Ice Age Floods, DEM and Lake Lewis

In an effort to get a handle on some land forms, I set the approximately 350 meter contour and below as blue. The elevation is the approximate height of the highest overflow path at Wallula Gap, the narrow point in the above digital elevation map where the Columbia River cuts through the Horse Heaven Hills. The contour approximates shores of Lake Lewis, the temporary lake that backed up behind the narrow gap during the ice age floods. The pool of blue down stream of the gap would not have filled that deep with flood water; however, the water did back up in that area as well due to the constriction in the Columbia River Gorger further down stream off the map.

Wallula Gap with highest flood overflow channel marked with blue on the left upon which the lake level projection is based.

Islands formed for brief periods along the fist anticlinal ridge south and west of Kennewick

 

Snipes Mountain a ridge within the Yakima Valley west of Sunnyside and east of Granger stuck up above the water in this scenario.

 

The water backed up the Yakima River including through Union Gap forming a pool in what is now Yakima and perhaps a small pool north of Selah Gap. The flood matches fairly well the inundation in the Yakima area.

 The water pooled to the east as well inundating the Walla Walla valley.

Water also backed up the Snake River canyon all the way into Idaho on the east of the above map. Keep in mind water from Lake Missoula was likely still poring into the lake from the north. The river north of the Snake is the Palouse and its old channel, now Washtucna Coulee. That sharp canal-like line connecting the Palouse to the Snake is the current route of the Palouse as it follows a joint carved by the flood (the-palouse-river-leaves-its-valley).  The back of water portrayed in the image is matched by thick silt units that fill the valley of the lower Tucannon River, a tributary that flows into the Snake from the south. 

 

The back up from Wallula Gap extended far up stream. This image is the Columbia River at Wenatchee with water backed up the Wenatchee River into the Cascades. The blue area extending across the lower right is Moses Coulee, one of the numerous flood route paths.