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<br />. <br /> <br />0141 <br /> <br />. <br /> <br />. <br /> <br />the alluvial section reaches a maximum width of about three-fourths <br /> <br /> <br />mile. The conformation of the drainage basin has developed a stream <br /> <br /> <br />pattern in which tributary streams are comparatively short and drain <br /> <br /> <br />small areas. <br /> <br /> <br />13. The average stream slopes vary from several hundred feet per <br /> <br /> <br />mile l.n the headwater reaches upstream trom Veston, Colorado, to about <br /> <br /> <br />26 feet per mile l.n the reach from the mouth of Long canyon to Trinidad. <br /> <br /> <br />In the alluvial section slopes decrease to about 14 feet per mile.. <br /> <br /> <br />These slopes are conducive to rapid runoff and high channel velocit1.es. <br /> <br /> <br />The Purgatoire River is perennial at Tr1.nidad and the greatest portion <br /> <br /> <br />of the annual runoff occurs during t.he period when snow melt begins in <br /> <br /> <br />the mountains and continues during the months when thunderstorms occur <br /> <br /> <br />most frequently. Records indicate that flood hydrographs resulting even <br /> <br /> <br />from general storms are extremely sharp with high peaks and varying <br /> <br /> <br />volumes. A good example of the varying conditions occurred during the <br /> <br /> <br />general storm of April 1942 when a mean daily flow of 12,400 cubic feet <br /> <br /> <br />per second and a peak discharge of 35,000 cubic feet per second were <br /> <br /> <br />computed by the United States Geological Survey at Trinidad for April 23. <br /> <br /> <br />The volume of this flood was 68,200 acre-feet for the period April 19- <br /> <br /> <br />29 inclusive. On August 12, 1942, the recorded precipit.ation at <br /> <br /> <br />Trinidad and North Lake was 0.15 inch and 0.07 inch, respectively. <br /> <br /> <br />However, a local thunderstorm occurred between these two stations and <br /> <br /> <br />resulted in a peak discharge of 12,100 cubic feet per second with a <br /> <br /> <br />mean daily flow of only 768 cubic feet per second at Trinidad. <br /> <br /> <br />14. Estimated channel capacities. - Channel capacities, or the <br /> <br /> <br />capacity at which general flooding begins, for the Purgato1.re River <br /> <br /> <br />below the Trinidad dam site at Trinidad and vicinity, is 15,000 cubic <br /> <br /> <br />feet per second. This is also the estimated minimum capacity dO~1n- <br /> <br /> <br />stream from Trinidad through the reach to San Francisco Creek which <br /> <br />~ <br /> <br />APPENDIX B <br /> <br /> <br />. <br />