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ABSTRACT <br />Weighted criteria are used to assess the impacts of altered stream- <br />flow regimes on a stream habitat. They are developed primarily for <br />those habitat parameters most closely related to stream hydraulics: <br />depth, velocity, substrate, and temperature. <br />Species for which criteria are developed may be classified into <br />five groups: economic objective species, indicator species, endangered <br />or threatened species, nongame species, and forage species. For each <br />species, criteria are developed for spawning, adult, juvenile, fry, and <br />egg incubation. Passage criteria may also be developed for certain <br />species. <br />Probability-of-use curves are based on the assumption that indi- <br />viduals of a species will tend to select arezs within the stream having <br />the most favorable combinations of hydraulic conditions. It is furthF~r <br />assumed that they will also utilize less favorable conditions, with the <br />probability-of-use decreasing with diminishing favorability of one or <br />several hydraulic conditions. Finally, it is assumed that individuals <br />will elect to leave an area before conditions become lethal. <br />Weighted criteria are presented in the form of probability-of- <br />use curves, the peak of which represents the optimum condition for a <br />given hydraulic parameter. These curves are developed through four data <br />analysis techniques, either singly or in concert: (1) frequency <br />iii <br />