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35. Flood frequency.-Floods, L 0., discharges exceeding bank- full stages, may be expected more frequently in particular re,aches because of the varying channel capacitios. A curve of probable flood occurrences was constructed for Purgatoire River at Trinidad by standard methods, using the period of record from Jime 1922 through June 1942. Rcliable and historical flood records indicate that five floods exceeding a discharge of 30,000 cubic feet per second have . occurred during a period of 76 years. An inspection of the probable flood occurrence curve for Trinidad indicates that five or six floods equaling or exceeding this magnitude may be expected per 100 years (cbart No.6, appe,ndix B).' As a period of 21 years and 17 years elapsed between the 1904,1925, and 1942 major floods, and five major floods occurre,d which exceeded 30,000 cubic feet per second during 76 , years of record, it was conclu(led that the probable flood occun'ence curve. constructed from the period of record from June 1922 to June 1942, is represcntative of flood occurrences per 100 years. ' 36. Designjlood criteria.~The maximum flood of record at Trinidad eccurrcd in September 1904 when a peak discharge of 45,000 cubic feet per second and a volume of approximately 50,000acre,feet were estimated.. Frequ~ncy investigations at Trinidad indicated that .a flood of thiS magllltude would occur about, once III 100 years. It IS considcred that the design of flood-control improvements should be based on providing adequate protection against a flood of this magni- tude if found economically feasible. Such a degree of protection would avoid inducing a false sense of )lecurit,y which might be engen- dered by improvements designed to provide flood protection for floods of lesser magnitude. Plans investigated and recommended are dis, cussed in paragraphs 60 to 78. 37. Maximum probable flood.-The maximum probable flood was developed from consideration of the maximum probable rainfall occur- ring when minimum loss rates prevailed in the drainage area under consideration. A peak discharge of 127,000 CUbic feet per second and run,off volume of 175,000 acrecfeet were computer! for these conditions. The maximum probable flood hydrograph is shown on chart No.9, appendix B.' The development of the maximum probable flood is given in appendix B.' 38.' Extent and character of jlooded area-General.-Although there are areas subject to ovorflow in other portions of the main valley and on the tributaries, the most impertant are along the main stem upstream from the head of the canyon section, near Alfalfa, to the Sopris Dam site. The probable overflow area is indicated on maps Nos. 2 to 7, inclusive,' appendix A. In this portion of the flood plain, which includes that in Trinidad, there is a total of 5,000 acres of which 370 acres are urban lands. Between Alfalfa and the mouth of the river, there are some cultivated areas, but they are relatively unimportant. Flood damages in this portion of the valley are not extensive, and crop damages are limited largely to the vicinity of Higbee and the agricultural Rrea below the mouth of the canyon. Major floods cause some damage to the irrigation ditches, diversion dams, and the bridges. There are no important overflow areRS in Long Canyon. 39. Extent and character of jloodedarea-1'rinidad.-An area of 370 acres in Trinidad is in the overflow plain. This contains rail, 1 Not printed.