Red Rock Pass and the Bonneville Flood

This unremarkable notch in the hills was once the trigger for one of the largest floods in the history of the Earth. Around seventeen thousand five hundred years ago, an enormous Ice Age lake called Lake Bonneville filled much of what is now Utah, covering roughly twenty thousand square miles, far larger than the Great Salt Lake that is its shrunken remnant. As the lake rose, its waters reached Red Rock Pass and breached the natural dam of alluvial debris that held them back.

The lake tore through here and emptied northward, releasing thousands of cubic kilometers of water into the Snake River plain. It was among the largest floods known in the geologic record. The torrent carved canyons, stripped soil, and scattered boulders for hundreds of miles downstream, reshaping the landscape of southern Idaho in less than a year.