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convective-type cloudburst storms occur in summer. The largest amounts of precipitation occur from January through April, and in August. In the river valleys, mean maximum temperature varies from the mid-30 degree Fahrenheit (OF) range in January to approximately 900F in July, and the mean minimums vary from looF in January to 500P in July. The growing season is 136 days and lasts from mid-March through late October. Native vegetation in Garfield County varies from salt desert shrub, and associated growth in the southwest comer to alpine and tundra-type cover in high mountain areas. Agricultural operations and urbanization have drastically modified native vegetation in the main river valleys, the adjoining benchlands, and the lower portions of small tributary valleys. The surface cover of western Colorado ranges from substantially barren rock to deep fertile friable loams and clays of good to excellent quality for plant growth. Along the north side of the Colorado River is a prominent escarpment known as the Roan or Book Cliffs near Rifle. Most of the area has fairly deep soils and steep slopes. Mancos shale outcrops follow West Rifle Creek. Sedimentary formations are nearly horizontal in the upper part of the Rifle Creek basin. Soil characteristics for the Glenwood Springs area consist of clayish to loamy textures with variable amounts of gravel, cobble, and stone throughout. 2.3 Principal Flood Problems Most of the annual precipitation in the Colorado River Basin occurs as snow resulting in a deep snowpack in the higher regions. General rainstorms can occur in the area from late spring through late fall, and convective type cloudburst storms can be expected frequently during the summer. Major floodflows on the" Colorado and Roari"ng Fork Rivers result from rapid melting of the mountain snowpack during the period from late May through early July. Snowmelt t1ood~ are characterized by moderate peak flows, large volumes and long durations, and marked diurnal fluctuation in flow. Rainfall on melting snow may accelerate the rate of snowmelt adding to floodflows. Major floods on the dry washes, gulches, and smaller streams under study, especially, those with much of their drainage area below 8,000 feet, are generally caused by cloudburst storms. This type of storm is characterized by a high peak flow. and short duration. Due to the storm's small areal extent and other factors, cloudburst runoff would not significantly affect flooding along streams as large as the __ Colorado and the Roaring Fork Rivers. Flooding from general rain does not constitute a significant problem in Garfield County. The 100- and 500-year floods would result from cloudburst storms on the small washes, tributary gulches, and tributary creeks; from snowmelt on the Colorado River, and from snowmelt that could be augmented by rain on the Roaring Fork River. Garfield County is known to have a long history of snowmelt and summer cloudburst floods, but limited definitive data on specific floods are available because flooding largely occurred on farmland and was seldom reported. Records indicate that 12 snowmelt floods and 15 cloudburst floods occurred in Garfield County since the turn of the century. Two snowmelt floods were recorded during the last two decades of the 19th century.. 7