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<br />CHAPTER 3
<br />COLORADO FLOOD CHARACTERISTICS'
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<br />3.1 General Flood Characteristics
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<br />From observation of the effect of topography on precipitation, it is evident that floods occur most
<br />frequently on the eastern slope of the Front RangEl both in the South Platte and Arkansas River Basins,
<br />In the mountains farther west. precipitation is insufficient to cause floods. except during periods of
<br />melting snow in May and June, At that time, the severity of the resulting floods is dependent not only
<br />on the rainfall, but also, and to a greater degree, on the depth of snow cover and the rate of seasonai
<br />rise in temperature, Hence, only floods caused by rainfall on melting mountain snow occur
<br />simultaneousiy throughout the greater p"rt of the State, During the storm of June 2.7,1921, the
<br />com bination of melting snow and rainfall causecl severe floods on all the major rivers except the
<br />Arkansas, Over the Arkansas River Basin, tne storm, influenced by the topogr"phy, was concentrated
<br />in a series of cloudbursts in the foothills just east of Canon City, These cloudbursts caused the most
<br />severe flood on the Arkansas River ever recorded, and at a time when the water from melting snow
<br />from the higher areas was insignificant in "mount
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<br />The tributary streams in the South Platte and Arkansas River Basins are subject to heavy rains of
<br />cloudburst intensity which cause the cloudburst type of flood, the peak discharge of which is greater
<br />than that caused by rainfall during the period of melting snow, Whenever heavy rain occurs along the
<br />Front Range, particularly in the lower foothillS, it is usually concentrated in cloudbursts at various points,
<br />The cloudburst flood flow is of such short duration, the peak only lasts for a few minutes, and the total
<br />quantity of water during the flood is so small, that when it reaches a larger stream, channel storage
<br />quickly reduces it to a stage which is not a serious flood,
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<br />3.2 Meteorological Conditions Causing Floods
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<br />During tne Spring and summer storms, the chief source of moisture is the Gulf of Mexico, The warm,
<br />moisture-laden air from the gulf, being deflected upward by the Front Range, is cooled and precipitates
<br />its moisture not only on the Front Range, but also on the adjacent plains area, Although the storms
<br />may continue westward, the remaining moisture is insufficient to produce heavy precipitation west of
<br />the Front Range, and does not cause floodS on the streams in the mountain Mea, Very rarely, as in
<br />September 1909, and October 1911, late summer and early fall storms bring in moisture.laden air
<br />from the Pacitic Ocean, which being defected upward by the mountain mass bordering the high plateau
<br />in the western part of the State, causes heavY rainfall in the southwestern part of the State and lesser
<br />amounts farther east 1 This Chapter taken from USGS "Floods in Colorado", 1948
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<br />During the winter, the moisture comes chiefly from the Pacific Ocean; it is deflected upward by the
<br />western mountain chains and is precipitated in the form of snow, usually heaviest on the western
<br />slopes of the highest ranges and decreasing eastward, Along the Front Range, somE' precipitation,
<br />even in winter is caused by moisture from the Gulf. In g,meral, however, the snowfall is heaviest 0 the
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<br />1 This Chapter taken from USGS "Floods in Colorado', 1948
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<br />Colorado Flood
<br />Hydrology Manual
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<br />:3.1
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<br />DRAFT
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