Thursday, July 21, 2011

Touchet River - Another Entrenched Meander

Entrenched meanders on the Touchet River on the left portion of photo
The river is flowing south away towards the upper portion of the photo and
the Walla Walla River. The Columbia River and curves through Wallula Gap in the upper right.
Note the longitudinal dunes aligned towards Wallula Gap to the right of the Touchet River

The Yakima River (antecedent-yakima-river) is not the only entrenched meander river in eastern Washington. A quiet, off the beaten path section of the Touchet River flows through a set of entrenched meanders on its way to the Walla Walla Valley. Although I have driven the road along the this section of the Touchet River, I had never thought of it as an entrenched meander until passing high overhead on a recent trip.

The Touchet River was inundated with flood waters from the great outburst floods from Glacial Lake Missoula as the flood water backed up at the narrow passage of Wallula Gap forming a temporary lake (Lake Lewis) that filled the valleys of the lower Columbia Basin in eastern Washington.

Map showing highest flood levels from the Missoula Floods 

Silts carried by those floods were deposited throughout Lake Lewis particularly in quiet back water areas. Each flood left a distinct layer of sediment and at least 40 distinct layers have been identified. These layers are well exposed in the lower Touchet Valley and the sides of the Walla Walla Valley in the vicinity of Touchet, Washington and are called the Touchet Beds.

Despite the flood sediments the entrenched meanders remained in tact. This particular area is not an area of uplift so the entrenched meanders may be a result of lowering of the Columbia River system starting approximately 3.4 million years ago.

On their return journey, Lewis and Clark followed the Touchet River on horseback as the valley was a trail from the Wallula Gap area to the Snake River near Clarkston. The party passed along the entrenched meanders of the Touchet on the west side across an area called Eureka Flats. Clark noted the sand dunes on Eureka Flats in his journal.

No comments: