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1.0 INTRODUCTION 2.0 STUDY AREA 1.1 Purpose 2.1 Location This report was prepared by the CWCB in order to assist local officials at the City of Delta, at Delta County, at the City of Grand Junction and at Mesa County in their floodplain management activities. The report delineates the 50-year and 100-year floodplains for the reach of the Gunnison River from Delta downstream to its confluence with the Colorado River. The report allows interested parties to determine preliminarily whether particular sites are in the 50-year and 100-year floodplain of the Gunnison River. The study reach of the Gunnison River extends from Delta downstream to the ~onfluence with the Colorado River. Although portions of the Gunnison watershed are located in other Colorado counties, the entire reach of the Gunnison River study (between Delta and Grand Junction) is divided almost equally between Delta and Mesa Counties in the west-central region of Colorado (see Figure 2). 2.2 Studv Reach 1.2 Acknowledgements Larry F. Lang Chief, Flood Control & Floodplain Management Section, Colorado Water Conservation Board (CWCB); The focus of this study on the Gunnison River is from downstream of Delta (approximately 43 miles upstream of the confluence with the Colorado River at Grand Junction) to the confluence (at Grand Junction). At the upstream limit of the study (Delta) the stream elevation is approximately 4920 feet and at the downstream limit of the study the stream elevation is approximately 4550 feet. The study reach contains two USGS gages. One is located at the upstream limit of the study (at the US Highway 50 bridge in Delta) and one is near the downstream limit of the study at Whitewater, Colorado (at the Highway 141 bridge crossing approximately 9.5 miles upstream of the confluence). Information for those two gage, as well as for one gage on a tributary stream is listed in Table 1. The assistance and cooperation in supplying information of the individuals, agencies, firms, and organizations listed below are appreciated and gratefully acknowledged: Brian Hyde Resource Scientist III, Flood Control and Floodplain Management Section, CWCB; Jerry (Ed) Vail Hydrologist, United States Geological Survey TABLE 1 GAGE DATA Bob McCabe Colorado State Engineer's Office, Division 5 U.S. Army Corps of Engineers, Sacramento District Federal Emergency Management Agency 09144250 384501,1080406 GUNNISON RIVER AT 19 5628 DELTA (uts HWY 50 BRIDGE) GUNNISON RIVER 9 79 7928 MI U/S GRAND JUNCTION AT WHITEWATER UNCOMPAHGRE 57 1115 RIVER AT DELTA 1.3 Related Flood Studies The Federal Emergency Management Agency prepared Flood Insurance Studies (FIS') with accompariying Flood Insurance Rate Maps for the City of Delta and unincorporated Delta County, both dated 8/91 (References 1 & 2) and for the City of Grand Junction and unincorporated Mesa County, both dated 7/92 (References 3 & 4). The FIS' include peak flow values for 50-year and 100-year floods in the vicinities of Delta and Grand Junction, Colorado, along with detailed floodplain mapping. The flow values from these studies and the accompanying detailed floodplain delineations have been incorporated into this report. 09152500 385900,1082700 09149500 384431,1080449 The reports referenced above are available at the offices of the Colorado Water Conservation Board, at the Federal Emergency Management Agency (Region VIII), at the Cities of Delta and Grand Junction, and at Mesa and Delta Counties. . 1 2