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Cross section data for Little Dry, Comanche, Little Comanche, Northfield, Niver, and Box Elder Creeks were field surveyed and were located at close intervals above and below culverts, bridges, and drop structures in order to compute the effects of backwater. For Big Dry Creek, cross section data were taken from previously published detailed topographic mapping (Reference 20). For the South Platte River, field-surveyed cross sections included 49 field-surveyed cross sections and 30 supplemental valley cross sections measured on the topographic maps used for the South Platte delineation (References 21 and 40). When necessary, USGS topographic maps at a scale of l:24,000, with a contour interval of lO feet (Reference 34) were used to supplement the field-surveyed data. Channel roughness factors (Manning's "n") for these computations were assigned on the basis of field inspection of the floodplain areas and engineering judgment. Bridge geometry and elevation information was obtained from the Colorado State Highway Department, when available, or measured in the field. Starting water-surface elevations for the tributaries of the South Platte River were taken from previously computed stage-discharge relationships when available (Reference 21). In many cases, control elevations were shifted upstream to bridges or culverts ~ Where no other information or control structures were available, the starting water-surface elevations were computed by the slope- area method option of the HEC-2 program (Reference 39). These analyses indicate that flood flow from Little Dry Creek is divided into two flow paths in the area downstream of the Colorado and Southern Railroad crossing. The Little Dry Creek profiles show the individual elevations for both of these separate flow paths. Flooding from Northfield Creek above Devonshi re Boulevard does not necessarily follow the natural stream channel. For this area, flood profiles were developed using hydraulic flow lines, labeled as base line of flow on the maps and profiles. The reach of Sand Creek between Vasquez and Brighton Boulevards within Commerce City was revised to reflect a revised hydraulic analysis carried out by Simons, Li and Associates, Inc., based on new topographic information (Reference 41). Tributary M of Niver Creek Cross sections for the backwater analyses for Tributary M of Niver Creek were obtained from topographic maps, provided by the City, at a scale of l:l,200, with a contour interval of 2 feet (Reference 35). All bridges and culverts were field surveyed to obtain elevation data and structural geometry. 30