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~:31 00 'I ~ T 11 0 POL IT ,\ N W ,\ T E R ]) 1ST HIe T A somewhat similal" though so far as can be verified from the available data, shorter and less severe drought period was recorded in 1897-1904 on the Colorado River. The curve of Fig, 6 is there. fore computed and plotted on the basis of the mean annual runoff records (and estimates) for the period 1905-1946, due solely to statistical con8ideration~J to avoid. the inclusion of two severe drought periods and one abnormally wet period in the approxi- mation to the long-time mean, Also merely on a statistical basis, it is probable that such major dry periods as noted will recur with a frequency of about twice in a century, or slightly oftener, and with flows nol exceeding the Compact "guarantcc" for a decade or longer. There is much valid evidence that longer and more severe droughts have occurred in the past, prior to stream gaging, than during the relatively sho,'t historical period of record. It must therefore be assumed that such dry periods will recur in the future. E\'en on the basis of the historical record, however, the Compact minimum flow will not. ue dependably and permanently available without. the ultimate construction of additional large up. st.ream reservoirs, which in t.he aggregate must equal or even exceed Boulder Canyon re~~rvoir in capacity. Columdo Rive)' ,,,nter q/lalit)! The outflow from Boulder Vam during the water year t.o Sep- tember 30, 1946 was slightly improved in qualit.y, being lower in dissolved solids and total hardness as compared with the previous year, according to available preliminary dat.a. At. the aqueduct intake near Parker Dam. monthly samples show a total hardness averaging 326 parts per million for ]946 compared with 343 parls per million for 1945 and a record maximum of 420 parts per mil. lion in April 1941. Similarly, natural Colorado River water at t.he District softening and filtration plant at La Verne showed a re- duction in total hardness of 10 parts per million from 363 for 1944.45 to 353 for the current year H145-46. The specific con- ductivity "K", which is an approximate index of total dissolved selids, avemged 102,5 in 1946 at the aqueduct intake, compared with 104,5 for 194;; and 1944, Tot.al dissolved solids averaged about 650 parts per million at the aqueduct intake in 1946 for a decre,,,e of 190 parts per million, compared with April 1941. In Lake Mead, just above Boulder Dam, a dozen wat.er samples at 50-foot depth intervals give the foil owing average anaiyses for August 1946 and for one year previously. These averages are weighted in proportion to the storagc volume represented by each .