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been to prevent extirpation due to water resources development, pursuant to pro- visions of Section 7 of the Endangered Spe- cies Act. This level of protection may be interpreted as maintaining population sur- vival (or viability), and it is based on the premise that the numbers of individuals in a population are sufficient to maintain ge- netic diversity and that the environment will be suitable. However, population size is not known for Colorado River fishes and their habitat requirements are not well un- derstood. The determination of survival flows needed by these fishes has been based on recent habitat use even though present habitats are presumably suboptimal. Pro- vision of existing conditions for these im- periled species is intuitively incorrect and ostensibly may further their decline. As suggested by Prewitt and Stalnaker (1982), careful consideration and a struc- tured approach are required to determine flow requirements of endangered Colora- do River fishes. Because of river-system complexity, habitat changes, and fish com- munity alteration, a careful and structured approach has been difficult. As an example, Miller et al. (1982) provided flow recom- mendations for survival of Colorado squaw fish stocks and noted that flows ex- ceeding survival flow levels were neces- sary to maintain fish habitats. Determina- tion of such flows was constrained by the limited availability of habitat use infor- mation. More recently, Tyus and Karp (1989) used empirical data and profession- al judgment to recommend flow regimes needed for maintaining habitats of the en- dangered fishes and noted that other man- agement practices would be needed to ar- rest population declines. To date, no study has identified the specific measures that will recover local stocks of these fishes to a nonendangered status. Instream flows have been determined by standard setting or incremental methods (Wesche and Rechard 1980; Trihey and Stalnaker 1985). Standard setting meth- odologies usually involve the determina- tion of one specific flow, such as a low flow that would occur at some exceedence value based on the historic flow record. Incre- mental methods include some modeling techniques that are useful in evaluating tradeoffs between developmental alterna- tives. Some of the most valuable assess- ment tools, the Instream Flow Incremental Methodology (IFIM) and Physical Habitat Simulation (PHABSIM) system (Bovee 1982; Gore and Nestler 1988), have had wide application. The IFIM was developed as a water management tool; PHABSIM is useful in relating flow changes to the avail- ability of certain habitats. In PHABSIM, the relationship between fish habitat use is usually determined by generating "weighted usable area" (Bovee 1982) to represent certain physical param- eters (usually water depth and velocity, and stream substrate) used by the fish. In the application of the method, alternative flow scenarios that provide a larger amount of such "physical habitats" are judged more acceptable than those that do not. PHAB- SIM does not determine whether or not a valid relationship exists between physical habitats and standing crops of fishes, and its utility has been widely debated (Condor and Annear 1987). Some workers have established relation- ships between fish abundance (i.e., usually 1 species and life stage) and physical hab- itats in small, coldwater streams with 1 to 10 fish species (Nestler 1990). Little rela- tionship has been found in large warm- water streams and other such complex bi- ological systems that may have 30 or more fish species (Orth and Maughan 1982; Ma- thur et al. 1985; Baltz et al. 1987; Layher and Maughan 1987). It has been difficult to establish a positive linear relationship between standing crops of warm water fishes and physical parameters that are of- ten used in habitat simulation models (e.g., water depth and velocity and stream sub- strate type) (Mathur et al. 1985; Nestler 1990). The relationships between flow, habitat, and fish production are not well understood (Reiser et al. 1989). Instream flow requirements for endan- gered Colorado River fishes can only be determined by the integration of life his- tory information with instream flow needs. Life history information can be used to identify habitat use by various life stages, and flow requirements determined by us- ing different techniques in such diverse habitats as main river channels, backwa- ters (little or no water flow), and seasonally flooded bottomlands. Recent attempts to determine flows for certain life history stages of the endangered Colorado River I~ 30 January 1992 Rivers · Volume 3, Number 1 'I