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mitigating effects of tributary flows and ambient warming of the river with in- creased distance from Flaming Gorge Dam. Flows that were associated with various life history needs of endangered fishes in the Green River basin were evaluated by FWS and used in part as a basis for a Draft Biological Opinion on the operations of Flaming Gorge Dam pursuant to Section 7 of the Endangered Species Act and for rec- ommending flow needs for fishes in the Yampa River (unpublished FWS report on file in Denver, Colorado). However, only in stream flows that are the minimum amount needed can be protected under Colorado water law (Col. Rev. Stat. S 37- 92-102[3]), and in other states as well. Al- though general flow needs have thus been established, it is difficult to determine min- imum flows because the system is no lon- ger natural; flow, temperature, and other physical features are changing in response to water management. In addition, endan- gered fishes are exposed to a myriad of introduced forms, and these introductions have resulted in a new, more complex fish community, whose structure and attributes are poorly understood (Minckley 1982). Habitat requirements of the endangered Colorado River fishes have been deter- mined in areas that are occupied by the many introduced fishes, and their habitat use has presumably been affected by them. For example, predaceous northern pike (Esox Lucius) and channel catfish (Ictalurus punctatus) occupy habitats of adult Colo- rado squaw fish (Wick and Hawkins 1989; Tyus and Nikirk 1990). Habitats used by young Colorado squaw fish are dominated by introduced fishes (Haines and Tyus 1990) and aggressive behavior of some of these toward Colorado squawfish (Karp and Tyus 1990b) suggests adverse interactions for food and space. Juvenile Colorado squaw fish have been consumed by intro- duced fishes (Coon 1965; Hendrickson and Brooks 1987; Osmundson 1987). Popula- tions of humpback chub may be adversely affected by channel catfish in the Green River basin (Karp and Tyus 1990a). The apparent lack of recruitment in razorback sucker has been related to predation by common carp (Cyprinus carpio) and other nonnative fishes (Marsh and Langhorst 1988; Marsh and Minckley 1989). It is common practice to determine flows I~ 32 that would provide habitat for a target fish species. However, the Colorado River sys- tem has been permanently modified by the introductions of so many nonnative fishes (Minckley 1982) that recovery efforts for the endangered fish must include evalua- tions of flow needs of all inhabitants. In this context, instream flow needs must not be considered in a vacuum, but should be incorporated in a comprehensive program that involves evaluating the limiting fac- tors, propagation and genetics manage- ment options, and field testing of alter- native management practices, including different flows. Because project compliance with agreed-upon instream flows is often lax, it is important to monitor specified flows to ensure that they are provided (Hubert et al. 1990) and to evaluate effects of such flows on fish populations (Deacon 1988). Another issue associated with flow rec- ommendations involves the importance of seasonally flooded lands to the survival of the Colorado River fishes. The role of bot- tomlands for fish production has been doc- umented in warm water, floodplain rivers worldwide (Welcomme 1979). Many such areas in the upper Green River have been lost by river regulation (reductions in the magnitude and duration of high spring flows) and by impoundment (removal of fish access) for agriculture, waterfowl pro- duction, and other purposes. Large bot- tomlands along the upper Green River are examples of these, and they are under eval- uation for their role in endangered fish management. These lands should be fur- ther studied for the relationship between fish use and various types of spring flood- ing events. It is not known whether the Colorado River fishes can be recovered in the re- maining habitat. To accomplish this re- quires adoption of a recovery philosophy that relies on the interpretation of empir- ical data and ecological perspectives. As demonstrated by others (e.g., Murphy et al. 1990), the persistence of threatened an- imal populations depends on genetic, de- mographic, environmental, and other fac- tors, some of which may act synergistically to affect population viability. Although it may not be possible to construct and val- idate population models for the complex Colorado River system, water management Rivers . Volume 3, Number 1 January 1992