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Incremental Flow Modeling In spite of the inherently variable nature of lotic ecosystems, the need to describe continuous functions between flow and habitat is widely perceived, along with the assumption that aquatic biota in rivers are primarily limited by availability of physical habitat. Physical variables, such as temperature, velocities, size of gravel, cover, etc., obviously vary with flow. So models were developed in an attempt to describe change in these habitat variables in increments of flow. This vastly more complicated approach still implies that as habitat increases so will fish carrying capacity, and hence, fish populations. By far the most used (Reiser et al. 1989a) and most sophisticated incremental method is that developed by the U.S. Fish and Wildlife Service (Bovee 1982). This concept is called the Instream Flow Incremental Methodology (lFIM) and is a collection of computer programs and analytical procedures designed to predict changes in fish or invertebrate habitats in a "representative" stream reach due to flow changes. The IFIM has three major components. I) Transects across a "representative" reach are divided into cells (intervals) in which depth, velocity, cover value and often substratum roughness or quality are measured or simulated. These variables are assumed to be independent of one another. 2) The range of velocities, depths and cover or substratum used by the biota are determined by relating occurrence of various life history stages (e.g., YOY, juveniles, adults, spawners) of target species to the "hydraulic" variables. In other words, life stages of target biota are sampled or otherwise monitored (fish preferences are often detennined from animals fitted with radio transmitters) across the range of the hydraulic variables to derive "habitat suitability curves." Intuitively this is a logical approach, but it is often biased by sampling error, especially in large, deep and often turbid rivers where the biota are difficult to capture or see. 3) The net suitability of use of a given locality (transect cell) is quantified by a parameter called weighted usable area (WUA), which is a derived relationship between plan area of the transect cell (area available) and the habitat preference indices (from suitability curves) for velocity, depth and substratum. The WUA is calculated cell by cell and summed for the entire reach and over a range of discharges. Hence, increments of WUA for a stream become a continuous function of 48 " .