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I I; I Ii , Ii If : I! I! r Ii II 11 Ii HYDROLOGY REPORT FLOOD INSURANCE STUDIES IN MESA COUNTY. COLORADO I. Introduction The study area for these on-going Flood Insurance Studies (FIS) includes, from upstream to downstream, the Town of Palisade, the City of Grand Junction, the Town of Fruita, and unincorporated areas of Mesa County. The major source of flooding in all four areas is the Colorado River. Three small tributaries are also sources of flooding in Grand Junction and Fruita. Figure 1 shows the relative location of these communities along the Colorado River. The hydrologic analyses of streams in the study area involved developing peak discharge estimates for a 33-mile reach of the Colorado River and reviewing previous studies by others for Big Salt Wash, Little Salt Wash, and Horizon Drive Channel. II. Previous Studies A. Colorado River r I! In 1976 the Corps of Engineers prepared Flood Hazard Information reports for Palisade, Grand Junction, and Fruita (1, 2, 3 *). These reports include estimates of 100-year and 500-year floods, based on regional frequency analyses of available data. The Flood Insurance Studi~s for the Town of Palisade (4), City of Grand Junction (5), Town of Fruita (6), and unincorporated areas of Mesa County (7) used the results of the hydrology studies performed for the 1976 Flood Hazard Information reports to prepare floodplain maps of the Colorado River and some tributary streams. I Ii , r ? r I' I I . The table below lists the discharges used by FEr~ in the earlier FISs. Palisade Grand Junction above Gunnison below Gunnison Fruita Drainage Area (sq. mi.) 8,700 8,800 16,800 17,100 Discharges 100-year 500-year 63,000 82,000 82,000 107,000 107,000 63,000 82,000 82,000 *Numbers in parentheses indicate references listed in Section VI. -1-