Laserfiche WebLink
<br />DISCUSSION <br /> <br />The previous Yampa River interim flow recommendations proposed by the <br />U.S. Fish and Wildlife Service (USFWS 1990) were based largely on relationships of <br />the hydrograph to reproductive behavior and nursery habitat needs of endangered <br />fishes (Tyus and Karp 1989), This approach is appropriate because insufficient <br />recruitment is the major factor in the decline of the "large river" endangered fishes in <br />the Upper Colorado River Basin (e.g Behnke and Benson 1980, Minckley et at. 1991). <br />The natural hydrograph is necessary to cue spawning migrations (Tyus and Karp <br />1989), provide habitat conditions necessary for spawning and survival of early life <br />stages (Tyus 1986, Modde et al. accepted for publication), and the construction of <br />habitat features (Le. river channel geomorphology) needed to maintain quality <br />spawning, nursery and adult habitats (O'Brien 1984, Mussetter and Harvey 1994). <br />Despite the demonstrated relationship of natural flow patterns to the biology of the <br />"large river" endangered fishes of the Yampa River, little information exists.on the <br />habitat available to endangered fishes among flow scenarios. Given this situation, we <br />assume that the conditions favorable for the recovery and maintenance of the <br />endangered fishes of the Yampa River, and affected areas of the Green River, are <br />best estimated using virgin flow conditions (Tyus 1994) when fish were abundant <br />(Quartarone 1993). <br /> <br />The comparison of Yampa River historical and virgin hydrographs indicated <br />present peak flows tend to be lower and baseflows higher than virgin flows. These <br />characteristics were evident in both monthly average flows and both high and low (3 d <br />through 90 d) duration records. Deviations in high and low flows dampen natural <br />variability. Several studies have reported that moderation of flow patterns in streams <br />and rivers resulted in replacement of native fishes with introduced exotic fish species <br />(e.g. Maheshwari et al. 1995, Deacon 1988, Moyle 1986, in Baltz an Moyle 1993). <br />The fish fauna of the Upper Colorado River Basin are largely endemic (Tyus et al. <br />1982a) and evolved in a very harsh climate and hydrology (Miller 1961). Although <br />tolerant to_ high levels of environmental variability, the largely endemic fish fauna of the <br />Colorado River Basin have not been effective competitors with introduced nonnative <br />fishes (Minckley and Douglas 1991, Johnson et al. 1993). Among the states of <br />Colorado, Utah, and New Mexico (data from Arizona not provided), 53% of the listed <br />species were considered to be largely the result of nonnative fish interactions while <br />71 % where considered largely the result of habitat alterations (Horak 1995). <br /> <br />Despite the encroachment of nonnative fishes in altered western rivers and <br />streams, native species have dominated in those systems able to maintain a relatively <br />natural hydrograph. Hawkins and Nesler (1991) described several instances in which <br />high spring peaks have resulted in the decline of nonnative fishes in the Upper <br />Colorado River Basin: Native fish distribution in the Yampa River is consistent with <br />observations of Hawkins and Nesler (1991) in that native fishes continue to dominate <br />in the areas with the greatest environmental variation. In the Lower Colorado River <br />Basin flow variability was observed to be the dominant factor in maintaining native fish <br />communities in several streams and rivers (Deacon 1988, Minckley and Meffe 1987). <br /> <br />25 <br />