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CHAPTER 3 COLORADO FLOOD CHARACTERISTICS1 , 3.1 General Flood Characteristics . From observation of the effect of topography on precipitation, it is evident that floods occur most frequently on the eastern slope of the Front Range both in the South Platte and Arkansas River Basins. In the mountains farther west, precipitation is insufficient to cause floods, except during periods of meiling snow in May and June. At that time, the severity of the resuiling floods is dependent not only on the rainfall, but also, and to a greater degree, on the depth of snow cover and the rate of seasonal rise in temperature. HBnce, only floods caused by rainfall on meiling mountain snow occur simuilaneously throughout the greater part of the State. During the storm of June 2-7, 1921, the combination of meiling snow and rainfall caused SBvere floods on all the major rivers except the ArKansas. Over the Arkansas River Basin, the storm, influenced by the topography, was concentrated in a series of cloudbursts in the foothills just east of Canon City. These cloudbursts caused the most severe flood on the Arkansas River ever recorded, and at a time when the water from meiling snow from the higher areas was insignnicant in amount. The tributary streams in the South Platte and Arkansas River Basins are subject to heavy rains of cloudburst intensity which cause the cloudburst type of flood, the peak discharge of which is greater than that caused by rainfall during the period of melting snow. Whenever heavy rain occurs along thB Front Range, particularly in the lower foothills, it is usually concentrated in cloudbursts at various points. The cloudburst flood flow is of such short duration, the peak only lasts for a few minutes, and the total quantity of water during the flood is so small, that when it reaches a larger stream, channel storage quickly reduces it to a stage which is not a serious flood. 3.2 Meteorological Conditions Causing Floods . During the Spring and summer storms, the chief source of moisture is the Gulf of Mexico. The warm, moisture-laden air from the guit, being deflected upward by the Front Range, is cooled and precipitates its moisture not only on the Front Range, but also on the adjacent plains area. Ailhough the storms may continue westward, the remaining moisture is insufficient to produce heavy precipitation west of the Front Range, and does not cause floods on the streams in the mountain area. Very rarely, as in September 1909, and October 1911, late summBr and early fall storms bring in moisture-laden air from the Pacnic Ocean, which being defected upward by the mountain mass bordering the high plateau in the western part of the State, causes heavy rainfall in the southwestern part of the State and lesser amounts farther east.' This Chapter taken from USGS "Floods in Colorado". 1948 . 1 This Chapter taken from USGS "Floods in Colorado". 1948 . Colorado Flood Hydrology Manual 3.1 a:w:r