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I I I I I I I I I I I I I I I I I I I EXECUTIVE SUMMARY Gunnison County is currently preparing floodplain information for use in updating the Federal Emergency Management Agency (FEMA) Effective Flood Insurance Study (FIS) and associated maps. In doing so, Gunnison County identified eight river reaches for inclusion in the updated FIS. This report presents an unimpaired and natural flow analyses for the natural flow of the eight stream reaches in Gunnison County. Flood frequency analyses are completed for the 10, 2, 1, and 0.2 percent chance exceedence events for each reach. The study is part of the U.S. Army Corps of Engineers', Floodplain Management Services Program, and is being conducted for the City of Gunnison, Gunnison County, and the Colorado Water Conservation Board. The Gunnison River and Tomichi Creek within the boundaries of the City of Gunnison have been included in two previous hydrologic studies. The first study was conducted by Engineering Consultants, Inc. in 1977 for the City of Gunnison, Gunnison County, and the Colorado Water Conservation Board. The second study was conducted by FEMA in 1982. These two reaches are the only reaches included in this current study that have been analyzed in the past. Since the time of those studies, the area has experienced some above average annual peak flow values that affect the hydrologic analysis and resulting floodplain inundation, thus warranting a revised study. Two of the eight stream reaches are analyzed using available peak flow information from streamgage records. The other six reaches have insufficient streamgage data and are therefore analyzed using expanded data series and correlations with available streamflow data from adjacent reaches. Procedures are presented in Section 4.0. Results are presented in detail and summarized in Section 5.0, and noted in Table ES.1. The study also includes a review of the recorded streamflow data to determine the attenuation effects on the flow data caused by the Taylor Park Dam located on the Taylor River. This is a Bureau of Reclamation irrigation storage reservoir for the Uncompahgre Valley Project. Construction of the dam and reservoir began in 1935 and the first releases were made in 1937. The primary purpose of Taylor Park Dam is irrigation supply. The operating schedule is not designed for flood control; however, the storage capacity of the reservoir does impact flood flows in downstream study reaches along the Taylor and Gunnison Rivers. Thus, stream flow records are adjusted to remove the influence of the reservoir on flood peaks using a combination of flow records and reservoir storage records. A review of the nature of flood events is also completed in regards to snowmelt and rain on snow event flows. The review shows that the hydrology of the streams in this system is driven by snowmelt runoff, the peak of which typically occurs between March and June (with a few peaks occurring in early July). Rain on snow, defined as 0.5 inches of rainfall or greater for this study, occurs infrequently. For example, the Tomichi Creek, with a large percent of watershed in high mountain valley, had eight out of sixty-three years of peak values caused by rain on snow event flows. Two analyses are performed: one with data sets including the rain on snow peaks, a second using only snowmelt peaks. The results, while similar, indicate that rain on snow event flows have a discernible effect on the resulting flood frequency statistics for the reach. Therefore, the 0.5-inch precipitation threshold was used to filter the data for the remaining reaches for rainfall on snow events. This study provides the necessary flood flows to complete the analyses of the hydraulics for the FEMA FIS study along each of the eight reaches for the associated flood events. v