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Methods are described in the report for estimating the magnitude of a flood of any frequency between 1.1 and 50 years for any site, gaged or ungaged, on most unregulated streams in Wyoming, within the limits of basin size for which records have been collected. Flood-frequency relations are not defined for a large part of the headwaters of the North Platte River in south-central Wyoming because of the paucity of streamflow data. Relations of the mean annual flood as the de, pendent variable to certain basin characteristics as independent variables are defined. Wyoming is in the headwaters of four main river basins; the independent variables used in each basin are as follows:
Missouri River basin: only drainage area for part of the basin; drainage area and mean basin altitude for part of the basin.
Green River basin: drainage area and altitude at site; separate drainage area relation curve for main stem of Green River.
Bear River basin: drainage area and mean basin altitude.
Snake River basin: drainage area, mean annual precipitation, and a geographic factor.
Curves are presented showing the ratio of floods of selected frequencies to the mean annual flood used as an index. A separate ratio curve is shown for the main stem of Green River. The ratio to mean annual flood varies with mean basin altitude for a small part of Snake River basin along the west-central border of the State.
Data are tabulated for the maximum known flood at each gaging station for which records are used in the analysis.