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Base flows also are important for maintaining populations of the endangered fishes. Generally, rime habitats are shallow, well oxygenated, free of fines, and highly productive in macroinvertebrate biomass. Because they are shallow, rimes also are sensitive to changes in stream flow; Modde et al. (1999) found that available rime habitat declined rapidly below 93 cfs. During lower flows, larger fishes may abandon ephemeral shallows and retreat into deeper, more persistent, though less productive, pools. Colorado pikeminnow are less mobile at this time (Modde et al. 1999); therefore, their ability to pass through shallow rimes is not critical to their survival. However, higher summer temperatures and overcrowding combined with a reductions in their food supply could affect their viability. Adequate base flows also are needed to freshen pools and moderate temperatures. The Little Snake River is the largest tributary to the Yampa both in terms of watershed area and volume of discharge. The Little Snake River subbasin, including about 1,331 square miles (35%) in Wyoming, encompasses roughly half of the total watershed area upstream from the Deerlodge Park gage on the Yampa River. However, its average annual discharge at Lily Park is only 27% (428,000 AF) of the average annual discharge ofthe Yampa River at Deerlodge Park. The Lily Park gage is located on the Little Snake River 9 RM upstream from its confluence with the Yampa River, or 14 RM upstream from the Deerlodge Park gage. Annual flow maxima at Lily Park have ranged from a low of 865 cfs in 1934 to as much as 13,400 cfs in 1984, with a 76-year average peak of 4,607 cfs (1922-1997). During this period, flow maxima at Lily Park ranged from 20% to 57% of their respective peak flows at Deerlodge Park 1 with an average of32%. The Little Snake River also contributes a significant quantity of sediment, considered important for building and maintaining spawning bars and nursery habitats for the endangered fishes in Yampa Canyon and the middle Green River (Andrews 1978, 1986; Elliottetal. 1984; Muthetal. 2000; O'Brien 1987; USDI 1998). ( ( / ( / ( ( ( ( Moreover, the Middle Green River, which extends from the confluence of the Yampa River downstream to the confluence of the White River near Ouray, Utah, is ecologically inseparable from the Lower Yampa in that the Green River not only benefits from the flows of the Yampa River, but also benefits Yampa River populations of the endangered fishes by providing them important nursery habitats from which fish are recruited as sub-adults into the Yampa River. Flaming Gorge Dam, operated by the U.S. Bureau of Reclamation (USBR) primarily for irrigation water supply and power generation since 1964, diminishes spring peak flows, while increasing base flows during the remainder of the year. However, this plan is not intended to, nor will it, mitigate the impacts of the dam on endangered fishes. Nevertheless, the Green River downstream from the Yampa River is the product"Ofboth rivers (Figure 4). Therefore, for, the Middle Green River, this plan recognizes the impacts of the dam, as well as the opporturuties it affords for flow management in the Green River, both above and below the Yampa River. Water can be released from Flaming Gorge to reinforce or extend the peak flow of the Green River below the Yampa confluence. By increasing base flows, the dam can partially offset impacts to the Green River due to depletions from the Yampa. However, the dam cannot mitigate depletion impacts to the Yampa River itself. Moreover, re-operation of Flaming Gorge Dam to mitigate its impacts on the endangered fishes in the Green River and support their recovery will be addressed in a separate biological opiruon and, therefore, its re-operation is not part of this plan. 1 Comparison of annual flow maxima. Prior to 1982, daily flows at Deerlodge Park were synthesized as the sum of gaged flows at Maybell and Lily Park. Flow maxima at Lily Park did not necessarily occur on the same day as flow maxima at Deerlodge Park. Management Plan for Endangered Fishes in the Yampa River Basin 8