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Show material used to increase the height of the causeway from 1983 through 1987 and subsequent compaction of the causeway. It may also have been influenced by the effect of the large head differential that existed between the two arms of the lake which minimized the potential for return- flow to the south arm. Gunnison Bay The salinity of the north arm does not exhibit as direct an inverse relationship with lake elevation as does the south arm. This is because the north arm receives small quantities of fresh surface water inflow and large quantities of salty water inflow from the south arm. ( See Figure 3.) Evaporation from the surface of the north arm is sufficient to maintain the north- arm salinity at a high concentration. From 1966 until about 1982, the salinity of the north arm remained within the 310- 350 grams/ liter range ( 25.7- 28.4 percent). Due to this high salinity, a layer of sodium chloride precipitated on the lake's bottom during this time. North- arm salinity dropped to only 160- 170 grams/ liter in 1987 ( 14.5- 15.3 percent), as evaporation was unable to keep up with increased, dilute inflows from the south arm. Since the high- water years, the north- arm salinity has climbed back into the 290- 310 grams/ liter range 24.3- 25.7 percent. ( See Figure 10.) Brine stratification was not present in the north arm of the lake from 1966 until about 1983. When the lake began its rapid rise from about 4200 feet in 1983 to its historic high of 4211.85 feet in 1986- 87, however, a layer of less- dense brine formed on top of the very- dense north arm brine due to increased precipitation, and the enormous inflow of less- saline, south- arm water as the railroad causeway was breached in August 1984 ( see later discussion), and the large, bi- directional exchange of brines between the north and south arms through the breach opening which followed. ( See Figure 4.) By mid- 1991, the level of the lake had dropped below the 4199.5- foot bottom elevation of the breach opening. Because of this, the constant flow of south- arm brine into the upper light- brine layer in the north arm nearly ceased, and the stratified- brine condition in the north arm soon disappeared due to vertical mixing. Net Northward Movement of Dissolved Salts from the South Arm to the North Arm To help alleviate the flooding of the 1980s, the state implemented two flood- control measures which affected the dissolved- salt distribution and the total salt load within the lake. Breaching the Northern Railroad Causeway In August 1984, the state created a breach in the northern railroad causeway consisting of a 300- foot- long opening near Lakeside ( See figure 4). At the time the breach was opened, the water elevation of the south arm of the lake was about 3.5 feet higher than the north arm. After the breach was opened, great quantities of less- concentrated south- arm brine flowed northward into the north arm, while large quantities or dense, north- arm brine flowed southward into the depths of the south arm as bidirectional flow. As a result of this bidirectional interchange of brine, the south arm density and salt load increased, while those of the north arm decreased. Bi- directional flow continued until the end of 1988 when the lake dropped to 45 |