The Nahanni is one of the most unique rivers in the world, it is the Nahanni that created the canyon and not the canyon that created the Nahanni.
The land mass that the Nahanni River flows over is the impact zone of two of the world's tectonic plates. When they collided, they rose the floor of the ocean that was there hundreds of millions of years ago and washed the water off. The river formed as mountains rose out of the sea, acting like a gutter for all of the water coming off these new towering cliffs.
This river, the Nahanni, is an antecedent river that carves the landscape like a knife. The combined flow of the waters is so powerful, and the mountains rise so slowly, that the river literally cuts through the ground to create the canyon. In other parts of the world, canyons are caused by a rising land mass breaking apart and cracking in great fissures. Where this happens, these cracks dictate where the water flows. However, the Nahanni has washed its way through the thousands of meters of rock crust of the earth.
It is here that we can see a prehistoric snapshot of what the earth looked like hundreds of millions of years ago. The Nahanni is the same shape today that it was when the mountains were just bumps in the ground.