in the Eagle River watershed with Instream flow rights, 47 are engaged, and the remaining segments have Instream flows that are not met at widely varying frequencies. Nine of the 19 segments that have been gaged for at least 10 years have Instream flow rights that have not been met at least one time per year in each year of the record since appropriation. Eagle River Inventory and Assessment ES-5