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. 18 FLOODS IN COLORADO Precipitation, in inches, in the SanJuan region, Sept. SB to Oet.S and Oct. ,4.-8, 1911 Altitude T()tal Total Station (lee,) sW;;'>- Oct. 4 Oct. 5 Oct.S Oct. 4--6 . . - - - Exposed to IOUthweat: 7,610 ~30 ..... US Fort Lewls__u_____________________________ -----r7f .nrir Durango.. ___ ___uunU___n__________."h ..... ~.. .n_____ a<. Terminal Dam___n______________n________ "300 3.80 .72 ~80 --'-~ir 3.62 C___"mn'n""'__hnnunm_'.. '.000 ... .10 H. ~72 saverton_o_____un_.._______n_____._.____ 9,400 ~80 .20 '.00 --Tr:-- .. .. Gladstone. _ __. n___ _u __.. 0_ on._ __ __ __. .-. 10,400 ... .11 8.00 8.1. b=~~~~::::::::::::::::::::::::::::: 7,108 ~.. .16 as7 ----:49" ~82 lo,OUi 4.16 ..OIl I." LB3 Sheltered rrom southwest: 1.62 Mancos...._________...__________...._______ 7,0311 .... . OIl '.04 nnn__ EllI'6ks_n......__n___n__._______un...__ 10,000 1.11 .-.o'-sf .47 -"'.-i5- .47 Rico..____..n___..___________...________u_ 8,824 ~03 1.11 .... Telluride.. .___~~_._.~_._______u_..nu.___ "7'" .... ... 1.117 .20 1.79 Chromo.. .__u_nn_~._n.___._n_U...n._ 7,600 .... ... .... ..1 ~" lrontou_nn...uuu___u........n__nu.. 9,800 U. .21 U. . OIl ... Nortbeastern Slope of San lUlU1 Mountains: 9,610 ... .71 1.94 ~60 Wagon Wheel Qap Expedtnont Sta.tion".h ....:04. Platoro. ._~_uu_n_...._____u&._. ~~.u____ 0,800 1.78 ... 3.25 .... The San Juan Mountains in southwestern Colorado are much higher than any mountains southwest of them, hence the moist air was deflected upward and, on cooling, deposited much of itS moisture in that rcgion, The amount of precipitation was influenced not only by altitude but by the de~ee of exposure to the storm. Mancos is sheltercd by Mesa Verde; and Eureka, although on the south slope of the San Juan Mountains, is in a deep, narrow valley cut off by high mountains. North and east of the San Juan Mountains the precipitation was much less, averaging about 1.5 inches for October 4-6, except along the Sangre de Cristo Mountains, which form the eastern boundary of the San Luis Valley. The part of the San Juan Mountains forming the western boundary of the valley is not as wide as the northern part in the Silverton region and caused less loss of moisture from the air crossing that part. Consequently, when the air currents reached the Sangre de Cristo Mountains and Were deflected upward, heavy precipitation resulted. Although no rainfall records for the western slope are available, there is evidence of severe floods on all streams draining that slope, indicating heavy rainfall. At Hermit Lake, at an altitude of 10,000 feet on the eastern slope just below the crest, the precipitation October 4-6 was 3.68 inches, At La Veta Pass it WIIS 2.01 inches. The only other record of precipitation in the State that exceeded 2 inches for the period October 4-6, 1911, WIIS 2.35 inches at Marble, at an altitude of 7,950 feet on the north slope of the Elk Mountains, a high mountain IIllI88 somewhat west of the central part of the State. . METEOROLOGIC CONDITIONS 19 JUNE 2--7, 1921 The general storm of June 2-7, 1921, was the only storm of record that caused severe floods and unusually high water in nearly all parts of Colorado. The daily weather maps for the first week in June 1921 show that an area of high pressure exceeding 30,5 inches appeared June 1 over the Province of Alberta. By June 2 it had displaced a low pressure area over the upper Missouri Valley, and on JlUle 3 it was over Manitoba and the northern part of North Dakota. Between June. 3 and 4 the movement of the high pressure area was very slow, and .t ad vaneed only as far as northeastern Minnesota. By June 5 it had reached a point over the Great Lakes. During this period an area of low pressure, 29,7 inches, had de- veloped over western Arizona and remained over the southern Rocky Mountain plateau for several days." Warm, moist air from the southern States and the Gulf of Mexico was drawn into the low- pressure area. When this moist air met the cold front from the north it was forced upward, and being cooled, its moisture was released as precipitation. The effect of the cold front was increased by the foothills, espe~ially in the vicinity of Pikes Peak, which is a semi- detached mountain mass rising abruptly from the plains. The following table shows the rainfall during the period June 2-7 along the foothills and the Pikes Peak region. Stations are listed from north to south, Rainfall, in inches, aWng foothills and Pikes Peak region in South Platte and Arkansas River Basinst June S-7t 19111 Alti. June StatIon tude Total (reet) . . . . 6 7 I- - 1- - - - South Platte lUvor Basin: li--~~~~~ ..... 0." 0.47 1.88 0.47 0." -0:00- ..7 .,... ... 1.07 1.46 ... ... .. ~... ..7 1." 1.88 ... ... .6. .. .,O(J() d:ar .... ... .44 ... .0' ~82 ....7 .14 1.19 .71 ... ... .... 6,000 .01 ... .W .20 .20 .17 1.74 0,450 d:23- ~16 ... ... .110 .00 .." "120 ... I." 1." ... .37 .... 6,492 .14 .17 .... ... ... .16 .... 8,140 .23 1.20 U, .., .... .47 ~... "20. ... ~78 .110 Tr. .18 ." 4,27 6,'" ... .1. ~23 1.07 ..0 .20 4.42 Ark:J~~u;{~e-t-jjasID:r.n.-~....-.---...~~~.-.- ..... _.~._- 1.10 ... .1. ... .20 3.10 ~k~uM~~fne=:::::::::::::::::::::::::::::: 7.200 ... -.~66- ~.. .82 ,00 ... '.23 10,265 --.-~- 3.'" 1.40 .18 Tr. ". vtemont EJ:periment StatioIl._nn......_ ..... un.. ~" 20. 1.43 ... .03 7." ~~l~~~~~~it~~iiiiiiiiiiiiiiiiiiiiiiiii 10,100 u:ii" .03 ~0Il I." .37 ..1 .... ..... ~.. ~.. --:Or --'-i5- .... '.086 1.94 I." 1.12 ... .... ~... n.... .30 ... .76 ... --:21- ..'" ~... --..~- .20 ... .30 Tr. 1.32 . 84 88 I Corrected record. II Follansbe6, Robert, and :tones, E. E., Tbe Arkamal RIver ftood of lUIle 8-1, 1021: '0. S. GeoJ. Survey, Water.Supply Paper 0187, pp. 1l~120 1022.