Yup, some kind of ice dam of a huge lake over in that region. (i've read)
I need to pull out those details again. They are truly impressive. Repeated ice dams in Montana that broke, I think, and the boulder strewn floods that ripped down the Columbia river passage and ground out the channels through the Cascade mountain gorges is stunning in it's sheer violence. The water was something like 500' deep(don't quote me) when it roared through what is now Portland Oregon on it's way to the Pacific.
Did you know that the largest floods to occur on the planet happened here? During the last ice age, ice sheets covered much of Canada. One lobe of ice grew southward, blocking the Clark Fork Valley in Idaho.
This 2,000 foot (600 meters) high ice dam blocked the river, creating a lake that stretched for hundreds of miles. When the lake was full, it contained 600 cubic miles (2,500 cubic kilometers) of water. How much is that? Imagine a block of water a mile high (as high as the mountains around Bonneville Dam), a mile wide, and stretching from Bonneville Dam to San Francisco!
Eventually, water traveled under the ice dam. The water drained out of the lake in two or three days, flooding eastern Washington. The flood, moving up to sixty miles per hour, scoured out hundreds of miles of canyons called coulees, created the largest waterfall to ever exist, and left 300 foot (90 meter) high gravel bars. At Bonneville, the water crested at 650 feet (200 meters). If you look on the cliffs southeast of the dam, you will see a transmission tower (the one with three poles) that is 200 feet (60 meters) above the high water mark.
During a period of 2,500 years as many as 100 of these floods scoured the Gorge.