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. . ..6 FLOODS IN COLORADO ~ !ll !5l ~ . ~ ~ ----- ~ I , I" 0 ji I - I ~ I !l I '2 . ~ '0 ~ a ~ 'S '9 ~ i ~ t ~ .. ] !\ .a . 0 -ffi 1 '8 0 .. it ~ I ~ ... . S I 0 , ~ r;; ~ I y r- . ~ !5l ; ~ ~ OCCURRENCE OF FLOODS . 7 Front Range some precipitation, even in winter is caused by moisture from the Gulf. In general, however, the snowfall is heaviest on the western ranges, and its melting in June has a greater effect on the flow of the streams in that regiou than the melting of the snow over the eastern ranges has upon the streams draining the Front Range. In late spring, precipitation not only along the Front RR.nge but also farther west sometimes augments the melting snow sufficiently to cause floods. Precipitation in the plateau region is so light during the summer that floods rarely occur. East of the Front Range general precipite.- tion is rarely sufficient to cause floods. AREAS SUBJ'ECT TO FLOODS 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 the Arkansas River Basins. In the mountains farther west precipitation is insufficient to cause floods, except during tbe period of melting snow in June. At that time tbe severity of the resulting floods is dependent not only on the rainfall but also, and to a greater degree, on the depth of snow cover and the ratc of the seasonal rise in temperature. Hence, only floods caused by rainfall on melting mountain snow occur simultaneously throughout the greater part of the State. The most severe floods of record on the Rio Grande, South Platte, Colorado, and Gunnison Rivers have resulted from this cause, as have the outstanding floods that occurred before gaging-station records began, During the storm of June 2-7, 1921, the combination of melting snow and rainfall caused severe floods on all the major rivers except the Arkansas. Over the Arkansas River Basin the storm, influenced by the topogral'hy, 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 melting snow from the higher areas was insignificant in amount. ~"; The tributary streBJIl.8 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 the Front Rangc, 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 lasting only 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. This is well illustrated by the typical stage hydrographs shown in figure 2.