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. I-~ o ~ CoO . . EVALUATIONS OF EXISTING SALINITY CONDITIONS (continued) Tables I to 20 with each table number corresponding to a stat ion number. The concentrat ions as shown were determined on a flow-weighted basis. To simplify tabulation, monthly values of flow and total dissolved- sol ids loads were rounded to the nearest 1,000 acre-feet and 1,000 tons. Th is resu 1 ted in some differences between the recorded and t he computed monthly concentrations when the flows were low, for example, below 1,000 acre-feet in the San Rafael and Duchesne Rivers. Similarly, minor differ- ences from published data in monthly concentrations occur in some instances for other stations. The water qual ity at the Lees Ferry and the four other key stations on the main stem of the Lower Colorado River has been affected by abnormal condit ions during the 1959-78 period because of low runoff in 1959, 1960, 1961, 1963, 1964, 1966, and 1967 and the storage of water in Lake Powell in 1963-78. Figure 3 shows the historical flow weighted average salinity concentration fOt" these five statiuns and the combinat ion of the Green River at Green River, Utah; San Rafael near Green River, Utah; Colorado River near Cisco, Utah; and the San Juan River near Bluff, Utah. The Grand Canyon stat ion has the longest water qual ity record on the Colorado River, 1926-78. It is of interest that the average salinity concentration for the period 1941-78 is only 51 ightly higher than the average salinity concentration for the period 1926-40, 0.B4 to 0.81 tons per acre-foot (618 to 596 mg/L), respectively. Generally the salinity concentration increases at each succeeding downstream station as a result of depletions by diversions; reservuir and stream evaporation; consumptive use by irrigated crops and phreatophytes; and by 'salt loading from natural springs) streams, solution of salt from reservoir basins, and by irrigat ion 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 concentrat ion from the below Hoover Dam stat ion to the below Parker Dam stat ion. Figure 3 shows the concentrat ion changes between the five lower stat ions and the combined four stations above Lake Powell 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 station~. This is especially noticeable for the high salinity concentrations of 1963 at the Lees Ferry and Grand Canyon stations. D. Present Mod if ied Cond it ions The 1941-78 period average present modified flow and quality at any station, as defined in this report, is the average of the flows and qual ity that would have resulted if the present (1978) level of deplet ions and salt loading instead of actual depletions and loading had occurred each year of the period. This average present modified flow and quality, therefore, represent an average condition based on the 1941-78 water supply 43