Geological History of the Columbia Gorge
The Columbia Gorge was formed over millions of years by the combination of several different kinds--tectonic shifts and uplift, massive lava flows, landslides of a magnitude we can imagine, and collapsing ice dams releasing floods with eight times the flow of all the rivers in the world.
The formation of the Columbia Basin by uplifting of the Earth's crust was completed about 40-60 million years ago.  It then began subsiding, creating a large inland sea.  The land uplifted again and repeated massive lava flows built up the surface about a mile thick, creating the present day Columbia Plateau.
During the ice age, beginning about two million years ago,  Glaciers up to two miles thick pushed south and covered the Columbia River.  At the same time, uplift created the Cascades, a relatively young mountain range.  The Columbia River eroded a gorge during the uplift and maintained its escape route to the Pacific Ocean.  Near the end of the ice age, the high volcanoes of the Cascades formed.
About 18,000 years ago, Lake Missoula formed behind an ice dam in what is now western Montana.  It stretched for several hundred miles and contained 600 cubic miles of water.  When the dam broke, floodwaters covered the Columbia Basin--about 800 feet deep by Pasco and about 400 feet in at Portland.  It was the largest flood  and the largest waterfall ever on the planet.  The water was traveling about 60 miles per hour. There were about 100 of these floods during a relatively short 2,500 year period.
About 500 years ago, five square miles of Table Mountain sloughed off into the Columbia River to form a temporary dam.  This event is likely the origin of the Native American legend about the Bridge of the Gods.  The dam held for a few months until its newly formed lake overtopped it.  The resulting flood, 100 feet deep at Troutdale, created the new river channel a mile south of its previous location, and left boulders behind to make Cascade Rapids.  Those waters became smooth in 1938 with the completion of Bonneville Dam.
The results of all these cataclysms are visible today as you travel through the Columbia River Gorge.