WSP10396 - Laserfiche WebLink

Home Browse Search

B38 DROUGHT IN THE SOUTHWEST, 1942-56 in August 1929, A plot of the monthly runoff against sediment load (Thomas, Gould, and Langhein, 1959) shows a rather consistent straight-line (exponentIal) relationship for the months of August through March in the period 1926 through 1950, During the months of maximum runoff (April through July) the sediment concentration was,generally less than would he expected from the sediment'runoff relation defined hy the months March through August. In 7 years of low runoff (1931, 1933-35,1939-40,1946) the sediment concentration gen- erally was greater than in corresponding- months of other yea.rs, Thus in these drought years the sediment concentration was higher during the annual freshet and during other months of the year than in years of more abundant precipitation, In each of the years since 1942, the annual sediment load at Grand Canyon has heen 50 to 100 million tons less than would he expected on the basis of a curve established by data for the period 1926 to 1941, although the annual runoff and the seasonal distrihution of run- off since 1941 have not heen significantly and consist- ently different from those for earlier years, On the hasis of data to 1950 it was tentatively concluded hy Thomas, Gould, and Langhein (1960) that this change in relationship Was an effect of the Southwest drought, Lake Mea.d is within the drought area, as is most of the sediment-producing area of the Colorado River hasin-the basins of the Virgin, Little Colorado, San ,T uan, and smaller trihutaries that enter the Colorado helow the mouth of the Green River, On the other hand, the principal sources of water flowing- through the Grand Canyon a.re far to the north, in a region where precipitation was, generally average or ahove, at least until 1952, 'rhus the reduced proportion of sediment . to runoff since 1942 is attrihuted to reduced streamflow in sediment-producing trihutaries and a corresponding reduction in their contrihution of sediment to the main stem, A douhle-mass plot of cumulative annual runoff against sediment load at Grand Canyon (fig. 11) indi- cates a fairly consistent relation during the years of high discharge 1926-30 and a new relation, with less sediment in proportion to runoff, during the drought years of the 1930's, In the period 1943-54 there was still less sediment in proportion to the runoff, although the runoff for several years was greater than the long- term mean, During the succeeding years of pronounced drought in the headwaters (1955-56), the sediment- runoff ratio was comparable to that during drought years of the 1930's, DISSOLV~ SOLIDS IN STREAM WATER The dissolved matter in surface water is derived from the soluhle minerals in rocks and soils with which the water comes in contact, When this contact is brief, as in the case of direct runoff from rainfall or melting snow, the resulting surface water is generally low in dissolved solids, Water that enters ground-water res- ervoirs generally is suhject to prolonged intimate con- tact with solid mineral matter and attains a higher dis- solved-solids concentration than it would in overland flow across the same materials, As a result, the water in a stream normally has the greatest concentration of dissolved solids when the stream is receiving all its water from eftluent ground-water seepage, as during rainless periods. The hase flow of some streams includes a component from one or more sources of highly mineralized water, and the difference in concentration at high and low stages may he marked, Streams whose qualities are strongly affected by drought are those that have large and relatively constant inflows from saline springs, Outstanding examples of such streams are the Salt River in Arizona (Thomas and others, 1963c) and the Pecos River in New Mexico (Thomas and others, 1962). The salt content of the Pecos River increases consid- erahly at Malaga Bend in southeastern New Mexico, . where springs discharge hrines (chiefly of the sodium chloride type) into the river channel (Thomas and others, 1962), as shown hy the records from gaging stations upstream and downstream from the springs (fig. 12). In 20 years (1938-57), the mean discharge at the downstream station (Pecos River near Red Bluff) has heen ahout 2112 percent greater than that at the up- stream station (Pecos River east of Malaga), corre- sponding to a 2-percent increase in drainage area, In only 4years (1941-43, 1955) has the mean annual dis- charge exceeded the 20-year average, and in each of those years the quantity of inflow hetween the gaging stations was less than 2 percent of the total streamflow, In years of less than average streamflow, the inflow hetween the stations is a larger proportion of the flow measured at the downstream station; in several years this inflow ex- ceeded 10 percent, and in 1954 it was more than 30 percent of the total. The increase in dissolved solids in the Pecos River hetween the Malaga and Red Bluff stations averaged ahout 500 tons per day in the drought years 1943-57, and more than 1,000 tons per day in the wet years 1941- 42. Storm runoff from a drainage area of 350 square miles was douhtless responsihle for some of this increase in dissolved solids, particularly in wet years such as 1941 and 1942, and return flow of ilTigation water also contributed some; hut the saline springs and seeps in the hed of the river at Malaga Bend prohahly are re- sponsihle for most of the increased mineralization during periods of low flow, or during droughts, This water rises under artesian pressure from underlying heds of halite (Rohinson and Lang, 1938), presumably