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(8 :;:~q,lETROPOLlTAN WATEH DISTRICT I l is on the order of 10 per cent for the various items. With Lake Mead now refilled with relatively pure flood runoff, this trend of improvement in the water diverted into the aqueduct should con- tinue during at least the next two years. In contrast to such adverse changes in Colorado River water quality at the aqueduct intake during fiscal year 1951-52, as com- pared with the previolls year, the natural water reaching the La Verne softening plant after long storage in Lake Mathews (table 6, page 41) was practically identical for the last two years. Hardness and sulfate content both showed a slight but worth-while redllc- tion. On the basis of the present softening to 125 ppm, the present hardness removed is 187 ppm or 71 per cent of the 262 ppm with 1943 conditions (when thc hard 1940-41 aqueduct diversion, reached the softening plant). This represents a substantial economy in softening costs and necessary plant capacity. In Lake Mead, at a long-used sampling point just lIpstream from Hoover Dam, between the intake gate towers, a dozen water sam- ples at 50-foot depth intervals gave the following average analyses for September 30, 1952 (the must recent available data) and for com- parable dates for previous years. For comparison there is listed al~o the most saline analysis on record for the re.c.;ervoir, as of March 31, 1941. These representative averages are weighted in proportion to the storage volume corresponding to each sample, as the reservoir contents near the dam are sharply stratified in summer after the inflow of relatively soft flood runoff, and the top 50 feet of storage, for example, though only about 11 per cent of the water depth, includes 24 per cent of the present total storage and 27 per cent of the active storage contents of the reserwir. This involves to some extent an over-simplification of the analytical data now available for Lake Mead, as the more distant sections of the reservoir do not necessarily follow the same stratification pattern as is observed near Hoover Dam. But the accumulated laboratory data are now so nllmerous, variable and complex as to be practically useless unless by some such statistical simplification, the annual trend of changing quality of the I'esenoir water can be disclosed. There is now indicated (1952 change since 1951) a very sub- stantial cllrrent decrease in salinity and hardness of Lake Mead water, a definite reversal of the recent 1949-1951 trend that re- sulted directly from the marked deficiency of runoff for 1950 and 1951. Not only was this season's greater inflow gofter and less saline but the larger holdover content of the reservoir now gives greater weight and effect to the pllrer upper strata, in comparison i ,j