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Comparison of USC;S TM 1 and CDM Dilta The USGS regional oq'lations are plotted in comparison with the CDM curves for 10, 50, 100 dnd SOD-year discharges on Figures 1, 2, 3 and 4. Also showTI are the data from Table 2 for comparison. It can be readily noted that there is little, if any, difference between the USGS and Cm! curves for 10 and 50 year events but there is some difference at the 100 year level and a significant difference at the 500 year event. Analysis of Runoff ~ecords Generally, the regional ~nalysis given in Ref. 1 is quite good. A sub- stantial aIJour.t of data has been analyzed in cowpiling the H'gional esti- :r.ates. The CDH report, in separating the data, took a different path than IM III ,,;here it was judged that insufficient records existed to separate snO<mIelt and rainfall floods. (Ref. 1, p. 43) To the writer, the separation of rainfall and snowmelt floods as done in the CD:1 report must be regarded as highly auesti-""~9J:e because the data reported by USGS generally do not reveal whether a peak flow in, say April, Mayor June, is from sno,,-melt, rainfall or a combination of both. Still, the technique is useful for it sometimes results in rather large positive s\e,,, coefficients "hi.ch then extrapolate to high values for the 500 year flood. As an example, see gages 1450 and 1720 on Table 2. These were ~eveloped from positive skew coefficients (0.45 and 0.81 respectively) 2~~ the SOD-year estimates are considerably larger than the USGS regional ~r-edictions. ~his ext~apolation to large SOO-YG2r values hel_ps to explain serne of the larger, less frequent flmvs ':vhich t.\~ill plot considc:rably above curves '!..;ith ske'!.\1s ~;:ar 0 or negative. The '!.\Titer feels that the use of this "rr;ixed pepulation" a.pproach has an intuitive appeal and provides a pr::t.ctical ./ :~'::,""; :~j2thod to help explain som2 of the high flm\1s experienced. On the other -. 'Cc:.. ."J 0""', .~,-~ \ d' fl' O. . t' . 1 ~ 0 s,zc. '14..,:L .:.::'i.1 , tne ~se 0 _ar:;e posltlve S1<'"-21:..:5 goes 8ga:inst lle reg1.0na 5,,2",,-5 glven 'cP-.... ~a.." -en Reference 5 (1,RC TB 17) which gives values ranging near -0.1 to -0.3 ~ ";"~ Cor the area. ~e Dust admi.t that these extrapol~tions of 10 to 25 years of data to 500 .,....':dr return periods is highly question,J.ble. The. facts are, hOi~oever, that o,i.;h rlm~'s do occur on a regular c2.sis at different locations in the Rocky :'~:2~t~i~ region and that our statistical approa~hcs, with all t!leir re- :in( ~c~ts, ~ust have ~ore data before accurate forecasts can be ~ade. c~all Ecsin Floods ~~ei ther 1:1 1 ,lor the CJ):.l report p-covide g\1idance for EstiT:'.ating floods :~OM the s~311 ~asins (0-10 sqG3re miles). It is strongly felt by the ~~rit~r t~at t~2se s~all basins will 2x?eri~nce rainfall flooding and the ,only recourse is to apply rainfall-runoff modeling. The SCS ;nethod "'as 2pplied for this purpose. -3