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plain areas. A swmnary of Manning's "nll values used is given in Table 5. The results obtained from the HEC-2 computer program were adjusted in the vicinity of bridges to more accurately represent actual flooding conditions (Reference 13). Starting water-surface eleva- tions for the San Miguel River were determined by the slope-area method. For the Uncompahgre River, the starting water-surface elevations were obtained from the U.S. Army Corps of Engineers Flood Hazard Information report for Delta County (Reference 14). The starting water-surface elevations for Cedar Creek, Dry Cedar Creek, and Montrose Arroyo were obtained from previous studies prepared for the Colorado Water Conservation Board (References 5 and 15). For Happy Canyon Creek, the starting water-surface eleva- tions were based on hand calculations at control sections. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the flood profiles are thus considered valid only if the hydraulic structures remain unobstructed, operate properly, and do not fail. Near the City of Montrose, flow on the Uncompahgre River backs up against North LaSalle Road. On the left overbank, a limited amount of this backwater flows over the road. As this water spreads out across the flood plain, it becomes shallow 100-year flooding with a depth of less than 1 foot. Sheetflow away from the main channel occurs on Montrose Arroyo near the southwestern and northwestern corporate limits of the City of Montrose, resulting in shallow 100-year flooding. This flow is the result of the backing up of floodwaters behind insuffi- cient culverts in the City of Montrose. Similar flows occur on Happy Canyon Creek. Low ground near the mouth of the creek causes sheetflow away from the channel, resulting in a reduction in discharge downstream. At the State Highway 90 culvert, flow on Happy Canyon Creek backs up against the road and flows around the main channel where the road fill is lower. This flow then re-enters the main channel downstream of the highway. Therefore, channel flow is reduced between the point where this flow exits the channel and the point where it re-enters the main channel. For all approximate study reaches included in this study, the approximate 100-year flood elevations were determined by normal- depth computations of typical sections based on field observation. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the maps. 21