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. ~ o w -~ ~ QUALITY OF WATER COLORADO RIVER BASIN PROGRESS REPORT SUMI>lARY . This report prepared and updated every 2 years presents the various important water quality aspects of the Colorado River. Although several water quality parameters are discussed, the major part of the report is allotted to salinity (total dissolved solids) because it is presently the most serious quality problem on the river system. The historical, present modified, and future salinity conditions of the water~ of the Cotorano River down to Imperial Dam are presented in this report. The historical is represented by a tabulation of the recorded or e~timated pA~t condition at 20 quality of water stations for the 1941-78 period. Although various other studies have included different periods of records such as the 1906 to present period, this report has used the 1941-78 period because most salinity records in the basin do not extend back beyond about 1941. Thc present (1978) modified condition includes adjustments to the historic condition hased on the assumption that all developments existing in 1978 were in operation for the full 1941-78 period. Estimated future conditions are shown for the years 1990, 2000, and 2010. They are estimated projec- tions, including past projects, presently authorized developments, projects proposed for authorization, and other future anticipated projects. Under historic conditions the flow weighted average concentration of dissolved solids of the Colorado River at Lees Ferry was' about 560 mg/L, below Hoover Dam about 692 mg/L, below Parker Dam about 696 mg/L, and at Imperial Dam about 769 mg/L for the 1941-78 period. Under present modified conditions (that is 1941-78 historical flows modified to reflect present depletions), the concentrations would llave been about 621, 760, 788, and 878 m~/L, respectively, at the three stations. For the data in Table D the projection of future water quality conditions was based on 1941-78 averages rather than a year-by-year study. These data were developed usin~ the present modified method as in the past. The Colorado River Simulation System (CRSS) was also used for future water quality projections. A description of the model is given in Part X, Special Studies, alon~ with the projections. We feel that the model will produce better water quality forecasts due to its ability to more accurately describe the river basin and its greater degree of flexibility. We anticipate that all future progress reports will show only the CRSS results. . The CRSS flow and salinity projections in Table F and Table G, Part X, Special Studies section are considered preliminary for several reasons. The hydrologic data base for the computer model was not completed beyond 1974. It was intended to use a data base from 1906 to 1978. Also, time did not allow thorou~h checking of the projections against the input data to insure proper operation of the model. The projections found in Table D based on annual averages remain our best estimate at this time. I