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Show Salt- Load Balance Through Time South- and north- arm salt- load data obtained from the USGS, prepared for their salt balance model ( in progress), are presented in figures E and F for the south- and north- arms of GSL over the 1963- 1998 time period. lO^- degree polynomial fit lines are drawn through the data points just to illustrate the general trends of the data, and notations are given to explain the rise and fall of the salt loads in each arm. As of the end of 1998, it can be seen that the south arm of the lake is declining in its dissolved salt load while the north arm is increasing. More complete information, including the total salt load over time, and the amounts of precipitated salt can be obtained from the USGS and from Appendix H. Lake " Hysteresis" The term hysteresis has been used to explain the northward movement of dissolved salts through the SPRR causeway during rising lake conditions, and the reverse or southward movement of salts during declining lake conditions. From the time of the causeway's construction in 1959- 1960, this principle was apparently operating as can be seen by the long, solid, upper lines on both figures G and H. This concept is supported, as the south and north- arm lake levels have increased and declined throughout this time period, they have not deviated greatly from these paths. With the breaching of the causeway and the additional of more fill to increase the height of the causeway during the lake's 1980s high water years, however, the causeway's long- standing physical parameters have changed. Most notable are the free flow of brine through the breach opening above an elevation of about 4199.5 to 4200 feet, and an apparent reduction in the permeability of the causeway fill below the breach- bottom elevation. Graphical evidence for the reduction in the causeway's permeability is given in figure A, and the associated discussion. With the breaching of the causeway in 1984, there was a significant redistribution of the dissolved salt loads between the south and north arms of the lake ( Gwynn and Sturm, 1987) and a dramatic departure from the pre- breach " hysteresis patterns in the two arms of the lake, as seen in figures G and H. In the south arm, starting in 1987, the salinity first increased above the 1966- 1987 path, due to influx of north- arm return- flow brine through the breach opening, and then from about 1989 to late 1994, a reduction in its salinity below the 1966- 1987 path. This reduction in salinity was brought about by a net loss of salt to the north arm due to nearly- exclusive south- to- north flow. From late 1994 to the present there has been a continued net loss of salt from the south arm to the north arm ( figures E and F) as south- to- north flows through the breach opening have increased due to rising lake levels, and little or no north- arm return flow until 1999. In the north arm, starting with the breaching of the causeway in 1984, the salinity was initially greatly reduced by the large influx of dilute south- arm brine. This occurred as the head differential across the causeway was reduced. Even during the rapid decline in lake level from 1987 to 1995, the path of the north arm was below the 1966- 1987 path due to the continued influx of south- arm brine until 1994- 95. The north- arm's path during this time period was well below the 1966- 1987 path. Through the 1994- 1997 375 |