Magruder corridor; Magruder Corridor Guide - Idaho | AllTrails.com
Just so you know, back about the last few ice ages there used to be great glaciers that covered the land. One of these ice sheets formed a dam which blocked drainage from Montana to the west and the dam was around Lake Pend Oreille in northern Idaho.
When the melted water would get deep enough it would float the dam and drain the 200 mile lake behind the dam. In a week! Imagine 500 cubic miles of bashing, roaring, grinding water ripping though Idaho and Western Washington. These cataclysmic flows created what we know as the Channeled Scablands. One such channel we call Grand Coulee.
There are places where the ripple deposits from the floods are 24 ft in height and 360 feet apart, they lie 150 to 250 feet above the current river height and contains boulders 4.5 feet in diameter. Geologist figure the water was 650 feet deep and moving at 50 to 80mph.
There was so much water that when it hit the Columbia River gorge it backed up and was 1,100 feet deep.
Self guided tours of the Channeled Scablands is on my list.