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. . . . 22 FLOODS IN COLORADO MAJOR FLOODB--SOUTH PLATrE RIVER 23 trast between the surface air and the upper air is the greatest. The upper air is still. very cold to the north, but the surlace air has begun to heat up rapully toward the south. This maximum contrast in temperature causes the heaviest general precipitation. -varies greatly within short distances. Fortunately, the local residents .are "cloudburst-conscious," and frequently measure the precipitation .of cloudburst storms by means of various receptacles standing on their premises. Although many such records may not have a high 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 Weather Bureau station in Denver recorded 1.35 inches of rainfall. Three miles distant, at the author's residence, a bucket in an open spnce was nearly filled during that storm, indicating a rainfall of at least 6Y, inches after allowance was made for the slightly flaring sides. Other evidence showed that the precipitntion was mnch greater in that vicinity than at the Weather .Bureau station. OLOUDBURSTS A type o~ storm confined chiefly to the eastern foothills region below an altItude of about 7,500 feet and extending eastwnrd 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 Moun- 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 brcathe. 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 tempern- ture within a relatively small area. This condition OJ..-ists 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 altitudcs near tho heads of the canyons. For this reason cloudbursts gencrally o~ur in the afternoon or early evening of an unu"ually warm day. On rare occasions rainfall of cloudburst intensity occurs as far east as the eastern edge of the State, An outstanding example was the storm of May 30-31, 1935, which took the form of a serics of clondbursts along the path extending from the Pikes Peak region to the Colorado- Nebraska State line in the Republican River Basin. At the higher altitudes the diffcrcnces in temperntnro are usually insufficicnt nnd the mass of warm air too small to cause cloudbursts," although on rare occasions they have occurred at high altitudes during unusually warm weather. PRECIPITATION RECORDS MAJOR FLOODS SOUTH PLATTE RIVER The headwaters of the South Platte River have their sources in the mountainous region 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. The general course of the stream is eastward to Lake George, thence through Pl",tte 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 Rivcr at North Platte, Nebr. Beyond Platte Canyon the South Plattc River enierges from the foothills and flows across \he plains in a shallow valley for a distance of 190 miles to the Colorado-Nebraska State linc. 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 general storm of June 2-7, 1921, causcd 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 in 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 subject to floods: Cloudbursts cover such small areas that only rarely have they occurred where the rainfall could be measured at a Weather Bureau station; moreover, within these small areas the depth of precipitatiou It Follansbee, Robert, and Hodges, P. V., Some OoodB In the Rockf Mountain region: U. S. 0001. SUrvey Water-Suppl, Paper 520, p. 107, 1925.