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Show Waddell and Fields- Model for Evaluating the Effects of Dikes on the Water and Salt Balance of Great Salt Lake 3 areas would have on the water and salt balance existing during the 1931- 73 base period. Hydrology of the Lake An understanding of prior changes in the water and salt balance of Great Salt Lake is important for an understanding of the current hydrology of the lake and of the model. Madison ( 1970, p. 9- 19) described the hydrology of the lake through the 1969 water year and Waddell and Bolke ( 1973, p. 2- 6) described changes during 1970- 72. The synopsis that follows is taken from these previous reports, updated for trends since 1972. The hydrologic characteristics of Great Salt Lake are typical of a closed lake. The water surface rises and falls in response to the balance between evaporation and the amount of water contributed to the lake by surface runoff, ground- water inflow, and precipitation on the surface. The annual peak water- surface altitude generally is in the late spring, and the minimum water- surface altitude generally is in the early fall. Also, the general trend of the water- surface altitude may rise or fall for several years ( fig. 1) as part of a long- term cycle. The causeway was constructed in 1957- 59 by the Southern Pacific Transportation Co. for its railroad track across Great Salt Lake. It extends between Promontory Point and Lakeside, where the lake is about 18 mi ( 29 km) wide ( fig. 5), and it divides the lake into north and south parts. A little more than one- third of the lake lies north of the causeway. The causeway is permeable and is breached by two open culverts, each 15 ft ( 4.6 m) wide. Although few data are available to substantiate the chemical characteristics of the lake prior to construction of the causeway, the restricted circulation effected by the causeway resulted in significant changes in the salt balance during the following years. According to Madison ( 1970, p. 7): " Prior to construction of the causeway, the dissolve d- solids content and the chemical composition of the lake brine were controlled primarily by volume changes resulting from inflow and evaporation. The causeway created two separate but interconnected lakes with different water- surface elevations and densities. As a result, brine flows in both directions through the causeway, with less dense brine from the south part moving northward through the upper part of the causeway and more dense brine from the north 4. 5 4.0 - -• 3.5 - 3. 0 2. 5 2 . 0 1 . 5 Total dissolved load ( north and south parts) - 4.0 - 3.5 - 3.0 2.5 ° - 2.0 1 .5 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 Figure 2. Graph showing variation of load of dissolved solids in Great Salt Lake, 1964- 74. 1 974 |