The HABSP program computes a composite suitability for each microhabitat cell used in PHABSIM, by comparing the suitabilities of each cell at paired discharges. This allows the calculation of effective habitat for organisms, such as benthic macroinvertebrates, that cannot effectively move laterally to find suitable microhabitats under conditions of rapidly varied unsteady flow. The use of the model is demonstrated on the Williams Fork River, Colorado, using microhabitat criteria for a mayfly (Rhithrogena hageni) and a caddisfly (Cheumatopsyche sp.). Many evaluations of hydropeaking operations tend to focus on fish habitat, whereas the most important impact may be exerted on the food base. This may lead the investigator to adjust only the low release portion of the hydropeaking cycle. Use of effective habitat allows the investigator to evaluate the combined impact of both extremes on macroinvertebrate habitat.