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<br />nll1"f~08 <br />(Jy '..' \; ',_ <br /> <br />GENERAL EFFECTS OF DROUGHT ON VVATER RESOURCES <br /> <br />B37 <br /> <br /> <br />Continllous records of the sediment or dissolved <br />luatter in streams are obtained at fe'" places in the <br />Southwest.-far fewer than records of t.he quantity of <br />surface water. Most of the records of quality were <br />begun during- the recent drought, and very few begin <br />as early as 1942; we have, therefore, little basis for com- <br />paring- the quality of surface water in the droug-ht years <br />with that in earlier and wetter years. Data that show <br />the effect of drought upon the quality of g-round water <br />firc even mOre meager, and all inferences drawn from <br />the data must be tentative. <br />The meagerness of quality data imposes still another <br />handicap upon this report.. Practically the only data <br />suitahle for evaluation of the treuds in quality wit.h <br />time are from It few stations on the Colorado River, Rio <br />Grande, and Pecos I~iver, and such evaluations belong <br />properly in the detailed discussions of the effects of <br />dl'oup;ht in individual river basins. But in order to <br />draw any conclusions at all on the g-eneral effects of <br />droug'ht npon water quality, it is necessary to cite some <br />of those details to elucidate t.he following- discussion. <br /> <br />QUALITY OF SURFACE WATER <br />The long-est. record of t.he qualit.y of surface water in <br />the Southwest is that. for the Colorado River at Grand <br />Canyon, Ariz., which indicat.es both the phyical and <br /> <br />" <br />w <br />~ <br />'" <br />'" <br />u 20 <br />.. <br />~ <br />0 <br />'" <br />z 18 <br />0 <br />:J <br />~ <br />" <br />;; 16 <br />u: <br />~ <br />0 <br />z <br />" 14 <br />'" <br /> <br />EXPLANATION <br /> <br />Runoff <br /> <br />Dissolved solids <br /> <br />Sus ended sediment <br /> <br />1931-40 drought <br /> <br />1\ <br />I \ Runoff, 1926-57 mean, <br />12.4 million acre-feet <br />1\ <br /> <br />12 <br /> <br />"' <br />z <br />re <br />~ 10 <br />o <br />"' <br />z <br />o <br />j B <br />" <br />;; <br />u; 6 <br />o <br />:J <br />iil <br /> <br /><M <br />\':: <br />~j <br /> <br />8 4 <br />> <br />~ <br />o <br />"' <br />"' <br />is <br /> <br />1926 <br /> <br />1930 <br /> <br />1935 <br /> <br />1940 <br /> <br />FIGUHE lO,-Annual runoff, dissolved solids, and sllspended-sediment load of the Colorado River near Orand Canyon, Ariz., 1926-57. <br /> <br />1945 <br /> <br />1950 <br /> <br />1955 1957 <br /> <br />chemical quality of the water and also provides essen- <br />tial dat.a for the interpret.ation of changes in the quality <br />of wat.er in Lake Mead. Beginning in 1925, this record <br />spans periods of greater than average runoff and <br />drought periods when runoff was significantly less than <br />the long-term mean. Graphs showing- the annual run- <br />off, in millions of acre-feet, and the total annual sedi- <br />ment. load and the total dissolved minerals, in millions <br />of t.ons, are presented in figure 10. These g-raphs indi- <br />cate t.hat t.he total load t.ransported by the river is <br />generally great.est in years of high runoff and less in <br />years of low runoff. Runoff was less than the long-term <br />mean in 9 of the 14 years 1943-56, and during those <br />9 years the suspended-sediment load and the dissolved <br />load also were less than average. Except in the 4 years <br />1953-56, the effect of this drought was less intense than <br />t.hat ofl931-40 (I" B28). <br />Although t.he sediment load, like the streamflow, is <br />far less during drought yea.rs than during- years of <br />normal precipit.at.ion, the proportion of sediment to <br />water is commonly increased during drought years. <br />This is best. shown by records of the monthly suspended <br />sediment at Grand Canyon, which has ranged from <br />156,000 tons in 298,000 acre-feet of water in January <br />1944 to 134 million tons in 2 million acre-feet of wat.er <br /> <br />(--I <br /> <br />I~ 1953-56-4 <br />drought <br /> 520 <br /> "' <br /> z <br /> 0 <br /> 440 >- <br /> ~ <br /> 0 <br /> "' <br /> z <br /> 360 0 <br /> :J <br /> ~ <br /> " <br /> ;; <br /> 280 >-' <br /> z <br /> w <br /> " <br /> is <br /> 200 w <br /> "' <br /> 0 <br /> w <br /> 0 <br /> z <br /> 120 w <br /> "- <br /> "' <br /> " <br /> "' <br /> 40 <br />