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. ~ ~ ~ -1 . . .... EVALUATIONS OF LXISTI~G SALI~ITY CO~~ITIONS The Grand Canyon station has the longest water quality record on the Colorado River, 1926-74. It is of interest that the average salinity concentration for the period 1941-74 is only slightly higher than the average salinity concentration for the period 1926-40, 0.84 to 0.81 tons per acre-foot. respectively. Generally the salinity concentration increases at each succeeding downstream station as a result of depletions by diversions, reservoir and stream evaporation. and consumptive use by irrigated crops and phre- atophytes. and by salt loading by iuflowing springs, streams, solution of salt from the streambeds and reservoir basins, and by irrigation return flows. The flows of the Bill Williams River often dilute the flow of the Colorado River in Lake Havasu which sometimes results in a decrease in the salinity concentration from the below Hoover Dam station to the below Parker Dam station. Figure 3 shows the concentration changes between the five lower stations on the Colorado River. Note also that Lake Mead has a dampening and delaying effect. about 2 years. on the salinity concentrations at the downstream stations. This is especially noticeable for the high salinity concentrations of 1963 at the Lees Ferry and Grand Canyon stations. D. Present Modified Conditions The 1941-74 period average present modified flow and quality at any station, as defined in this report, is the average of the flows and quality that would have resulted if the present (1974) level of depletions instead of actual depletions had occurred each year of the period. This average present modified flow and quality, therefore. represent an average condition based on the 1941-74 water supply period occurring at the present (1974) time. This is shown for each station on Table E in Part VII. Adjustments to the historic flow that were made to develop the present modified flow included: (1) adjustments for the increase in depletion in 1974 over that for years prior to 1974; (2) adjustment of records below large reservoirs by adding the historical storage and subtracting storage releases to obtain unregulated flows at each station; and (3) adjustments for historic evaporation as compared to average present evaporation. The large reservoirs considered in these adjustments were the Colorado River Storage Project. Navajo and Fontenelle Reservoirs in the Upper Basin and Lake Mead. Lake Mohave. and Lake Havasu in the Lower Basin. Average present evaporation from the Colorado River Storage Project Reservoirs plus Navajo and Fontenelle Reservoirs was est~ted to be 568.000 acre-feet per year. (Note: This is the latest evaporation estimate pending results from additional investigations being conducted.) This would include evaporation from Lake Powell of 460.000 acre-feet; Flaming Gorge. 50.000 acre-feet; Curecanti Unit Reservoirs. 10.000 acre- feet; ~avajo. 26.000 acre-feet; and Fontenelle Reservoir. 22,000 acre- feet. These figures were chosen to represent present conditions rather 44