"A principle stating that profits or benefits gained from something will represent a proportionally smaller gain as more money or energy is invested in it."
It can be applied to other things apart from economics. The real point is that the relatively low flow and pressure of a stream across a basically flat surface is not going to carve out the Grand Canyon. Which, as you no doubt know, was at one time sea bed. Mount Everest and the Alps, as well as the Australian outback were also under the sea at one time. I wonder when.
I've been to the Grand Canyon. It is astounding. One of the things that geology does not explain is the sediment layers. If they were deposited over billions of years, there should be evidence, such as forests, shrubbery or plant life between the layers. There is none. You can see the same effect if you part fill a glass tank with water and put enough dirt in it to fill it about half dirt, half water. Stir it up so that the dirt is suspended. It will very soon settle into layers, taking hours, not millions of years. The Mount St Helen's eruption produced the same effect from the vast amounts of material released. It settled into layers, not in millions of years, but in a few decades. The material settled into Spirit Lake.
If you have an open mind, this link will explain what happened.
The explosion loosened the already unstable northern side of the volcano, and not only changed the visage of that portion of the Cascades, but deeply shook the foundation of the geological timescale.
answersingenesis.org