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The value-based process (Jackson et al., 1989) for protecting instream flows has been used successfully by the U.S. Bureau of Land Management for more than 10 years. Over the last decade, the methodology has evolved significantly as it has been applied to increasingly complex river systems. In the original process, which was somewhat linear, hydrologic characterization was an early step and development of a flow-protection strategy was a late step, and the focus was primarily on resource values and their flow dependency. The procedure has evolved into a trichotomous process in which legal/institutional analyses and hydrologic characterization occur concurrently with and frequently overshadow the assessment of flow-dependent resources. For complex river systems, the legal/institutional analysis usually includes historical analysis of water management in the basin, a geographical description of projects and facilities controlling streamflows, a review of operating principles and other institutional constraints, development of potential flow scenarios, and identification of other management opportunities. Similarly, hydrologic characterization may include detailed ground-water and geomorphological investigations to support assessment of flow-dependent resources, and extensive historical analysis to support the legal/institutional framework. Thus, whereas the resource assessment identifies flows needed to support resource values, the legal/institutional analysis and hydrologic characterization identify both the physical and institutional limits that will influence management decisions.
Instream flow methods have been developed predominantly by biologists and hydrologists working for agencies having regulatory responsibility related to water development and management (Stalnaker 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 progression we also saw the application of a water budget which set the stage for having the fisheries manager 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 toward what the future might hold.