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23 MAJOR FLOODS-SOUTH PLATTE R1e showed that the at the \V cather 1 residence, a open storm, indicating a rainfall of n t made for the slightly flaring sides. Other evidence precipitation was much greater in that vicinity than Bureau station :MAJOR FLOODS SOUTH PLATTE RIVER The headwaters of the South Platte River have their sources in the mountainous rC'gion surrounding the large basin near the center of the State, known as South Park, and in the long eastern slopes of the high mountains forming the Continental Divide~ TIle general course of the stream is eastward to Lake George, thence through Platte Canyon northward to its junction with the Cache la Poudre River near Gree- ley, and thence eastward again to its junction with the North Platte River at North Platte, Nebr. Beyond Platte Canyon the South Platte River emerges from the foothills and flows across the plains in a shallow valley for a distance of 190 miles to the Colorado-Nebraska State line. Through the can- yon the river has an average fall of 55 feet to the mile, but across the plains the fall decreases from 15 to ,7 feet per mile. The South Platte River is rarely subject to floods .above the canyon section, although the generalstonn of June 2-7, 1921, caused a flood that did serious damage. Below the canyon the river is subject to floods, caused chiefly by the tributary streams draining the Front Range, and rarely by the tributaries from the plains area. , The floods LTJ. the South Platte River Basin described in this report were on tributaries that drain the eastern slope of the Front Range of the Rocky Mountains, with the exception of Cherry, Kiowa, and Bijou Creeks, which are plains streams. ' . , Gaging stations have been maintained at the following points on the South Platte River in the areas subiect tb floods " , e " " ~. I ~I ~ varies greatly within short distances Fortunately, the local resident- are "cloudburst-conscious," and frequently measure the precipitatiol of cloudburst storms by means of various receptacles standing 0], their premises. Although many such records may not have a higL degree of accuracy, they are of value in that they constitute the only record of the intense rainfall that causes the cloudburst floods. A field search in connection with investigations of floods frequently brings such records to light. An experience of the senior author indicates the wide variations in rainfall within a comparatively short distance. On August 11, 1936, the \Veather Bureau station in Denver recorded 35 inches of rainfall. Three miles distant, at the au thor's bucket in an space was nearly filled during that least 6}~ inches after allowance was ~ , , trast between the surface air and the upper air is the greatest. The upper air is still very cold to the north, but the surface air has begun to heat up rapidly toward the south. This maximum contrast in temperature causes the heaviest general precipitation. IN COLORADO FLOODS 2:2 A type of storm confined chiefly to the eastern foothills region below an altitude of about 7,500 feet and extending eastward from the mountains for a distance of about 50 miles, is the so-called cloudburst, which is a rainfall of great intensity confined to a very small area and lasting usually a very short time. The intensity of cloudbursts is indicated by two incidents reported on reliable authority. The first occurred during the cloudburst that caused the Bear Creek flood of July 25, 1896. The daughter of a rancher was riding on Green )'Joun- tain, looking after the stock, when the'storm started. By the time she reached the barn she was practically unconscious on her horse. and had to be revived by means used for resuscitating victims of drowning, as the intensity of the rain made it almost impossible for her to breathe. The other incident occurred during the series of cloud- bursts that caused the Arkansas River flood of June 3, 1921; a horse was drowned in an open field. , Cloudbursts occur only where there is a marked range in tempera- ture within l1 relatively small area. This condition exists chiefly in the foothills, where the warm air from the plains drifts toward the moun- tains, is deflected upward, and cools rapidly at the higher altitudes near the heads of the canyons, For this reason cloudbursts generally occur in the afternoon or early evening of an unusually warm day. On rare occasions rail1full of cloudburst intensity occurs as far east as the eastern edge of the State. Ari outstanding example was the storm of ~Iay 30-31, 1935, which took the form of a series of cloudbursts along the path extending from the Pikes Peak region to the Colorado- Nebraska'State line in the Republican River Basin. At the higher ~ltitudes the differences in temperature are usually insufficient and the mass of warm' air too small to causecloudbursts,14 although on rare occasions they have occurred at high altitudes during unusually warm weather~ CLOUDBURSTS PRECIPITATION RECOF-DS Cloudbursts co'Ver such small areas that only rarely have the~ occurred where the minfallcould be measured a,t a Weather Bureau station; m9reove~, within these sIlla:U areas the depth of precipitation II Follansbee; Robert, lUld B<!dges, P. V., Some floods In the Rocky Mountain region:'U. S. GellI. SurveY'Water~Supply Paper 52!\,p. 107, 1925.' '" , ' ,