SEDIMENTARY CONSTRAINTS ON THE MAGNITUDE OF HOLOCENE LAKE LEVEL CHANGE IN BEAR LAKE, UTAH-IDAHO

A wide variety of sedimentological, chemical, and biological proxies in Bear Lake indicate climatically driven lake-level fluctuations through the Holocene. Mixing of sediments due to pervasive sediment focusing hinders the interpretation of proxy data. Relative fluctuations are readily apparent, but absolute lake levels are more difficult to determine. Analysis of sedimentary features places boundaries on lake levels and, therefore, provides a clearer picture of the impact of climate change. Shoreline deposits provide the best indication of absolute lake level, but they are commonly affected by erosion and reworking of older materials and generally are difficult to date. Wave models that are based on effective fetch and minimum bottom shear velocities for sediment movement constrain possible depths, but do not account for other factors such as gravity-driven sediment transport. Comparison of sedimentary characteristics (e.g. grain size and textural features) of the modern lake sediments to older deposits provides a sense of change. This empirical approach is hindered, however, by a variety of problems including sediment mixing, inherited boundary conditions, and geographic variability. Despite these problems, we have constructed a lake-level curve that depicts our interpretation of lake-level history, and hence the impact of climate-change through the Holocene. Several droughts in the early Holocene lasted centuries dropping the lake more than 25 m below the modern level. Lake levels a few meters above the modern highstand occurred three times between 2.4 and 4.7 Ka and about 6 Ka. A lake transgression to over 14 m above the modern lake occurred about 8.5-8.0 Ka when the Bear River re-entered the basin during a prolonged wet period.