What is Isostatic Rebound:
Isostasy is "The condition of equilibrium, comparable to floating, of the units of the lithosphere above the asthenosphere." (Bates et al., 1984) The rigid surface layers, the lithosphere, of the earth "float" on a denser subsurface layer, the asthenosphere. The "floating" body will displace its own mass in the subsurface layer (Fowler, 1990). Therefore, a lack of mass is found beneath mountain chains, while extra mass is found beneath the oceans. This also occurs when a load, such as a glacier or large water body, is put on the Earth's surface. The load causes a downward deflection of the Earth's crust due to the weight of the load. After the load is removed there is a rate of recovery, or isostatic rebound, that occurs. This rate is dependent on the viscosity of the mantle and the elasticity of the lithosphere in the region (Fowler, 1990). Over millions of years the mantle behaves as a viscous fluid, but in a very short time period the crust and mantle act elastically. Glaciers and lakes fall in between these time periods and show the transition between viscous fluid and elastic solid and therefore are used to look at specific qualities of the mantle (Bills et al., 1999). There are some advantages to using large lake loads to glacial loads when trying to determine properties of the mantle.

Lake Loads and Isostatic Rebound:
According to Bills et al. (1994), there are two advantages to using lakes for studying isostatic rebound: the load is well known and lakes record climatic history better than glaciers. The disadvantages to using lakes are: glacial loads are larger in weight and in lateral extent. Loads which are large and wide show the properties of the upper and lower mantle while narrow, smaller loads show the properties of the upper mantle very well. Therefore, Lake Bonneville studies show the properties of the upper mantle better than many glacial studies (Fowler, 1990).

Lake Bonneville:
The tilting of Lake Bonneville shorelines was first noticed by Gilbert (1890) with simple instruments. When shorelines are created they are horizontal but Gilbert (1890) noticed the shorelines near the center of the basin were higher than the shorelines at the periphery of the basin. He theorized the tilting was caused by the load of the lake and therefore hypothesized a liquid substrate beneath the rigid crust. He believed with more knowledge of the chronology of the lake and ability to do the mathematics involved the characteristics of substrate could be determined. Due to better dating and surveying methods, we have an improved hydrograph and accurate elevations on shorelines throughout the basin. Using this data and computer technology, models are created that Gilbert could only theorize about. The volume of Lake Bonneville was approximately 10,000 km3 at its highest level. The weight of this much water depressed the Earth's crust by about 1/3 the depth of the water (Bills et al., 1999) and when the water left the crust began to rebound appearing to dome the ground. This is apparent by viewing and measuring the elevations of Lake Bonneville shorelines around the basin. The Bonneville shoreline is as much as 74 meters higher in the Lake Side Mountains, toward the center of the basin, than at Red Rock Pass, on the periphery of the lake (Oviatt, 1997). The Provo shoreline is as much as 59 meters higher (Oviatt, 1997). According to Bills et al. (1994) "the crust was deflected downward by 60-70 meters" under the weight of Lake Bonneville.