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<br />CHAPTER 3 <br />COLORADO FLOOD CHARACT'ERlSTICS1 <br /> <br />3.1 General Flood Characteristics <br /> <br />From observation of the effect of topography on precipitation, ff is evident that floods occur <br />most frequently on the eastern slope of the Front Range both in the South Platte and <br />Arkansas River Basins, In the mountains farther west, precipffation is insufficient to cause <br />floods. except during periods of melling snow in May and June, At that time, the severffy <br />of the resulling floods is dependent not only on the rainfall, but also. and to a greater <br />degree, on the depth of snow cover and the rate of seasonal rise in temperature, Hence, <br />only floods caused by rainfall on melling mountain snow occur simullaneously throughout <br />the greater part of the State, During the storm of June 2-7. 1921, the combination of <br />melling snow and rainfall caused severe floods on all the major rivers except the Arkansas, <br />Over the Arkansas River Basin, the stolm, influenced by the topography. was concentrated <br />in a series of cloudbursts in the foothills just east of Canon Cffy, These cloudbursts <br />caused the most severe flood on the Arkansas River ever recorded. and at a time when <br />the water from melling snow from the hinher areas was insignificant in amount. <br /> <br />The tributary streams in the Souttl Platte and Arkansas River Basins are subject to heavy <br />rains of cloudburst intensffy which cause the cloudburst type of flood. the peak discharge <br />of which is greater than that caused by rainfall during the period of melting snow, <br />Whenever heavy rain occurs along the Front Range, particularly in the lower foothills, ff is <br />usually concentrated in cloudbursts at various points, The cloudburst flood !low is of such <br />short duration. the peak only lasts for a few minutes, and the total quantffy of water during <br />the flood is so small, that when ff reaches a larger stream. channel storage quickly reduces <br />ff to a stage which is not a seriouS flood, <br /> <br />3.2 Meteorological Conditions Causin" Floods <br /> <br />During the Spring and summer storms. the chief source of moisture is the Gulf of Mexico, <br />The warm. moisture-laden air from the gulf, being deflected upward by the Front Range, <br />is cooled and precipffates ffs moisture not only on the Front Range, but also on the <br />adjacent plains area, Allhough the storms may continue westward. the remaining moisture <br />is insufficient to produce heavy precipffation west of the Front Range, and does not cause <br />floods on the streams in the mountain area, Very rarely, as in September 1909, and <br />October 1911, late summer and early fall storms bring in moisture-laden air from the <br />Pacific Ocean, which being defected upward by the mountain mass bordering the high <br />plateau in the westem part of the State, causes heavy rainfall in the southwestern part of <br />the State and lesser amounts farther east.' This Chapter taken from USGS "Floods in <br />Colorado", 1948 <br /> <br />1 This Chapter taken from USGS "Floods in Colorado", 1948 <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />3,1 <br /> <br />a:w=r <br />