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/ L I ,~ ~[ '-1 IQgs C~t0Y; vtud. 01:,60 =t- _I . Fisheries Habitat Dynamics in the Upper Colorado River John G. Carter Richard A. Valdez Ronald J. Ryel and Vincent A. Lamarra Ecosystems Research Institute 975 South State Highway Logan, Utah 84321 ABSTRACT . An investigation in the upper Colorado River near Parachute, Colorado revealed temporal variations in the amount of important fish habitats. Quanti tative measures of habitat area were rel ated to flow. The relationships developed between flow and surface area of specific habitats revealed the existence of critical thresholds. The impl ications of these results could be important in the management of the Colorado River system for wildlife and human consumptive and non-consumptive uses. INTRODUCTION The Colorado River system drains one-fifteenth of the area of the United States. Most of this basin is arid to semi-arid, and the environment is harsh with rivers of drastically fluctuating flows, temperatures and turbidities. Many of the fishes of the Colorado River have developed unique adaptations to these conditions. These conditions and a long isolation from other major~iver . systems have resulted in 73% endemism, the highest percentage of a-ny----rIver basin in the United States (Mill er 1961). Three of these 1 arge ri ver fi shes are 1 i sted as endangered under the Endangered Species Act of 1973. These are the Colorado squawfish (Ptychocheilus lucius), humpback chub (Gila cypha), and bony tail (Gila elegans). The razorback sucker (Xyrauchen texanusT is also listed asillCandidate species. All four of these species are protected by the State of Colorado as endangered. Nine other natj1e species of fishes occur in the upper Colorado River along with a number of exotics (Tyus et al. 1982). ~ Presently, there is a strong confl ict between water use and the needs of the endangered species within the river system. The habitat requirements of these fishes vary by life stage and time of year (Holden 1977; Seethaler 1978; Behnke and Benson 1983; Ulmer 1981; Kaeding and Zimmerman 1982; Miller et al. 1982a; Tyus et al. 1982; Valdez et al. 1982; Valdez and Clemmer 1982). Three factors are generally recogni zed as important in the decl ine of the endangered species within the Colorado River. These are (1) dams and reservoirs, (2) water withdrawals, and (3) the introduction of non-native fishes into the system. Resolution of the confl ict between endangered fishes and development may be difficult because of the many diverse interests, including federal and state resource agencies, municipalities, private industry and environmental concerns. There also remains large uncertainty as to the relative contribution of each development or water use to the problem, and the adequacy of the existing data base to address these problems is uncertain. The ongoing development of a data base through continued monitoring of fish populations may in time help define important cause-and- effort relationships, and help identify population trends. , - . 249 Journal of Freshwater Ecology. Volume 3, Number 2 - December, 1985 Copyright @ 1985 by Oikos Publishers. Inc.