7758 - Laserfiche WebLink

Home Browse Search

INSTREAM FLOWS TO ASSIST THE RECOVERY OF ENDANGERED FISHES 21 windows (Fig. 14) determined to maximize areas of backwater habitats in the alluvial nursery ar- eas of the Green River during summer and fall 1992, peaking operations still caused considerable diel fluctuation of river stage (e.g., Figs. 12-14). I infer that backwaters thought to be protected by these flow windows were in fact flushed or, at least, significantly fluctuated repeatedly during late summer 1992. Data presented by Grabowski and Hiebert (1989) indicate that the food webs in the backwater environments of the Green River are not very productive. As noted above, these backwaters should contain rooted aquatic plants and a biodiverse, productive invertebrate and fish food web. I realize that the fluctuations shown in Figs. 12-14 are considerably reduced from opera- tions in the past. Nonetheless, development of stable, productive food webs in the backwaters probably has not occurred as a consequence of reregulation of the Flaming Gorge releases. More- over, these backwaters probably will never be very productive unless flow fluctuations can be elimi- nated. Empirical information with which to firmly judge the productivity of backwater food webs as influenced by regulated baseflow regimes throughout the Upper Colorado River Basin is sorely needed and should be approached in the dynamic time and space context described above. Peaking operations at Flaming Gorge are at- tenuated in relation to distance downstream from the dam. Therefore, baseflow instability (Figs. 12 and 13) progressively worsens upstream from Jen- sen and may be severe in the Echo and Brown Park reaches. Elsewhere between Jensen and the dam, the river is constrained in canyons, and the problem may be somewhat ameliorated by geomorphology. However, peaking flows are known to interrupt insect emergences that feed the trout fishery in Red Canyon immediately downstream from the dam (my observation and Larry Crist, personal commu- nication). Similar effects were observed on the Mis- souri River below Holter Dam in Montana, and an outcry from fly fishermen caused load control op- erations to be shifted to another dam. The effect was a translocation of stream regulation effects from one river to another, thereby confounding management objectives (Stanford and Hauer 1992). This illustrates the potential difficulty of changing dam operations to meet the needs of en- dangered fishes in potamon reaches of the Upper Colorado River Basin, if rhithron trout fisheries might be influenced in the process. Stream Regulation Mediates Invasions of Nonnative Predators and Complicates Provision of Instream Flows to Protect Endangered Fishec Introduction of trout and other nonnative fish in regulated streams is an enormously confound- ing problem in the interpretation of the ecology of regulated streams because the native species vir- tually always seem to decline in the presence of exotics, especially if the river is regulated. This pervasive ecological problem has been reviewed thoroughly (e.g., Mooney and Drake 1986). Preda- tion of natives, including endangered fishes, by exotics does occur in the Upper Colorado River Basin, and red shiner, fathead minnow (Pi= mephales promelas), walleye (Stizostedion vitreum), northern pike (Esox lucius Linnaeus), channel catfish, largemouth bass (Micropterus salmoides), smallmouth bass (Micropterus dolomieui), and green sunfish are especially prob- lematic invaders (cf., Karp and Tyus 1990; Tyus 1991b; Tyus and Haines 1991). However, Meffe (1984) and Minckley and Meffe (1987) showed that intense flooding in rivers in the southwestern United States was positively correlated with di- versity and abundance of native fishes and nega- tively correlated with diversity and abundance of nonnative fishes. The strong inference is that non- natives are maladapted to survive intense and frequent (annual, at least) flooding compared with natives. Having fewer predators increases re- cruitment of natives and over time allows the natives to persist in greater abundance than non- natives (Fig. 15). The work of Meffe and Minckley included the Virgin River and other tributaries of the Colorado River but none in the upper basin. Thus, while the data are not directly applicable, the relationship probably holds. Hawkins and Nesler (1991) correlated lower ratios of nonna- tives to natives with high peak flows in the Yampa River, and red shiner populations declined after years of high spring flows in the Colorado River (Osmundson and Kaeding 1991). The prediction that flooding will limit preda- tion mortality of endangered fishes is used as one rationale in the recovery program for reinstate- ment of peak flows. However, introduced species, red shiner for example, are native in rivers that experience floods (of bankfull or greater) rather frequently, which suggests that flow augmenta- tion might not work very well in controlling some nonnative species. However, the complex interac- tions described above that are associated with