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source engineering profession. Looking at stream- flow through time (by constructing a hydrologic time series) allows one to compare the frequency and duration of wet and dry periods, to examine the difference between snow-melt and rain-driven sys- tems, and to determine the intensity and duration of short-term events such as cloud bursts and peak- ing cycles. To influence operating decisions within large-scale water development settings, a tool was needed that underscored conflicts and complemen- tary water uses, considered and evaluated each user's needs, and was understandable, acceptable, and easy to use by a broad clientele. Such decision arenas involve a diversity of disciplines, including engineers, hydrologists, biologists, recreation plan- ners, lawyers, and political scientists. The U.S. Fish and Wildlife Service Directorate requested direct input from other agencies in the development of this special methodology. Water resource professionals were assigned to work on this cooperative effort for periods up to 4 years. Engineering, water quality modeling, and plan- ning expertise came from the U.S. Bureau of Rec- lamation, Soil Conservation Service, Army Corps of Engineers, Environmental Protection Agency, and university scientists. Expertise in aquatic ecology, fishery biology, water law, institutional arrangements, and planning came from state agencies. The Intergovernmental Personnel Act provided the vehicle for these assignments from state organizations. This interagency effort led to the conclusion that an analytical methodology should handle a variety of instream flow problems, from simple diversions from the stream channel to complex storage and Overview of Incremental Methodology Yes Institutional analysis model Ol c: '5. o o C/) Iii o '2 .s:::. o (]) I- THE INSTREAM FLOW INCREMENTAL METHOLDOLOGY 5 release schemes involving hydropeaking schedules, pump-storage, and a network ofinterconnected res- ervoirs. For such a methodology to be suitable for evaluating alternatives, it had to be useful in iden- tifying, evaluating, and comparing potential solu- tions, be capable of being tailored to a specific stream reach, and be expandable such that reach information could be applied throughout a river basin. With this general charter, and building on historical planning practices using stream reach hydrology, IFIM has developed over a period of 15 years into a river network analysis that incorpo- rates fish habitat, recreational opportunity, and woody vegetation response to alternative water management schemes (Bartholow and Waddle 1986; Milhous et al. 1989; Auble et al. 1991). Infor- mation is presented as a time series of flow and habitat at selected points within a river system (Milhous et al. 1990). Figure 1.1 illustrates the general information flow within IFIM; we will dis- cuss the various components of IFIM throughout the remainder of this booklet. Optimizing for anyone use is contrary to the general philosophy of multiple use; efficiency of use is defined as the greatest return in the number and quantity of uses, with emphasis on simultaneous use. It is imprudent to use the simple, intermediate output (for example, flow/habitat or flow/recreation functions) to argue for a minimum release or flow standard chosen from the maximum value on a flow versus habitat graph. The timing of events across seasons is critical to the reproductive success and relative strength of year classes within fish popu- lations. The temporal distribution of strong versus weak year classes shows the well-being of the fish No Fig. 1.1. Overview of the Instream Flow Incremental Methodology.