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microhabitat and suitable macrohabitat conditions. This area is computed in the IFIM by conducting two separate analyses, superimposing the macrohabitat analysis on the microhabitat analysis, and computing the area that has suitable conditions in both categories. Before the evaluation of macro- and microhabitat conditions, it is neces- sary to evaluate the watershed conditions and relate these conditions to the macrohabitat characteristics of the stream. This step is needed for two reasons. First, all measurements are made at a point in time and assumed to be representative of the stream as the analysis is extended into the future. If a land use change or natural disturbance has recently changed the character- istics of the watershed, these changes are reflected in the stream macrohabi- tat at the time of measurement. Therefore, the measurements may not accurately reflect future conditions. Second, problems in the amount of total habitat available may be related to a land management factor, rather than water manage- ment. The most effective corrective action, and the benefits derived from such action, cannot be fully determined without identifying the source of the problem. Figure 1 shows the overall analytical strategy of the IFIM. Individual components in Figure 1 are discussed and expanded in the following section. Each application of the IFIM begins at the watershed level and proceeds through both a macrohabitat and microhabitat analysis. A proposed action may act on the watershed, indirectly affecting macrohabitat characteristics, or directly on one or more macrohabitat characteristics. These effects will be reflected in the total amount of habitat available. Based on this computation, the impact of the proposed action can be quantified, and a judgment made on the acceptability of the proposed action. If an action is determined to be un- acceptable, corrective measures may be suggested, model variables changed, and the analysis process repeated. 1.3 MAJOR HABITAT COMPONENTS AND ANALYSIS SEQUENCES 1.3.1 Step 1: Determine Watershed Influences on Macrohabitat Characteristics The first step in the habitat analysis sequence of the IFIM is the deter- mination of the present status of the watershed. Watershed and land use patterns largely control the yield of water, sediment, and chemicals to the river. Disturbances on the watershed may alter one or more of these delivery processes. An evaluation of watershed processes is necessary to distinguish those factors which can be modified by water management from those which can (or must) be modified by land management. The prevailing land use may limit the effectiveness of water management in controlling a particular problem. The investigator should evaluate the current status of the watershed to determine two conditions: the relative permanancy of existing water, sediment, and chemical yields as currently measurable in the stream; and the effective- ness of instream flows as a mitigation strategy. This step can be considered an early screening process to determine whether an instream flow study should be conducted immediately, deferred for a period of time, or not done at all. Undisturbed watersheds, or watersheds in which the disturbance is con- sidered permanent (such as agricultural lands), are usually amenable to 4