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<br />is quite variable due to the mixture of rock outcrops and dense <br />pine needle beds (Reference 3). <br /> <br />The climate is classified as generally semiarid, with an abundance <br />of sunshine, frequent wind, and low humidity. The prevailing <br />direction of air movement to the region is from the west. On <br />arrival, airmasses from the west are comparatively dry due to long <br />travel over land and loss of moisture over mountain ranges. <br />However, the primary sources of moisture are the Pacific Ocean and <br />the Gulf of Mexico. Generally, airmasses from the Pacific Ocean <br />dominate from October through April, while airmasses from the Gulf <br />of Mexico dominate during late spring and summer. <br /> <br />The mean temperatures in the Delta area range from approximately <br />26.40F in January to approximately 74.1oF in July. Cooler <br />temperatures prevail in higher mountain areas. The growing season <br />usually extends upward to 140 days (Reference 4). <br /> <br />Normal annual precipitation varies with elevation and ranges from <br />about 7.9 inches at Delta to an estimated 40 inches at higher <br />elevations in the stream basins (Reference 4). Most of the annual <br />precipitation in high-elevation areas occurs as snow between <br />December and May. Beginning in October, a deep snowpack <br />accumulates. Snowmelt begins in late April and continues well into <br />June or early July. General rain may occur over the Delta area and <br />convective-type cloudburst storms frequently occur in summer. <br /> <br />Floodplain development consists of residential, commercial, and <br />light industrial developments along the study reaches. <br /> <br />2.3 Principal Flood Problems <br /> <br />Major flooding in the City of Delta has been the result of rapidly <br />melting snow, sometimes augmented by general rain, between May and <br />August. Snowmelt flooding is characterized by moderate peak flows, <br />large volume of runoff, long duration, and marked diurnal <br />fluctuation of flow. Flooding from general rain is characterized <br />by high peak flows and moderate duration of floodflows. Flooding <br />from convective-type cloudburst storms is characterized by high <br />peak flows, short duration of flow, and relatively small volume of <br />runoff. Because cloudburst storms are small in areal extent and <br />short in duration, they do not constitute a flood threat on streams <br />as large a~ the Gunnison or Uncompahgre Rivers. <br /> <br />Historical references to floods on the Gunnsion River extend back <br />to 1884. In that year, early high temperatures in May and June <br />resulted in the largest known snowmelt flood on this river. At <br />Delta, the Gunnison River was reported to be 10 feet above bankfull <br />stage, with a width of approximately 0.75 mile. Many wooden <br />bridges along the river were destroyed by this flood. Peak flows <br />of the larger historical floods on the Gunnison Ri ver are shown <br />below (Reference 5). <br /> <br />5 <br />