Tammany Bar

January 06, 2015


Photographer: Tom Foster; Tom's Web site
Summary Author
: Tom Foster

This photo was taken at Tammany Bar - just south of Lewiston, Idaho along the Snake River. Ice Age flood deposits sit in the valley here - an amazing set of deposits recording truly spectacular floods that raced through the Pacific Northwest thousands of years ago.

The Bonneville Flood struck 17,400 years ago. Lake Bonneville in Utah - an Ice Age predecessor to today’s Great Salt Lake - spilled into southern Idaho’s Snake River Valley, surged through Hells Canyon, and then followed the Columbia River Gorge to the Pacific Ocean. At Tammany Bar, the Bonneville water was midway on its journey. All of the rocks below the geologist’s (Nick Zentner - Central Washington University) hand were deposited in just a few weeks during the singular Bonneville Flood.

At this location, twenty Missoula Flood deposits sit on top of the Bonneville deposit. Notice the lack of rocks and the repetitive look to these Missoula layers. Why do they look so different than the Bonneville layer?

The Missoula Floods did not surge through Lewiston, Idaho. Glacial Lake Missoula - the source of the floods in western Montana - flooded through northern Idaho and raced across Washington to Wallula Gap, the eastern gateway to the Columbia River Gorge. At the gap, the high-energy water was stopped, and the slack water slowly backed up river valleys as the Missoula Floodwater waited to pass through narrow Wallula Gap. The result? Sluggish, brown water slowly crept up the Snake River all the way to the Lewiston area. As the water sat in this valley, suspended silt in the water slowly deposited at the bottom of the lake. Photo taken November 12, 2014.

Photo details: CAMERA: Nikon D7000; Lens 18-200 at 27mm; ISO 200; f/18; 1/50s