The Other Ice-Age Flood: Bonneville Flood

The highway through Lewiston, Idaho passes by a gravel cut slope with a retaining wall at its base to catch material that ravels off the slope. The cut slope is a deposit from an ice-age flood. But this flood was not derived from the rapid draining of ice-age glacial Lake Missoula. Instead this thick deposit of gravel was derived from a massive ice age flood when Lake Bonneville in northern Utah overflowed into the Snake River drainage in southern Idaho approximately 15,000 years ago. When Lake Bonneville over topped the pass at Red Rock, the resulting flow of water rapidly down cut a channel draining the lake down 350 feet.

The flood greatly altered the Snake River and areas along the river in Idaho and left a series of large gravel bars and terraces within and downstream of Hells Canyon including deposits along the lower Snake River in Washington State. It likely left a mark all the way down the Columbia River to the Pacific Ocean; however, the much more intense repeated ice age floods from glacial Lake Missoula obliterated most if no all the evidence. The Bonneville flood deposits a bit upstream of Lewiston were deposited over back water sediments from the Missoula Floods and in turn later Missoula Floods deposited backwater sediments above the Bonneville deposits. Tom Foster has a nice write up http://iceagefloods.blogspot.com/2010_05_01_archive.html with some great images from the Lewiston to Pittsburgh Landing area of the Snake River.