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1.2 METHODOLOGY DESIGN An initial hurdle in the development of this methodology was how to describe habitat and the various factors influencing it. This problem was rooted in the diverse approaches to describing riverine ecosystems. One perspective is to examine a river from its headwaters to its mouth. Numerous authors have reported the addition or replacement of species as a function of stream order, stream size, gradient, or other descriptions of longitudinal gradations of environmental conditions (Shelford 1911; Burton and Odum 1945; Huet 1959; Sheldon 1968). This type of study considers the "longitudinal succession" of species as a function of variables such as mean depth, mean velocity, temperature, water quality,, or other characteristics exhibiting gradational change. This perspective might logically be defined as a macro- habitat approach to riverine ecology. A second approach is to hold the macrohabitat as a constant and examine resource partitioning by different species at a microhabitat level. Dettman (1977), and Alley and Li (1978) found that competition between rainbow trout and Sacramento squawfish was reduced by habitat isolation and different feeding habits. Everest and Chapman (1972) showed that young of the year and juvenile steelhead and chinook salmon utilize virtually identical microhabitats and food items. However, because the spawning cycle for the two species is approx- imately 6 months out of phase, there is little competition between like age groups of the two species. Thus, ecological segregation can occur on either a large and small scale and both spatially and temporally. The IFIM has been designed to incorporate both macro- and microhabitat concepts. Certain macrohabitat characteristics, such as temperature and water quality, define limits of suitability for different species. The net result of changes in these characteristics is a change in the longitudinal distribu- tion of species. These macrohabitat conditions determine the length of stream that could potentially be inhabited by a species. Other macrohabitat charac- teristics, such as geology, elevation, slope, and water supply, create longi- tudinal changes in the shape, pattern, and dimensions of the river channel. These, in turn, are major determinants of the types of microhabitats which occur at any location on the stream. Thus, the types and spatial distribution of microhabitats also grade longitudinally in response to geomorphic character- istics and processes. Fish and invertebrates do not respond directly to physical macrohabitat characteristics; instead, they respond to the microhabi- tat conditions associated with the macrohabitat. Because it is not feasible to measure all the microhabitat for the entire length of the river, it is necessary to measure the microhabitat in locations that reflect the longitudi- nal change in physical macrohabitat. A sampling strategy has been developed to aid the investigator in the selection of these microhabitat measurement sites. This strategy is discussed in Chapter 3. Two different functions are developed in the course of this analysis: a macrohabitat suitability function; and a microhabitat availability function. The relationship between temperature or water quality and discharge is gener- ally a simple linear function; the more water in the channel, the better the water quality and the more kilometers of suitable macrohabitat. The relation- ship between microhabitat and discharge is usually (but not always) nonlinear. The total available habitat occurs within the area of overlap between available 3