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I I I above John Martin Dam is 300 miles in length and drains an area of 18.915 square mill:s of .....hich ] 8,130 squan: miles are contributing drainage area. Above John Martin Dam. four principal tributaries entcr the main stem from the south. they are: St. Charles. Huerfano. Apishapa. and Horse Creek. Additionally. drainages in Bent Count~.. include Adobe and Gageby Creeks from the north and Rule Creek from the soulh. The stale line is about 58 miles below John Martin Dam. Caddoa and Mud Creeks are the principal drainages in Bent County below John Martin Dam. I I I Hi"lorical Floodin2 Situation Floods of Record: I I During the period of streambed records. major flooding occurred on the Arkansas River in the years 1921, 1942.1955. and 1965. The floods of 1921. 1955. and 1965 resulted from rainstorms; whereas. the 1942 flood was the result of snowmelt runoff augmented by rainfall runoff Brief descriptions of these floods are given in subsequent paragraphs. I I I I I Flood of June 1921- The flood of June 1921 was the result of the storm of2-5 June 1921. which centered near Pueblo and produced the flood peaks of record in Colorado above Las Animas. Weather records show that precipitation between Cannon City and Pueblo \I,"as from 3 to 5 inches for the 48 hour period ending on the afternoon of June 4. The streams that enter the Arkansas River between Cannon City and the mouth of Chico Creek. 16 miles below Pueblo. were the principal contributors to the flood. The flood crest increased rapidly as il progressed downstream. The flood ~ks of many of the tributary streams reached Pueblo at the same time and caused the maximum peaks of record at PUt.-blo, Nepesta. La Junta, and Las Animas. Above La Junta the effects of valley storage were observed by the inflow from the tributary streams. Below La Junta the tributary inflow was small and the effects of valley storage were evident. The peak at Pueblo, 103,000 c.f.s. increased to 180,000 c.t:s. at Nepesta and to 200.000 c.f.s. at La Junta. The peak reduced to 187.000 c.f.s. at Las Animas as a result of valley storage. I I Flood of April 1942 - The month of April 1942 was one or the Weltest months in the history of Colorado. The average precipitation of 4.35 inches was the second largest for the state at that time. This amount was exceeded only by the 5.96 inches of precipitation that occurred during April 1900. fleavy precipitation over the entire Purgatoire River watershed occurred 16-20 April: much of it in the form of snow. Rainfall during the 22.25 April period hastened the melting of the heavy snow deposits and resulted in a peak flow of 60.000 c.f.s. on the Purgatoire River near Las Animas. Precipitation OVI..'I' the Arkansas River drainage area abo..'e Las Animas produced a peak of 12.000 c.f.s. at Nepesta, 34,800 c.f.s. at La Junta. and 23.600 c.f.s. at Las Animas. Although there was major flooding at Trinidad. where the peak flow on the Purgatoire River reached 27.000 c.f.s.. the principal damage was to transportation and irrigation systems in the Arkansas River valley. I I Flood of May 1955 - During the period J7~20 May heavy precipitation occurred over the 100...er mountains and plains of eastern Colorado and northeastern New Mexico and the plans of western Kansas. The precipitation over the mountainous area was principally in the form of a dense wet snowfall but heavy rains fell over the plains. The storm began on the afternoon of 17 May in New Mexico and Kansas. Precipitation ended at most of the Colorado stations during the afternoon of 19 May. At Lake Maloya, New Mexico, 13.59 inches of precipitation occurred in 48 hours, with 11.28 inches measured during the 24-hour period at 4 p. m. on 19 May. This station recorded the greatest amount orprccipitation during the storm. I I Major flooding occurred in the Arkansas River from Pueblo to John Martin Dam and in the reach of the Purgatoire River from Trinidad to the mouth. The flood on the Purgatoire River exceeded the flows PAGE r