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4.0 HYDRAULIC ANALYSIS <br />4.1 Base Mqpping <br />' Base mapping selected for the delineation of the 50 -year and 100 -year floodplains on the Yampa <br />River consisted of 7.5 minute U.S.G.S. topographic quadrangle maps. A total of 14 quadrangle <br />maps were used in compiling the base maps for the Yampa River study reach. The contour <br />interval for the maps was either 20 or 40 feet (see Figure 8). Due to the small scale of the base <br />maps (1 "= 2000'), the 50 -year and 100 -year floodplain boundaries could not be shown as separate <br />in most cases. <br />4.2 Stream Reaches <br />Preliminary work on the floodplain mapping of the Yampa 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 15 separate stream reaches classified for a total of 57 river <br />miles. That is an average of 3.8 miles per reach. These 15 stream reaches are listed in Table <br />4 and they are shown on Figure 6. <br />fl <br />0 <br />1 14 <br />L <br />