Abstract
The outlet area of Lake Bonneville has had over a century and a quarter of science and speculation about its configuration and character and many disparate interpretations have appeared in the literature. In this chapter, we offer our interpretations of the outlet, which are based on field research begun in the 1980s and review of the pertinent literature. The Marsh Creek alluvial fan near Red Rock Pass, which formed the natural dam for Lake Bonneville, was built westward from the Portneuf Range to where it abutted a large landslide complex on the eastern slope of the Bannock Range. At the low point on the crest of the alluvial fan the Zenda threshold of Lake Bonneville determined the elevation of the Bonneville shoreline and was washed out by the Bonneville flood. The landslide includes deformed Neogene lacustrine sediments (the Salt Lake Formation) and blocks of Paleozoic bedrock. Landforms characteristic of large-scale mass movements on the Red Rock Pass landslide complex include hummocky topography and multiple scarps at the heads of smaller slumps. The older part of the landslide mass was emplaced long before Lake Bonneville in response to increasing tectonic relief during the Neogene and Quaternary. It sits higher on the eastern slope of the Bannock Range adjacent to a younger part of the landslide mass, which moved in response to catastrophic dewatering and erosional removal of lateral support by the Bonneville flood. Valleys with bowl-shaped heads and U-shaped cross-valley profiles on the north-sloping surface of the Marsh Creek alluvial fan, were formed by groundwater flow from Lake Bonneville late in its transgressive phase through the fan and other surficial deposits. Headward erosion of the valleys by spring sapping led to the Bonneville flood, a dam failure involving processes typical of failures of man-made and natural dams throughout the world.