Proposed Workplan for Yampa River Basin Sediment Monitoring Program with Notes

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0 0 In developing the recommendations, the peer review panel considered a number of issues related to man - induced factors that have affected the flow patterns and sediment delivery to the Yampa and Green Rivers, and the potential effects of those factors on the suitability of spawning and other life -stage habitat for the endangered fish. The water resources of the Upper Colorado River basin have been extensively developed for water supply, irrigation, and power generation. The net effect of the development has been to significantly modify the pre - development annual hydrograph through water storage in upstream reservoirs, and subsequent releases during the remainder of the year. Under pre - development conditions, peak flows from the snowmelt runoff were higher, and non- snowmelt runoff flows were lower than occurs under present conditions. Presently, peak flows are stored within reservoirs and are subsequently released during the remainder of the year. The Yampa River is the only large river in the Upper Colorado River basin in which flow patterns have not been substantially altered by water development projects (Tyus and Karp, 1989), and therefore, is probably the best river in which to evaluate the specific habitat requirements of the endangered fish species (Tyus, 1992). Closure of Flaming Gorge Dam in 1962 has not appreciably affected the mean annual water discharge of the Green River (Andrews, 1986), but has significantly reduced the magnitude of the flood peaks (e.g., the 100 -year peak flow has been reduced by about 21,000 cfs) (Harvey and Mussetter, 1994). The 90 percent exceedence flow for the pre- and post - Flaming Gorge periods are 1,500 and 2,300 cfs, respectively, which indicates that there has been a significant increase in the flow magnitude during the low -flow periods of the year. Because of the changes in the flow duration curve, the mean annual sediment load has decreased by about 54 percent at the Jensen gage and by about 48 percent at the Green River gage (Andrews, 1986). Andrews (1986) evaluated the effects of the dams on the stability of the Green River, and concluded that it was degradatonal upstream of the Yampa River, in approximate equilibrium from the Yampa River confluence to the Duchesne River, and aggradational in the reach downstream of the Duchesne River. He further concluded that, in the alluvial reaches downstream of Jensen, channel width between 1964 and 1978 decreased by about 13 percent (700 feet to 610 feet) and would have to decrease by a total of about 25 percent (700 feet to 524 feet) to reach equilibrium. Most bank width reduction was due to bank attachment of mid - channel bars, but the number of mid - channel bars through the reach increased significantly during the period. In contrast, Lyons (1991) concluded that an equilibrium channel width that was about 13 percent less than the pre -dam width had been re- established in the Jensen reach by 1974. Downstream of Green River, Utah, channel width had reduced by about 5 percent (474 feet to 454 feet) by 1981. In addition to those specifically mentioned above, many other studies have been performed during the last few decades to characterize the flow, sediment transport, and sediment storage dynamics of the Yampa and Green River systems. In preparing the summary of existing data for the peer review workshop, MEI also prepared a bibliography of studies that are potentially relevant to the proposed sediment monitoring program (Attachment 1). The two primary factors that have affected, and are likely to continue to affect, the habitat characteristics of the Yampa and Green Rivers are flow modification and sediment trapping by Flaming Gorge Dam on the Green River, and landuse changes and water development in the Yampa and Little Snake River basins. In addition to the effects of Flaming Gorge dam, the peer review panel identified several land -use and water development, activities that may have implications to the recovery of the endangered fish. These issues included: • Ongoing efforts by the Natural Resources Conservation Service (NRCS) and local 2