THE BONNEVILLE SHORELINE: WIND AND WAVE PROCESSES, ISOSTATIC SUBSIDENCE, AND PROLONGED SPILL ALL LED TO THE TALLEST LACUSTRINE SHORELINES IN THE WORLD

Lake Bonneville was the largest pluvial lake in the Great Basin during the late Pleistocene and consequently has the strongest shoreline development of any lake basin in the western U.S. and perhaps elsewhere. The focus of this presentation is the Bonneville shoreline, the highest shoreline in the basin, and the formative factors that led to its development. Results from wave modelling, using a range of wind speeds (5 – 30 m/s) and fetch lengths (50 – 200 km), indicate that waves of up to several meters high likely traversed the surface of Lake Bonneville and upon breaking on the shore, ran up the beaches (1 – 10° slope) to a height of 1 – 5 m above still water level, forming beach ridges and eroding cliffs. This relatively narrow vertical range of vigorous wave activity is in stark contrast to the vertical heights of the beach features themselves that range up to 15 – 20 m from base to crest, with the tallest beach ridges found near the center of the basin, which mimics the pattern of isostatic rebound. Another curious aspect of the shore features, whether they are constructional ridges, depositional platforms, or delta complexes, is that the coarse sediment in all of these features accumulated vertically, likely under the control of a slowly rising lake level. One hypothesis to explain this pattern of tall, vertically accreted shore features is that once the lake rose to its spill point and commenced overflowing, the basin continued to subside due to the enormous load of water. Rates of subsidence depended on the location within the basin but may have ranged from about 0.5 – 1.5 cm/yr, with the highest rates near the maximum load and almost no deformation at the outlet. In this scenario, the local rate of lake-level rise was dictated by the local rate of subsidence. If 5 m is considered the maximum “normal” thickness of large beach ridges found in pluvial lake basins, then the “excess” height of these features may have been influenced by subsidence. Based on gravel thicknesses and modeled rates of subsidence at multiple locations, the Bonneville shoreline may have formed over a period of about 500 – 1000 years, while the lake was spilling and prior to the Bonneville flood. Subsidence-assisted shoreline development may be more common than thought, particularly where there are thick accumulations of beach deposits in both modern and ancient lake basins.