Rarely is a river the same from beginning to end, because few regions are topographically consistent. Because of these flow variations from fast to slow within the same river, we use the following method to classify rivers. “Very young” or “very old” are, perhaps, misnomers in most cases, as some old rivers run quite fast out of the Blue Ridge and Smokey mountains. The key is understanding gradient. The steeper the drop, the harder the bottom will be, as softer substrates are washed away by powerful currents. In flatter terrain, the sediments settle, covering the rock with sand and silt. Most river stretches in North America fall within one of seven categories. River stretches, though, often exhibit transitional tendencies, just as a natural lake may have eutrophic bays while its main body is mesotrophic.