| OCR Text |
Show changes continuously. Short- term changes occur in an annual cycle of dry, hot summers and wet, cool winters. Long- term climatic changes occur with overlapping periods of about 20 to 120 years, and perhaps longer. The annual high- lake level, which normally occurs between May and July, is caused by spring- summer runoff. The annual low- lake level occurs in October or November at the end of the hot summer evaporation season. The average annual ( pre- 1983) fluctuation of the south arm of the lake, between high and low, was about 1.48 feet; the north arm fluctuation averaged 0.99 feet. The difference between the magnitude of the south and north arm fluctuations is due mainly to the flow- restrictive influence of the northern railroad causeway ( formerly the Southern Pacific Railroad ( SPRR) Causeway) and the lack of tributary inflow to the north arm. The highest recorded annual rise of the south arm, 5.05 feet, occurred in 1983. This exceptional rise in lake level was due to high snow pack and above- normal spring precipitation. Because of the broad, shallow nature of GSL, its surface area expands rapidly as its elevation increases. Elevations 4200 and approximately 4212 feet represent a common average lake level and the historical high- lake elevation, respectively. Between these two elevations, the area of the lake increases more than 46 percent from about 1,079,259 to 1,572,000 acres. Within this range, the potential of flooding exists. Above- normal annual fluctuations, such as those experienced during 1983 and 1984, result in extensive flooding. The low lying plain surrounding GSL is particularly susceptible to flooding and other related hazards. Regarding the flood plain, Lowe ( 1990a and 1990b) states the following: " Using the best available historical and scientific data on GSL, government policy- makers and lake experts have recommended that a beneficial development strategy should exist for lake- shore areas up to 4217 feet in elevation" ( DCEM, 1985). This strategy establishes BDA along the shore of GSL between 4191.4 feet ( the lake's historical low level in 1963) and 4217 feet. The strategy recommends that, within the elevation interval between 4191.4 and 4217 feet, development take place in a manner that will encourage the maximum use of the land for the people of Utah while avoiding unnecessary disaster losses. Pursuant to this strategy, UGS would provide technical information and maps showing geologic hazards; city- county surveyors would provide a BDA line which is at the 4217 foot elevation contour to the planning, zoning and permitting agencies of applicable city, county and state agencies. The naturally occurring water level fluctuations of GSL are termed " flooding" when the level of the lake begins to adversely affect structures and developments which are located within the flood plain. However flooding is a natural process and is mostly beneficial to species adapted to this dynamic environment. The impact of flooding is greatest around the shores of the south arm of the lake where the majority of the recreational, industrial, wildlife management and transportation facilities have been built. To minimize the impact of flooding, the present and past elevations of the lake and its anticipated short- and long- term fluctuation ( rises and falls) should serve as guides to determine " safe" construction areas. This should also identify areas which may be 23 |
