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<br />ABSTRACT <br /> <br />The Colorado Division, of Highways plans to build <br /> <br /> <br />I 70 through DeBeque Canyon near Grand ;Yunct.ion. l\ pre- <br /> <br /> <br />liminary alignment has :been proposed. The Hydraulics <br /> <br /> <br />Unit was requested by Dis.t:,ict III to estimate a design <br /> <br /> <br />flood, set maximum permissible encroachments, and re... <br /> <br /> <br />conunend the necessary st;abiliza t.ion for the Colorado <br /> <br /> <br />River through the canyon. <br /> <br /> <br />It was decided tha'~ a 50 y,'lar frequency flood would <br /> <br /> <br />be used for design. The design discharge was obtained <br /> <br /> <br />from analysis of stream gaging si:ation records. Excevt <br /> <br /> <br />for gage 9-0725, the x'ecords were genera.lly for two <br /> <br /> <br />periOds of time. The first period was in the early part <br /> <br /> <br />of the century when diver,d.ons [tad little effect on peak <br /> <br /> <br />discharges, and the second 1,vasnore recently "hen controls <br /> <br /> <br />and diversions had some efh,ct; ,:;n peak discharges" Gage <br /> <br /> <br />9-072!3, with records for bC'i:h p'"riods of time" showed a <br /> <br /> <br />33% reduction in the 50 year flood calculated for years <br /> <br /> <br />1934 - 1966 as compared to the 50 year flood calculated <br /> <br /> <br />for the years 1900 - 1933. <br /> <br /> <br />~'hree additional gages in the basin were found t;ha t <br /> <br /> <br />had enough years of record '1:0 include both time periods and <br /> <br /> <br />were not affected by controls or diversions. There <br /> <br /> <br />appeared to be an 8% drop in the statistical 50 year flood <br /> <br /> <br />computed for the two time periods. It was assumed that <br /> <br />i <br />