The problem with rivers on plains is that they wander. Take a gander at the Mississippi River river valley sometime. Why hasn’t the mighty Mississippi carved a canyon yet? Because there has been no uplift to lock the river in place for long enough. The Colorado cut the grand canyon because the plains on which that river used to wander began to uplift. This started the cut. The faster the uplift, the faster the cut. Nick has another one on the hells canyon geology that showcases this really well. Hells canyon is deeper than the grand canyon while being only half the age.

https://youtu.be/wxg6124BpZw?si=AVQcuTjyhgkkzYpB

The other part you’re talking about is massive erosion, and frankly the vast majority of rivers just don’t have the energy to do that. You’d need APOCALYPTIC amounts of water to carve canyons like that. That kind of canyon is called a coulee, and was formed during the last ice age floods. Nick does a LOT of ice age flood stuff too. He does a whole breakdown of how rivers have moved and interacted through the geological changes in the PNW over the last very very long goddamn time.

https://youtu.be/0IjLO9ABKYU?si=FG0mj7xj3YxNIDzN

Fascinating that the riverbeds of the past can be the mountaintops of today.