"The Cooperative Instream Flow Service Group has developed the Instream Flow Incremental Methodology, largely in response to requests from the water planning community for a set of procedures capable of quantifying the effects of increments of change in streamflow. A computerized system or family of models (PHABSIM) is the working component of the methodology."--Leaf [1].

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.