Which came first, the river or the canyon?

I watched a PBS special a few years ago that presented a contrary theory to how the Grand Canyon was formed.  In fact, it reversed previous notions about water carving out the sides. The new theory was that the canyon came first, in a cataclysmic event millions of years ago, and the water just followed the road of least resistance, just like it always does.

I mention this fascinating challenge to conventional wisdom, which I am inclined to embrace because it goes against the grain, since I just heard the same theory about how the Mississippi river formed. Apparently there was a big split in the continent and the water heading downhill just filled in the hole and became the river we know today. Same with the Ohio river that feeds into the Mississippi. There was probably a big quake back in the day, and when the land split, the water found the route of least resistance.

Speaking of which, Niagara Falls is backing up stream a few inches at a time every year. You've still got time to visit. These days, since more than a week has passed since I saw this concept on TV [so I know it's true], I can't remember why this phenomenon happens. PBS had a fascinating special which showed how the falls got started several miles away from where they are now. Then over thousands of years the weight of the water just kept chipping away at the earth. Over time, the falls kept moving farther and farther down the road. Or up the road. Guess I've got to hit the Google button, to prove that I'm not hallucinating any of this.

Not that anyone could tell the difference.