The Bonneville Flood and where it began

July 17th, 2010

DOWNEY, IDAHO–Lake Bonneville has been one of the geological themes of my short visit to northern Utah this summer. The remnant wave-cut platforms of its shorelines dominate the geomorphology of the Logan area, and the lake sediments are the basis for the rich soils of the Cache Valley. Today my parents and I visited Red Rock Pass in southern Idaho where this massive lake breached a weak area of limestones and shales 14,500 years ago and then catastrophically flooded the land to the north. The Bonneville Flood was not as large as the Missoula Floods of geological legend, but it left a very similar record of scoured land, scattered boulders, huge waterfalls, and thick gravel bars.

Red Rock Pass near Downey, Idaho. The rocky hill in the center was part of the dam of sedimentary rocks which gave way 14,500 years ago and released the catastrophic Bonneville Flood.

View north from the dam area looking down one of the flood channels. On the left is a rocky outcrop of the original dam. On the right along the side of the channel is a gravel bar running parallel to the current direction.

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