OCR Text |
Show subjected to inundation, wind tides, ice damage or shallow ground water problems. Long- term lake fluctuations result from a net gain or loss in lake elevation over a specified period of time. For example, between 1873 and 1963, the elevation of the lake fluctuated downward more than 20 feet, from 4212 feet to the historic low of 4191 feet. It then moved upward, while fluctuating within a 20- foot range, to the historic measured high of nearly 4212 feet in 1986. For planning purposes, it is important to know the maximum movement that might be expected during a given period of time. Based on historic estimated and measured lake levels, it is estimated that during six- year blocks of time from 1847 through 1982, the maximum measured one- year upward fluctuation is about six feet. A notable exception to this was seen during 1983- 84 when the level of the lake increased by nearly 12 feet during a five- year block. When the trend is downward, the maximum one- year downward fluctuation is about 2.5 feet. In addition to the historic record of lake level fluctuations, an extensive geologic record of prehistoric fluctuations is preserved as shorelines and other geomorphic evidence in the sediments underlying the lakebed and in the lagoons around the lake shore. This prehistoric record reveals that GSL has risen twice above the 4220- foot level in the last 10,000 years and numerous times to elevations between 4212 and 4217 feet. The rises above the 4220- foot level are exceptional. They result from significant departures from what is considered normal climate for the Great Basin in non- glacial times. The rises to the 4217- foot level occur with climate that is " normal" for the region. They result from a series of years with precipitation above average, but normal for the region. An initial high lake level coupled with consecutive years of above average precipitation will result in a high lake level. Great Salt Lake Planning Zones The 1995 GSL CMP adopted 11 four- foot elevation zones as a tool to aid in the planing process. These zones are shown in the following table ( See Table 1). Many of the figures used in this report show these zones along the elevation axis. 26 |