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<br />. <br /> <br />the Plum Creek, Cherry Creek, Boxelder Creek, Bijou Creek and <br /> <br /> <br />Beaver Creek basins. In addition, 31 inches of new snow was reported <br /> <br /> <br />at Squaw Mountain. The isohyetal pattern shown on plate 5 was drawn <br /> <br /> <br />from data as reported by regular National Weather Service observers. <br /> <br /> <br />These results indicated an average 24-hour rainfall value for <br /> <br /> <br />1,000 square miles of 5.5 inches. <br /> <br />STORM OF 31 JULY 1976 <br /> <br /> <br />During the afternoon of 31 July, a quasi-stationary front <br /> <br /> <br />oriented east to west through Missouri, Kansas and Central Colorado <br /> <br /> <br />and curving northward along the eastern slopes of the Rockies into <br /> <br /> <br />Central Wyoming caused development of intense convective cells <br /> <br /> <br />over the Front Range. The slow northerly movement of these forma- <br /> <br /> <br />tions coupled with rapid development of new cells to the south <br /> <br /> <br />resulted in very heavy precipitation over the Big Thompson basin <br /> <br /> <br />below Estes Park and northward into the Cache la Poudre basin. <br /> <br /> <br />The storm ended at about 11 p.m. The isohyetal map for this storm, <br /> <br />based on "bucket" survey data, is shown on plate 6. <br /> <br />Runoff Characteristics <br /> <br />Runoff in the South Platte River basin can occur from snow- <br /> <br /> <br />melt, rainfall, or a combination of both. Most of the disastrous <br /> <br /> <br />flood experience in the basin has been caused by intense convective <br /> <br /> <br />thunderstorm rainfall on tributaries which produces high peak dis- <br /> <br /> <br />charges that rapidly attenuate in the lower reaches of the tributary <br /> <br />. <br /> <br />17 <br />