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FLOOD HAZARD INFORMATION The Gunnison River is tributary to the Colorado River, joining that stream at Grand Junction, The Uncompahgre River joins the Gunnison from the south near Delta, Both streams rise at high elevations on the western slopes of the Continental Divide. Elevations in the drainage area range from about 14,000 feet (NGVD) in headwater regions to 5000 feet at Delta, The upper drainage areas are character- ized by high mountain peaks, steep-walled canyons, and narrow valleys, while broad expanses of low hills and mesas characterize the lower regions, Stream gradients along the reaches studied average about 10 feet per mile on the Gunnison River and 30 feet per mile on the Uncompahgre River, GUNNISON AND UNCOMPAHGRE RIVERS Delta and Delta County, Colorado CONTENTS Page INTRODUCTION "."., NATURE OF FLOOD PROBLEMS FUTURE FLOODS ",..., ,1 .2 ,5 NATURE OF FLOOD PROBLEMS APPENDIX Climate of the Gunnison-Uncompahgre Rivers region is arid to semiarid with normal annual precipitation varying from about 8 inches in the Delta area to 40 inches in headwater areas, Most of the annual precipita- tion in the high elevations of the Gunnison basin occurs as snow and a deep snowpack usually accumulates, Although general rain over large areas for extended periods can occur from July through October, and convective- type cloudburst storms occur from late May through September, runoff from these types of storm does not constitute a serious flood threat, Therefore, rapid snowmelt from about the middle of May through early July poses the most serious flood hazard along the study reach of the Gunnison River. Snowmelt flood condi- tions may be intensified by general rain during the snowmelt season, The average elevation of the Uncompahgre River basin is lower than that of the Gunnison River basin and deep snowpack normally covers a proportionally smaller part of the drainage area, Conversely, rain storms over the Uncom- pahgre basin cover proportionally larger areas than in the Gunnison basin, Flooding along the lower Uncompahgre River usually results from rapid snowmelt from mid-May through early July, but may also result from runoff of general rain, particularly in the Delta area, On the long- term average, rain floods occur less frequently than snowmelt floods, which may be intensified A PEAK FLOWS AND STAGES . . . . , , , , , . . , . , , , , , , , , . . , . , 7 PLATES PLATES 1 INDEX MAP 2-59 FLOODED AREAS 60-72 FLOOD PROFILES INTRODUCTION This report briefly covers the findings of a flood hazard information study of a 16.5-mile reach of the Gunnison River and a 13-mile reach of the Uncompahgre River, both in the vicinity of Delta, The study reach along the Gunnison River extends from the railroad bridge near the mouth of Roubideau Creek upstream to Austin, On the Uncompahgre River, the study reach extends from the mouth upstream to Olathe. The purpose of the report is to provide a basis for planning the best use of lands subject to inundation by the 100-year flood, Topographic maps and channel cross sections needed for the report were jointly provided by the Colorado Water Conservation Board, the Commissioners of Delta and Mont- rose Counties, and the Water and Power Resources Service, Data on past floods, as well as other information useful in making the study, were obtained from local residents and various local, state, and Federal agencies. Incl1 1 In the high elevations of the tributary areas, vegetation includes stands of pine and fir, Engleman spruce, aspen, and dwarf juniper, Cottonwood, sagebrush, other low brush and shrubs, and some native grasses predominate in the lower areas. The Gunnison River drains about 7,050 square miles (including Roubideau Creek) above the lower limit of the study area and 5,271 square miles above the Highway 92 crossing near Austin, which is about 6 miles downstream from the mouth of the North Fork. The Uncompahgre River drains about 721 square miles above Olathe and 1,129 square miles at its mouth, A general map of the study area and its tributary drainage is shown on the opposite page, by general rain, Snowmelt runoff attenuates along the lower Uncompahgre River due to channel storage and numerous diversions for irrigation. Although high flows on the Gunnison and Uncompahgre Rivers are largely contained in- channel and overbank flow is infrequent, these streams have a long history of damaging floods, Prior to settlement of the area, Chipeta, wife of the Ute Indian Chief Ouray, stated she had seen the Uncompahgre Valley at Delta "flooded from bluff to bluff," The largest known flood on the Gunnison River occurred in June 1884. It resulted from rapid snowmelt and was intensi- fied by heavy rain. Since the snowpack was especially deep that year, and covered large areas in western Colorado, it is probable that a great flood also occurred on the Uncompahgre River. In May 1884, that stream was reported to be rapidly rising and expected to overflow, Records show that, since. 1900, high flows occurred on the Gunnison and Uncompahgre Rivers in 1921,1927,1938,1941,1944,1947,1957, 1973, and 1975, High flows also occurred on the Gunnison River in 1905, 1920, 1969, and 1970, Peak flows of the larger historical floods on the Gunnison and Uncompahgre Rivers in the study area are shown in the tabulation on page 4, Historically, floods on the Gunnison and Uncompahgre Rivers have disrupted highway and rail traffic and damaged or destroyed 2