Tuesday, November 12, 2013

The Yakima River Changes Course

The views from the Hinge (View from the Hinge and picnic-at-hinge-in-horse-heaven-hills) also provide a view of the altering of the route of the Yakima River from the ice age floods. 

View of lower Yakima River valley and Benton City
The puff of steam on the horizon is the large nuclear power plant at Hanford

I liked this alignment of a turn right sign with the Yakima River below making a left turn.
 Prior to the ice age flood the Yakima turned right instead of left at this location.
Ice-age flood waters filled the former Yakima River valley to the right with sediment
After the flood waters exited the river followed a new coarse to the left instead of to the right

View of Badger Coulee, the former lower Yakima River valley 

DEM showing the turn in the river


Scott Schuldt said...

What happens at Prosser? When I canoed the Yakima, we went from cut bank meandering scrubland on the upriver side of Prosser, to what I think is channeled scablands boulder filled river on the downriver side. Quite a noticeable change from the inside of a canoe.

Alan said...

Fun images & discussion - but I wondered if the silting of the former valley would have been enough? Is it possible that there was an "ice dam" (or series thereof) of grounded ice - one can imagine the Wallula Gap being clogged by a combination log jam and ice jam and a resulting secondary jam at the entrance to the side valley.

My collection of flood literature seems week on ice effects other than just forming potholes and as a transport mechanism for glacial erratics, but it would seem that given the numbers of known erratics in the Willamette Valley, extrapolation suggests there must of been huge quantities of ice with less dramatic passengers, and it must have had a significant landscape-shaping effect beyond just the potholes and erratics.

Dan McShane said...

Scott: You are describing Chandler Narrows a place where the valley narrows plus bedrock is at the surface. Hence, that reach had greater velocity as the water drained out the valley as the ice-age flood subsided.

Alan: There certainly were ice bergs floating about in the flood. There are ice rafted boulders on the upper slopes of Wallula Gap and lots just north of the Yakima change in course as well as some on the valley sides of Badger Coulee. The volume of sediment deposited at the turn in the Yakima was significantly controlled by the temporary low velocity at the location as water backed up and formed Lake Lewis. Upstream on the Columbia are several massive 100s of feet high gravel bars from the flood waters as they entered the backed up water and lost velocity.