Bonneville level 16 - 14.5 ka - The shoreline levels raised until about 16 thousand years ago when, at 1552m (5090 ft), the water reached the rim of the basin at Red Rock Pass. For roughly a thousand years the lake level oscillated, forming the highest shoreline covering 19,800 square miles. Principle control was probably through overflow at Red Rock Pass, but leakage through the alluvium and karsts through the limestone as well as a balance between precipitation and evaporation stalled the lake level at this stage. Shortly before its break out, a short regression occurred that created the Keg Mountain oscillation. Between 15 to 14 thousand years ago a catastrophic failure of the alluvium fan at Red Rock Pass released a surge of floodwaters and lowered the over all lake level approximately 105m (350 ft.) Isostatic rebound has raised the shoreline left by the Bonneville level unevenly, and its current level varies by 61m (200 ft) of elevation.



This image shows the size and shape of the lake at the Bonneville stage, about 16 thousand to 14.5 thousand years ago. The red rectangle indicates the location of Red Rock Pass, where the floodwaters escaped.


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