Abstract There are no widely accepted methods to define minimum instream flow needs for trout in ice- and snow-covered streams during winter, as there is during ice-free periods of the year (Annear and Conder 1984). Accumulation of ice on stream channels changes hydraulics and prevents sampling in standard ways. Therefore, methods for determination of instream flow needs during ice-free periods are often not applicable to many streams during winter.
There is a history of using hydrologic criteria to define minimum instream flow needs during ice-free periods. In Wyoming, Wesche (1973) recommended 25% of average daily flow during the summer (July-September) to support good trout populations. Burton and Wesche (1974) found that streams where this criterion for instream flow was met or exceeded 50% of the time between July and September 30 had good trout fisheries. The most widely applied technique using hydrologic criteria is the Tennant method (Tennant 1976). Tennant recommended use of a percentage of the historic mean annual flow to estimate minimum instream flow requirements.
The purpose of this project was to assess the use of hydrologic data in the establishment of winter instream flow standards for trout streams in Wyoming. We utilized analysis of flow duration curves in our assessment. Flow duration curves identify the percent of time that specified discharges (defined as a percentage of average daily flow) are equaled or exceeded. Our objectives were to: (1) describe physical conditions and trout habitat at various locations in watersheds during winter; (2) determine the extent to which spatial and temporal variation in stream flow affects physical conditions and trout habitat in streams during fall and winter; (3) describe stream flows from fall through winter in streams with and without irrigation and water development in the watershed; and (4) determine if criteria based on flow duration curves can provide standards for minimum instream flow determina-tions to maintain trout fisheries during winter.
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