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6 TECHNICAL MANUAL Streambed slope, SS.nThe slope of the streambed, in feet per mile, at the study site measured from the best available topographic maps or by field surveys. Streambed slope is computed as the elevation difference of the contours upstream and downstream of the study site divided by the distance along the stream between the contour crossings measured by stepping "ith draftman's di- viders or with a opisometer or map wheel (fig. 1), Some'special problems that may arise are: 1. Where contours cross the stream at or just downstream from the study site, a slope needs to be computed for the reaches both upstream and down- stream from the site, and the average of the two slopes used as the streambed slope for the site. 2, For study sites at or very near the mouth of a stream, it may be necessary to determine the approximate elevation of the mainstream at the mouth for use as the downstream point in the computations. In other cases, the mainstream slope may best represent the streambed slope at the study site. 3. If a field survey is made, the streambed elevations should be ob- tained at points approximately equal distances upstream and downstream from the study site. The minimum distance between points is recommended as 500 ft. Extreme care should be exercised to avoid deep pools or potholes when meas- uring streambed elevations, Basin (channel) slope, S8.nThe slope, in feet per mi'le, measured be- tween two points along the main channel, one of which is located at 10 percent of the channel length and the other at 85 percent of the channel length (fig. 1), Channel length is measured upstream along the water course from the site to the basin divide, The channel slope can be measured from the best available topographic maps except for very small basins that may require field surveys. In mixed-population flood areas, the channel slope needs to be computed for each subbasin in the manner described above. Re9ression equations were developed for peak discharges and depths for the 10-, 50-, 100-, and 500-year floods for each of four flood-characteristic regions in Colorado. The four regions as shown on figure 2 are Plains, Moun- tain, Northern Plateau, and Southern Plateau. Descriptions of the boundaries of the four regions are given in the explanation to figure 2 and on pages 39 to 44, The equations for each of the regions are listed in tables 1-4, The equations for the 100-year flood discharges and depths are depicted graphi- cally in figures 3-10. ~- _.