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2 BIOLOGICAL REpORT 29 Chapter 1. The History of Instream Flow Problems and IFIM Instream flow methods have been developed pre- dominantly by biologists and hydrologists working for agencies having regulatory responsibility re- lated to water development and management (Stal- naker and Arnette 1976). Such efforts over the last 30 years have provided the impetus for detailed ecological studies leading to a significant growth in the understanding of the relations between stream flow and aquatic habitats. Most of the empirical evidence gathered to date has focused on fish and benthic macro-invertebrate habitat requirements, with recent emphasis on the relation between stream flow and woody riparian vegetation and river-based recreation (Gore 1987; Orth 1987; Brown 1992; Shelby et al. 1992; Scott et al. 1993). Water management problem solving has matured from setting fixed minimum flows with no specific aquatic habitat benefit to incremental methods in which aquatic habitats are quantified as a function of stream discharge. Within this historical progres- sion we also saw the application of a water budget which set the stage for having the fisheries man- ager be an integral part of an interdisciplinary decision-making system. This chapter will review the progression of circumstances and techniques leading to the development of IFIM and point to- ward what the future might hold. Minimum Flow Standards Provide Minimal Protection Following the large reservoir and water devel- opment era of the mid-twentieth century in North America, resource agencies became concerned over the loss of many miles of riverine fish and wildlife resources in the arid western United States. Con- sequently, several western states began issuing rules for protecting existing stream resources from future depletions caused by accelerated water de- velopment. Many assessment methods appeared during the 1960's and early 1970's. These tech- niques were based on hydrologic analysis of the water supply and hydraulic considerations of critical stream channel segments, coupled with empirical observations of habitat quality and an understanding of riverine fish ecology, most nota- bly the Pacific salmon and freshwater trouts. Col- lectively, the efforts led to a general class of in- stream flow assessment techniques (models) meant to help reserve a specific amount of water within the channel for the benefit offish and other aquatic life (Wesche and Rechard 1980; Morhardt 1986; Stalnaker 1993). Application of these meth- ods usually resulted in a single threshold or'mini- mum' flow value for a specified stream reach below which water may not be withdrawn for consump- tive water use. The minimum flow is almost al- ways less than the optimal or pristine habitat condition, yet these 'reservations' of water form the current basis for issuing water permits in many states. See MacDonnell et al. (1989) and Lamb and Lord (1992) for recent discussions ofthe status of state recognition and protection of flowing water for instream flow. Impact Analyses Lead to Increased Resource Protection Following enactment of the National Environ- mental Policy Act (NEPA) of 1970, attention was shifted from minimum flows to the evaluation of alternative designs and operations of federally funded water projects. Methods capable of quanti- fying the effect of incremental changes in stream- flow to evaluate a series of possible alternative development schemes were needed (Stalnaker 1993). This need led to the development of habitat versus discharge functions developed from life- stage-specific relations for selected species, that is, fish passage, spawning, and rearing habitat versus flow for trout or salmon. Corroborating research took the form of analyses correlating the general well-being of fish populations (usually in terms of measured standing crop) with various physical and chemical attributes of the stream flow regime and its interaction with the stream channel structure