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,,'.:".: ' ;',; oj,., ". ,~~i~j?r~Bt ':, ..:. .'..-';:'.,' '. .'.' '. : .~ ."i:~~ ,~::~ . . . , ':. ,~'. :.:;'.:?~:!.'t~f;::;,> ..' ',::,.:",..,' . ',...'. " .-,'. . oaOJ;)! :-'.. .._~ . (~-, -- 32 degrees fahrenheit varies from 28 April at Pueblo to 24 June st Lake Moraine. The average first-fall occurrence of frost at the same locations are 10 October and 23 August. See Table 4-3 for additional temperature data. '.: ~,; ::{':.,:.' '~ ':~ ;-" ',., ,'/:~~":-~.': :,. U. Evaporation. Evsporation records for the basin are rather sparse and incomplete. Pueblo City Reservoir has an average total Class "A" land pan evaporation for the period May through October of 57.72 inches. Sugarloaf Reservoir has an average total pan evaporation for the period July through September of 14.62 inches. Evaporation records are being kept at Pueblo keservoir and 4 years of record give an average total pan evaporation for the period May through October of 57.78 inches. See Table 4-4 for additional evaporation data. . ; 4-03. Storms and Floods. a. General. The Arkansas River floods are of two general tYpes: "Spring Floods" which result from snowmelt, often aug- mented by thunderstorm runoff; and "SulIDDer Floods" which result entirely from thunderstorm runoff. The spring floods are characterized by moderate flows of long duration, engendering a large volume of runoff. Most aummer floods are characterized by high discharges and relatively small volumes of runoff. b. Flood lIistory. lIistoricsl records include reference to many floods in the Arkansas valley above John Martin Reservoir. The earliest known flood in the area occurred in 1826, long before permanent settlement in the valley. Other reported floods oc- curred in 1859, 1864, 1869, l8B6, and 1894. These floods were generally confined to the reach from Pueblo downstream to below the present site of John ~~rtin Reservoir. "'. :{~); ':r". c. Flood of June 1921. Weather Bureau records show that during the period 2-4 June 1921, precipitation over the watershed between Canon City and Pueblo was from 3-to-5 inches for the 48 hours ending the afternoon of 4 June. The streams that enter the Arkansas River between Canon City and the mouth of Chico Creek, 16 miles below Pueblo, were the principal contribu- tors to the flood. Ibe flood wave increased rapidly as it pro- gressed downstream. Floodflows from many tributaries contri- buted to flooding at Pueblo and caused the flood of record. The peak discharge at Pueblo was 103,000 c.f.s., which increased to ::00,000 c.f.s. when it reached La Junta. Allove La Junta, ti,e .. ffect.. of vslley storage on peak ettrition we.. ohscl1r..d by trihutary inflow. lJelow La Junta, tho trihutary inflow waH Hlllall, and the valley storage effect more markl'd, a8 evidtllll'"d by the decrease in peak discharge to 1Il7,OOO c.f.s. at Las AnilllaH OIlU to 165,000 c.f.s. at Lamar. t:\ 4-5