Friday, February 24, 2012

More on Giant Ripples and Big Floods


Two ice-age flood flood bars from the edge of a terrace
Columbia River Valley downstream of Wenatchee, Washington

On a recent trip to Wenatchee, I had a nice view of some giant-ripples associated with one of the Ice Age Floods. These giant ripples are not the only giant ripple sites associated with the huge ice age floods, but they are one of the better sites for actually seeing them for what they are. Other giant ripple sites do not stand out as well.

My project site was on a high terrace on the side of the Columbia River valley near Wenatchee. This is a land of terraces and huge gravel bars deposited during the ice age. It is a very complicated landscape. While I knew the giant ripples were part of the ice age flood story, spending a little time looking at terraces and absolutely huge ancient landslides made me realize that the story of floods and terrace formation along this reach of the Columbia River is complicated.

Richard Waitt provides a good description of the various surficial features in the Columbia River valley near Wenatchee in Tabor and others (1982). Waitt has been one of the modern geology Missoula Flood detectives and has spend considerable time figuring the multiple Missoula floods and the complexities of the Columbia River Valley between Grand Coulee and Wenatchee. More recently  Richard Waitt (2009) provides a summary in a GSA program abstract on his thoughts on large ice age floods in the Columbia at Wenatchee.

After a few days of map reading and connecting terraces and correlating what Waitt wrote in 1982 with his more recent postulations as well as my own observations at the north end of this reach of the Columbia Valley, I made a few self edits to my previous post and was able to do a decent job of assessing the geologic conditions encountered at my project site. If I understand correctly, the giant ripples at West Bar are from a younger flood that the terrace I was working on at East Wenatchee. Waitt postulates that the giant ripples at West Bar (pictured above) may have been either from a late, smallish Missoula Flood that passed down the Columbia after the Okanagan ice lobe had retreated and was no longer blocking the Columbia River upstream, or that the ripples were from some other flood triggered by ice impounded lakes elsewhere. It is pretty easy to imagine surges of water coming down the Columbia Valley in this area from j√∂kulhlaups (surges of sub glacial ice water) from the Okanagan ice lobe or any other ice location.

The Missoula Floods so dominate the landscape of eastern Washington and the Columbia River that it is easy to forget that there were other large floods. For example the Bonneville Flood that surged down the Snake River and down the Columbia was huge, but evidence for that flood is nearly completed obliterated by the Missoula Floods features.    

No comments: