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8 FLOODS IN COLORADO s 0 '" ~ ~ ;:; c 0 c 0 ---- '" - ;; - --- --- --- ---j ~-- - V ~ "' ; c- ; 0 ~ ;; '- --- --- 1--- ~--- -~- ~,-- --- o '" ... '" ~ " , ~ , .. .. '" ... j ~ i o .. .l! ~ ! .. t ] ! ~ ~ ;;; .. '" " '" '" ,Uj UI'I'I61'1I dot! ,; '0 u Z; --- ;:; - c - --..,. --- --- -- -- 1---- 0 c 0 . '" - 0 . c / ~ . . 1; . / 0 0 is - --- --- ---, --- --- ---, - -- '" " '" ~ ~ ~ N 2 .. .. " '" o . ~..t ul 'lq~I'1I tOoO I. I METHODS OF MEASURING FLOODS 9 In the southwestern part of the State the severest floods, and the only ones of record, have resulted from the storms of September 1909 and October 1911. Precipitation during melting snow in June haa been insufficient to cause floods as severe, except during June 1927 when the resulting flood on the Animas River was more severe than. that during September 1909. In the North Platte River Basin the slight precipitation is not suffi- cient to cause floods. This basin includes North Park, a high, level area surrounded by mountain ranges, except at the lower end where rolling hills displace the mountains. The Green River Basin in Colorado, in the northwestern part of the State, lies on the western slope of the Continental Di vide, and is made up of rolling hills and some comparatively level country. The Yampa and White Rivers are the principal tributaries of the Green River. Here, as in the North Platte River Basin, the precipitation is insuffi- . clont to cause floods, The greatest flood of record in this part of Colorado was that of 1884, which was caused by rain at the time the very heavy snow cover was melting. METHODS OF MEASURING FLOODS For most of the earlier floods it has not been possible to estimate the actual discharge; the facilities 8Jld technique for making such cstimates either had not been developed at the time these floods occurred or were not available for application. Most of the later floods describcd have either been of such short duration as to make measurement nf them impossible, or if of longer duration, as on the larger streams, they have destroyed the approaches to bridgcs and other means whereby they might have been mel\Sured. Few dams exist suitable for use as weirs in computing flood discharges, and the same is true of bridges suitable f()r use in computing the dis- charge by the contracted-opening method. Discharges for the major floods of longer duration that have occurred since the establishment of gaging stations have usually been estimated from the extension of tho rating curves. However, some of these floods reached stages so far above stages of current-meter measurements that it was nooessary to obtain slope-area measurements of the peak discharges in order to extend the rating curves. This was particularly true of the Arkansas River flood of June 1921, whieh overflowed large areas. The ouly means by which most peak discharges could be measured for most 01 the floods deseribed in this paper was by the slope-area mcthod, which has been in use in Colorado since the ea.rly nineties. The proeedure gcncrally followed is to visit the stream soontr the flood, while the high-water marks are still visible, a.nd seleet ight, uniform reach as long as possible, up to 1,000 feet on the larger s reams.