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Show 4 Utah part moving southward through the lower part of the causeway. The chemistry of the lake is now controlled by the interchange of dissolved- solids load through the causeway, as well as by changes in the salt crust and by volume changes." In 1963, shortly after construction of the causeway, when the lake declined to its lowest recorded stage, both the north and south parts were probably saturated with respect to sodium chloride and a salt crust probably formed on the lakebed north and south of the causeway ( Madison, 1970, p. 12). As the lake rose during the following years, the south part began to freshen with the increasing lake volume and because of dissolved- load loss to the north part. The net dissolved- load movement to the north part, which probably was already saturated due to the low lake altitude, may have resulted in additional deposits of sodium chloride in the north part. The concentration of dissolved solids in the north part remained at or near saturation ( 355 grams per litre) through 1973. The dissolved- load loss from the south to the north part continued until about 1972, when the loss was only about 0.01 billion tons ( 0.009 billion t). Waddell and Bolke ( 1973, p. 2) indicated that the salt balance between the two parts of the lake was near equilibrium for inflow conditions like those of 1972. During 1973- 74, inflow conditions were similar to those of 1972, and dissolved- load computations based on water- quality data confirmed that dissolved- load losses to the north had ceased. This is indicated by the graph shown for the south part in figure 2. The dissolved load in the north part continued a general trend upward in 1972, even though the south part showed little or no change. This indicates that the salt crust in the north part was dissolving as the volume of the north part increased and freshened as the lake rose. In October 1974, the total dissolved load in the north and south parts was about 4.5 billion tons ( 4.1 billion t), representing a net increase of about 0.5 billion tons ( 0.45 billion t) since the low point near the end of 1971. During the fall of 1970 and 1972, the Utah Geological and Mineral Survey cored the bottom of the north part of the lake, and J. H. Goodwin ( written commun., 1973) estimated the salt crust at 1.14 and 1.33 billion tons ( 1.03 and 1.21 billion t), respectively. Also, the dissolved- solids load in the fall of 1972 was about 4.2 billion tons ( 3.8 billion t). On the basis of the 1972 estimates, the total dissolved plus precipitated salt load for the entire lake would be about 5.5 billion tons ( 4.99 billion t). Therefore, about 1 billion tons of salt crust ( 0.91 billion t) remained in the north part in October 1974. Geological and Mineral Survey Water- Resources Bulletin 21, 1976 In 1965, a lower layer of brine was observed in the south part of the lake ( Hahl and Handy, 1969, fig. 1). This lower layer had chemical characteristics similar to the brine in the north part. Madison ( 1970, p. 12) and Waddell and Bolke ( 1973, p. 35) also observed this layer and stated that its volume remained relatively constant. Additional data collected by the U. S. Geological Survey and the Utah Geological and Mineral Survey during 1973- 74 indicated that the volume of this lower layer and the altitude of the interface with the overlying brine was essentially unchanged, even though the lake altitude had increased by several feet. Madison ( 1970, p. 12) surmised that the apparent stability of the volume of the lower layer was due to equilibrium between the amount of brine moving south through the causeway and the amount of mixing taking place at the interface. Data prior to 1957 are insufficient to indicate whether density stratification was prevalent in the lake prior to construction of the causeway. WATER BUDGET The water budget for Great Salt Lake can be expressed in the following equation: AS = Is + Ig + Ip- Oe ( 1) where AS is change of storage, Is is surface inflow, Ig is ground- water inflow, Ip is precipitation directly on the lake surface, and Oe is evaporation from the lake surface. Now, let V ( t - 1) represent the volume at the beginning of time step ( t) and V ( t) represent the volume after the time step. Then V ( t) = V ( t - 1) + AS. ( 2) Altitude, area, and volume relationships are known for the lake ( see appendix); therefore, volume ( V) and area ( A) can be expressed as functions of altitude ( Al). Thus, by knowing the volume or changes in volume with time, a prediction of altitude can be made. Equations ( 1) and ( 2) are the basic equations used in the model in this study for computing the water budget for separate parts of the lake. This budget or mass balance technique is simple, but it requires knowledge of all parameters in the budget equation. In order to predict the effects of various diking proposals on the water and salt budget of Great Salt Lake, it is necessary to have a data base for the parameters in the budget through a pre- selected base period. A base period is necessary in order to observe the response of the lake to climatic changes that affect |