The Cascades of the Columbia were a series of rapids in the Columbia River just east of the current site of the Bonneville Dam. The name was later applied to the mountains on either side, and later the entire 1,000 mile long Cascade Range.
The Columbia River, which rises in the Rocky Mountains, must pass through the narrow Columbia River Gorge before it reaches the ocean. While this produces some incredible scenery, it is somewhat unstable from a geological point of view, because the river is undercutting the cliffs on either side, cliffs that were already dramatically scoured during the Missoula Floods over 10,000 years ago. Also, the entire Pacific Northwest region is prone to earthquakes, which makes the Columbia River's travel through the gorge a rather trouble-laden feat, from a geological viewpoint. One of these troubles happened several hundred years ago, probably around 1700 or so, when records of tsunamis in Japan attest to a gigantic earthquake in the Pacific Northwest. This Earthquake caused one of the mountains overlooking the Columbia to fall into the river, displacing earth on a scale of cubic kilometers. Native American legends speak of the Bridge of the Gods, an earth dam across the Columbia that allowed people to walk from the Oregon to the Washington side. Dams made of earth aren't that stable in the face of gigantic rivers, so before long, the river topped the dam, and what was left was a series of rapids where the river was continuing to cut through the remainder of the landslide.
Given enough time, the river probably would have done so, but when European settlers came to the area, the rapids were still in place, making travel up and down the river difficult. Of course, traveling from east to west was difficult in any route, since passing over the Cascade Range was also very difficult. In the late 19th century, the Federal government built the Cascade Locks so boats could travel up and down river while avoiding the falls. And in the 1930s, Bonneville Dam was built, submerging the entire area of the Cascades. As with most public works and technological advancement, this was a double edged sword, because while ships could now easily traverse through the Bonneville dam's locks, and the Portland area would have a source of cheap, clean electricity, the unique feature that was the Cascades of the Columbia is buried forever. And, as in almost all of The Americas, the original inhabitants weren't really consulted on the matter.
Today, the only signs that the area once would have been dramatic whitewater is a narrowing of the river in the area, and the fact that on a satellite map, the lobe of the landslide is still obvious. Many pictures still exist, but it will never be the same as actually being able to visit and experience such a site.