One way to think of this is that many major rivers have been dammed by natural causes in the past -- notably by landslides and lava flows. In relatively short periods of time, all is back to normal.

The Columbia River was dammed not far from the current site of Bonneville Dam by a landslide several hundred years ago (once thought in the 1400s, new dates are suggesting it was caused by the last Cascadia subduction quake of January 1700). Other than at the landslide site itself there's little obvious evidence that it happened.

At other times in the last few hundred thousand years the Columbia has been dammed much more effectively by lava flows and again, there's enough evidence for geologists to figure it out, but nothing obvious. The current Columbia Gorge is only a few million years old; in the Pliocene and earlier the Columbia had different channels ten or twenty miles south of the current path, now identifyiable by gravel deposits.

The Colorado too has often been dammed to great depths by lava flows, and even more commonly by landslides.

I suspect Hoover and Grand Coulee and the rest would leave fairly fleeting marks on the landscape.