Lake Missoula, aka Glacial Lake Missoula, was a large lake covering most of Western Montana during the last ice age. The story of its creation and destruction was so incredible the first time I heard it, that I didn't believe it.
During the last ice age, the Pend d' Oriel Glacier, which originated in the Canadian Rockies, got so large, that a tongue of it got far enough South to block the Clark Fork of the Columbia River, which was responsible for draining all of Montana west pf the Continental Divide. Being blocked, the Clark Fork just kept on backing up and backing up until it reached a height of 300 meters or so, all over the valleys of Western Montana. This was Lake Missoula. It was around the size of one of the Great Lakes.
Of course, once it reached a certain depth, it was able to float the ice dam. When it did this, the last sixty years of accumulated rain fall drained out in a matter of a few hours, or maybe a few days. During this time, the flow of the Clark Fork and the Columbia River was over 50 times greater then all other rivers in the world put together. This was known as the Missoula Floods.
After Lake Missoula drained, the glacier made its way south again, once more blocking the Clark Fork river, and causing the lake to form. The lake formed and drained several dozen times over the course of a thousand years or so.
Once I learned about these floods, features of geography in the Northwest that I had known about all my life started to make sense. For example, since the Willamette Valley was flooded to a depth of a hundred meters at least, some of the hills around that area have glacial erratics that were carried down in icebergs that originated in the Canadian Rockies.
In Western Montana, especially after a snow fall, it is often possible to see parallel horizontal lines in the hillsides. These were the shorelines of the consecutive fillings and emptyings of Lake Missoula.
One of the areas hardest hit was the channeled scablands of Eastern Washington, which were scraped down to the bedrock in many sections due to the force of the flood.
In Portland, Oregon, Mount Tabor is much more sheer on its eastern side, because when the Missoula Flood came through, it carved away that side.
Until we have another ice age, Lake Missoula is a thing of the past, although a remanant of it still remains in Flathead Lake.