http://www.iafi.org/floods.html
The above is a link to society for the ice age floods. As you head south to Hanford from Vanatage, when you go south across the river and then up that steep hill, look back to the north and there is a huge ridge (100 feet+ tall?) along the north side of the river that is an old sand bar.
I did a job near the mouth of the Columbia river. It was all basalt rock about 20 feet down, acres of basalt. They drilled down and after 200 feet or so they hit soft sand. (Not sandstone rock!) Not sure how that works!
Here is a site with images of what is called the Picabo Chain which currently ends at Yellowstone after 12 million years. There are also many other volcano and earth images and for some weird reason hamburgers, and other food stuffs.
This article was written by a geologist who has studied the Picabo Chain. He suggests that another super eruption may be a million years away. Smaller ones can occur as has happened about 70 times since the last big one of 640,000ya.