Beautiful blues in this image. I like the title, it is a very interesting question . In effect, both forces (the flow of the river and the resistance by the rock) shape each other. It is interesting to note just how important they both are for survival of land-based life forms... the land holds back or slows down the flow, creating essential catchments of water and preventing complete drainage; at the same time, erosion of the rock is essential for depositing silt downriver, without which the biologically rich and fertile plains wouldn't be there. It is a balanced process that gets thrown out of order every time we build a dam.