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<br />PURGATOIRE (PICKET WIRE) RIVER; COLO.
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<br />flood of September 15, 1934, at Nine Mile DllJIl, in the lower reaches
<br />of the river. The average annual discharge of Purgatoire Rivet at
<br />Trinidad for the period October 1920 to September 1941, inclusive,
<br />was 62,522 acre-feet, about 83 pe'rcent of which occurred during the
<br />months from April through September. In addition to this volume, it
<br />has been estimated that an average of approximately 25,000 acre,feet
<br />per year" has been diverted for irrigation purposes upstrellJll from
<br />Trimdad. During the period, the annual run,off has varied from a
<br />minimum of 24,730 acre,feet to a maximum of 131 ,450 acre-fee,t during
<br />the first 9 months of 1941, which exceeded the annual run-off during
<br />all other years of record. The monthly, annual, average monthly, and
<br />average mmual discharges of Purgatoire River for the periods of
<br />record at Trinidad, Nine Mile Dam, Highland Dam, and Las Animas
<br />are shown in tables Nos. 7, 8, 9, and 10, respectively, appendix B.'
<br />31. Run,off.- Purgat,oire River is perennial upstrellJll from Trinidad
<br />and the greatest part of the volume of run,off occurs during the period
<br />when melting of the snow blanket in the mountains begins and con-
<br />tinues during the months when thunderstorms occur most frequently.
<br />32. Studies of run,olf records of floods resulting from general
<br />storms indicatc that the shape of the flood wave is extremely sharp
<br />for a drainage basin which is narrow compared with the length. It
<br />nifty be deduced that ilood'producing precipitation eitller progresses
<br />downstmam causing synchronization of large discharges from the
<br />tributaries with the pealt in the river, or orographic influences control
<br />flood-produciug int,ensities within areas having comparable times of
<br />concentration, or a eomhination of both.
<br />33, In mountainous areas, where a greater portion of the soil profiles
<br />are shallow and undeveloped, the maximum run,off conditions may
<br />prcva.il during periods when storms producing maximum intensities
<br />do not occur. This apparcnt paradox is explained by the fact that
<br />loss rates l.'revailing during storms of long duration and of relatively
<br />low intensity, may be. much smaller, thus producing greater run,off
<br />than that produced during high,intensity storms when losses due to
<br />infiltl'ation rates are large. This anomaly may result in run,off flood
<br />waves of eomparable peak discharge, bnt when loss rates prevail,
<br />the rnn,off hydrogmph will indicate a brger volume of run,off pro'
<br />duced by a smallcr amount of precipitation at lower intensities over
<br />the drainage area, than for a, comparable peak discharge when infiltra,
<br />tion rates prevail., '
<br />34; Floods ~f rccord.-Although centinuous stage records of flood
<br />run,off are available for a period of only 20 years, historical records
<br />indica,te thc comparative magnitude of tho major floods which have
<br />occurred in the drainage area since 1866,thus extending the data for
<br />major floods over a period of 76 years. During the period of reliable
<br />records, major fleods occurred in Purgatoire River on September 30,
<br />1904; July 22, 1925; August 7, 1929; September 15, 1934; and April
<br />23; 1942. Available data indicatll that prior to the est,ablislunent of
<br />gages, or during pcriods when the gage readings of the crest stages
<br />were' not obLained, large magnitude floods occuired at Trinidad in
<br />November 1866, the summer 1883, and July 1886, andin the lower
<br />reaches of the.river on Septmeber 16, 1875, and, October 19, 1908.
<br />The majm flood occurrences are discussed in detail and relevant data
<br />for these and other floods are given in Appendix Bl
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<br />I Notprlnted.
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