To begin to grasp the age of the canyon we should also know something about the history of the Colorado River, for without this river (or at least some ancestor to it) there would be no Grand Canyon.Grand Canyon and Colorado River: The Colorado River, or some ancestor to it, has carved the Grand Canyon.
The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced.
Dating is not necessary to demonstrate that evolution is a fact.
Chronological sequence is all that is really required.
However, human beings love to see factual precision, and we want to know how old something is.
Please remember that all dating methods, even those termed "absolute," are subject to margins of error. That is a very small amount of possible error range. Modern studies almost always use two or more methods to confirm dating work and to build confidence in the results obtained.