Lake Lewis and the very narrow Wallula Gap
During the ice age floods that swept into eastern Washington, the flood waters were greatly slowed by Wallula Gap. This choke point caused the flood waters to back up the Snake River canyon all the way to Idaho, flooded the Walla Walla valley and backed up the Yakima valley as well as large tracts of the lower Columbia Basin.
Lake Lewis waters would have been filled with mud and silt from the fast moving flood waters stripping away entire landscapes of soils and was filled with glacial silts and floating ice bergs from the collapsed glacial ice dam that triggered the largest floods. The silts deposited in the Walla Walla valley and Yakima valley as well as elsewhere the Pasco Basin (Lower Columbia) left a legacy of rich soils.
The blue area at the south end of the figure is a residual of the elevation I picked, but there was also a temporary lake that formed downstream of Wallula Gap when the water became constricted at the Columbia River Gorge forming Lake Umatilla and additional excellent farm soils in the valleys the water backed up into.
The elevation for the lake I picked was approximately 1,200 feet in elevation. This would have been from one of the bigger floods. Not all of the floods would have created such a deep lake; however, the Walla Walla valley and Yakima valley were flooded possibly as many as 40 times or more. The depths of some of the later floods may have been somewhat reduced by the widening of Wallula Gap from the erosion of earlier floods.
One can see in the map above that the hinge in the Horse Heaven Hills (HERE) would have been a great view spot to watch the floods.