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4.0 HYDRAULIC ANALYSIS <br />4.1 Base Mapping <br />Base mapping selected for the delineation of the 50 -year and 100 -year floodplains on the <br />Gunnison River consisted of 7.5 minute U.S. G. S. topographic quadrangle maps. A total of 11 <br />quadrangle maps were used in compiling the base maps for the Gunnison River study reach. The <br />contour interval for the maps was either 20 or 40 feet (see Figure 8). Due to the small scale of <br />the base maps (I "= 2000'), the 50 -year and 100 -year floodplain boundaries could not be shown <br />as separate in most cases. <br />' 4.2 Stream Reaches <br />Preliminary work on the floodplain mapping of the Gunnison River included dividing the entire <br />study into separate reaches. The reaches were classified by estimating stream floodplain widths. <br />The width classifications were refined once cross - sections were plotted. The width classifications <br />' selected are based on the distance from one side of the river to the other at a ground elevation <br />15 feet above the low water level. Those classifications are: <br />Broad - >0.5 mile wide; <br />Medium 0.2 - 0.5 mile wide; <br />Narrow - 0.1 - 0.2 mile wide; <br />Canyon - <0.1 mile wide. <br />Based on these standards, there were 12 separate stream reaches classified for a total of 43.5 river <br />miles. That is an average of 3.6 miles per reach. These 12 stream reaches are listed in Table <br />4 and they are shown on Figure 6. <br />r <br />12 <br />1 <br />