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... I I I ! The average monthly temperature at Grand Junction ranges from approxi- I mately 260!F in January to approximately 78.30F in July. Grand Valley temperatuie extremes are evidenced by mean maximums ranging from approxima~ely 380F in January to approximately 940F in July, and by mean minimums ranging from approxmately 150F in January to 620P in July. coqler temperatures prevail in higher mountain areas. The lowest reqorded temperature in the county was -360F at Collbran on February 8, 1933. In Grand valley, record low and high temperatures are -340F 'and 640F for January and 380F and llloF for July, respec- . I , t1vely. ~he grow1ng season usually extends for approximately 190 I days. I I Mesa County is largely mountainous, and native vegetation is mainly in the pirlion-juniper and forest categories, with some salt desert shrub land in the west-central portion of the county and grass- covered vcilley land in the west. Agricultural operations and urbani- , . zation ha'{e taken place along several streams in the county, but have been conc~ntrated principally in the Grand Valley region. Valley areas notichanged by human encroachment retain desert shrubs, cotton- wood tree~, willow trees, and an understory of hardy grasses. Between I elevationi of 5000 and 8000 feet, pinon pine, juniper, oak, big sage- brush, anq Douglas fir trees are prominant. From an elevation of 8000 feetlto the timberline, vegetation consists mainly of sub-alpine fir, asperl, Englemen spruce, and lodgepole pine trees. Hardy grasses, sedges, artd alpine willow trees are found above timberline. ! I 2.3 Principal!Flood Problems ! As noted ~arlier, most of the annual precipitation in the higher regions of the Colorado River Basin occurs as snow, and a deep snow- pack acc~ulates. General rainstorms can occur in the region during I spring an9 summer. Convective-type cloudburst storms of small areal extent, w~ich account for approximately one-half of the normal annual precipitation in the Grand Valley area, can be expected anytime , , 1 dur1ng late summer and early fal . , Flooding tn Mesa County is caused mainly by snowmelt in the larger drainage ~asins and by cloudbursts over the smaller drainage basins. However, ~eneral rainstorms constitute the principal flood hazard on Roan Creek, while general rain on snowpack creates the most hazardous conditions in the basins of Plateau and Buzzard Creeks. The Dolores , River exp~riences flooding from both snowmelt and general rainstorms. Major floods on the Colorado and Gunnison Rivers result from rapid melting of the mountain snowpack during May, June, and July. Major floods onlthe other creeks, washes, and small streams in the study area, especially those with much of their drainage basin below an elevation of 8000 feet, are most often caused by cloudburst storms. Snowmelt flooding is characterized by moderate peak flows, large volume, artd long duration, and is marked by a diurnal fluctuation in flow. iRainfall on melting snow may hasten the melting process and incre~se floodflow. General rainfloods are caused by prolonged 9 ~'";"'.