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(Binns and Eiserman 1979). A set of these variables consistently was shown to contribute significantly to the variation in fish population and production. These variables were water velocity, minimal water depths, instream objects such as cover, bot- tom substrate materials (with particular emphasis on the amount of fines in the interstitial spaces within coarse bed elements), water temperature, dissolved oxygen, total alkalinity, turbidity, and light penetration through the water column (Gosse and Helm 1981; Shirvell and Dungey 1983). These variables were integrated into methodologies for analyzing the consequences of proposed water withdrawal or storage-release activities and were applied to many federal water projects operated by the Bureau of Reclamation, Army Corps of Engi- neers, and Tennessee Valley Authority (Nestler et al. 1989). During the late 1970's and early 1980's, an era of small hydropower development began. Hundreds of proposed hydropower sites in the Pacific North- west and New England regions ofthe United States came under intensive examination by state and federal fishery management interests. During this transition period from evaluating large federal res- ervoirs to evaluating license applications for small hydropower, the Instream Flow Incremental Meth- odology (IFIM) was developed under the guidance of the U.S. Fish and Wildlife Service (Trihey and Stalnaker 1985). This methodology attempted to integrate the planning concepts of water supply, analytical models from hydraulic and water quality engineering, and empirically derived habitat ver- sus flow functions. This methodology produced simulations of the quantity and quality of 'poten- tial habitat' resulting from proposed water develop- ment, illustrated through a series of alternative flow regimes. Such efforts involving incremental methods and analyses of alternatives through time were further enhanced during the next 10 years, driven by several hundred relicensing applications submitted to the Federal Energy Regulatory Com- mission. Most of these applications involved reser- voirs that had been in place for 30-50 years without any downstream instream flow considerations fac- tored into their operating procedures. Opportunities were seen by the natural resource agencies to restore riverine aquatic resources that had been impacted (occasionally eliminated) for many decades. Conversely, many hydropower com- panies wanted to shift their operating protocol to- ward hydropeaking and pump-storage to enhance the revenues at existing installations. Many of THE INSTREAM FLOW INCREMENTAL METHOLDOLOGY 3 these peaking projects operated by private power companies or public utilities were readily accessible for recreational use. The recreational interests seized on the relicensing opportunity as a means to enhance river recreational use by canoes, kayaks, and rafts. NEPA guidelines for examining alterna- tives and hydropower relicensing forced United States decision makers to balance potential con- flicts among users of the riverine resources. Incre- mental methods became the tools of choice for quan- titatively describing the consequences of alternative ways of managing flowing waters, set- ting the stage for negotiation among various inter- est groups and better informing the decision mak- ers in their role in conflict resolution (Stalnaker 1993). Water Budgets Establish Fisheries as a Legitimate Management Purpose AI; the multiple-use ethic emerged over the last two decades, it became clear that simply allocating part of the water supply to various uses is not sufficient to resolve conflicts. The same water can be used many times if it is managed so that the timing of release serves instream purposes while still being delivered to downstream consumptive users. This multiple-use management philosophy, exemplified by the Pacific Northwest Electric Power Planning and Conservation Act of 1980 and subsequent efforts by the Bonneville Power Authority, allowed federal, state, and tribal fishery biologists to identify management prescriptions for restoration and enhancement of the anadromous salmon runs in the Columbia River basin. A mini- mum flow did not provide sufficient 'protection for stream resources during drought cycles, nor did it provide the opportunity for optimal fish production during wet years. Water budgets allocating a por- tion of water stored in upstream reservoirs for fishery benefits reserve flows that could be released when they were most needed (Waddle 1991). When downstream water users are not calling for delivery through critical spawning or rearing reaches, the 'fish water' can be released to relieve any habitat- induced bottlenecks. A recent case study by the National Research Council (1992) recognized the value of reservoir release management in alleviating conflict and en- hancing multiple uses (including instream) within arid western United States river basins. The