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. . W~T!;~ ~!;SOU~(!;S (O~SULT'\NTS , I~C 2765 South Colorado Boulevard. Suite 218. Denver, Colorado 80222. (303) 756-3821 November 24'1197~-,c----, D . (",Y i."'~lI5'01'r-\~D~--1ml.. ," -". " "', n "-CJ..' ',_ . , [:'.'- '. j, -'~' '/ \".: ::. ~::~; ill i. NOV271978 .~:: ~_, ':i COLO-~ ",', ,j CO ,WAIt.ii - ifY.SERVATlON BOARD Mr. William P. Stanton Senior Water Resource Engineer Colorado Water Conservation Board 823 State Centennial Building 1313 Sherman Street Denver, Colorado 80203 WRC File: Subject: 1281/4 Supplemental Input to the Hydrologic Addendum Dear Mr. stanton: We have received your comments of November 22, 1978, regarding the Hydrologic Addendum for the South Arkansas River floodplain study near Salida, Colorado. We have adjusted the discharges for 050 presented in Table-3 of the Hydro- logic Addendum submitted to you on October 27, 1978. Four frequency-discharge curves for respective South Arkansas River reaches are attached. The adjusted values for 050 were based on a comparison of 050 as a per- centage of 0100 for eleven streamflow recording stations located in the Upper Arkansas River basin. These stations were those used in the South Arkansas River/Poncha Creek floodplain study. Utilizing the elevation stations, we found that the 050 as a percentage of 0100 ranged from a low of 82.3% to a high value of 98.2% with an average figure being 90.9%. The average figure for the three stations used to plot the peak flow-area-frequency curves in the South Arkansas River/Poncha Creek study was 87.3%. Similar values for the three stations ranged from a low of 83.8% to a high of 91.7%. Applying the 90.9% figure, we computed 050 values for the four stream reaches presented in Table-3 of the Hydrologic Addendum. The recommended peak flows (including the revised Q50 ) for the four reaches of the South Arkansas River are presented below. Plots of the frequency-discharge curves for the same reaches are presented in Figures 1 through 4. The curves plotted as a smooth function on the logarithmic probability graph paper.