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<br />" <br /> <br />B-1 <br /> <br /> <br />i <br />..:'-~-< <br /> <br />INTERIM REPORT ON REVIEW SURVEY FOR FLOOD CONTROL <br />ARKANSAS RIVER AND TRIBUTARIES <br />LAS ANIMAS, COLORADO, AND VICINITY <br /> <br />APPENDIX B - HYDROLOGY <br /> <br />WATERSHED DESCRIPTION <br /> <br />I. INTRODUCTION.- The drainage area of the Arkansas River water- <br />shed above John Martin Dam, Colo., comprises 18,915 square mi les of <br />which 785 square mi les are non-contributing. Las Animas is located <br />17.9 river mi les above the dam on the south side of the Arkansas River <br />immediately above the mouth of the Purgatoire River, a major tributary <br />entering from the south. The watershed area above the confluence of <br />the two rivers comprises 14,428 square mi les of which 441 square mi les <br />are non-contributing. The watershed extends approximately 215 mi les <br />west of Las Animas and varies in width from 120 mi les at that point to <br />25 mi les near Sal ida. Drainage area data are given in table I. <br /> <br />CLIMATOLOGICAL DATA <br /> <br />2. PRECIPITATION.- Most of the drainage area above Las Animas is <br />semiarid with the average annual precipitation varying froT 12 inches <br />in the valley and plains areas to over 20 to 40 inches in the higher <br />elevations of the headwaters. However, the intensity of summer storms <br />is generally high in the plains and valleys as shown by the fol lowing <br />I ist of maximum recorded 24-hour precipitation amounts: <br /> <br /> Amount <br /> I nc he s Date <br />Colorado Springs WB AP 4.27 Apr 1921 <br />Las Animas I N 3.60 Jul 1958 <br />Pueb 10 WB AP 3.77 Oct 1957 <br />Trinidad FAA AP 3.00 Sep 1904 <br />\~a I senburg Power Plant 3.37 Jul 1946 <br /> <br />The watershed above Las Animas has its maximum thunderstorm activity <br />in July and August. These months have an average of I I to 16 thunder- <br />storm days. Apri I has an average of 2 thunderstorm days, with a gradual <br />increase to the peak in July and August. September drops sharply to an <br />average of 4 thunderstorm days. Precipitation data for representative <br />stations are given in tabie 2 and detai led precipitation data for Las <br />Animas are given in table 3. <br /> <br />3. Heavy snowfal I is general in the mountainous areas. In the <br />higher elevations, snow sometimes remains on the ground throughout the <br />year. The average annual snowfal I at Pueblo is 32.4 inches. Also at <br />Pueblo, a maximum monthly snowfal I of 29.3 inches occurred in November <br />1946, and a maximum 24-hour snowfal I of 16.8 inches occurred in Apri I <br />1957. The average annua I snowfa I I at Las An i mas is 20. I inches. <br />