This image was taken looking north through Sentinel Gap. This gap cuts through the Saddle Mountains which separate the Pasco and Othello basins in Eastern Washington. The Columbia River runs through this gap on its way to the Oregon/Washington border. At the time of the Missoula Floods, the water level going through the gap reached the top of the left ridge, flowing into Lake Lewis which covered the Pasco basin.
The erratics in the foreground most likely are from the gap, carved away by the raging wall of water that flowed through it.