Why do some rivers have waterfalls near their source but not near their mouth?

The upper parts of a river are usually steeper because they begin in mountains or highlands. That steep gradient, combined with resistant bedrock, creates ideal conditions for waterfalls and rapids. Farther downstream, the river gradient becomes gentler and the channel widens into floodplains. By the time a river nears its mouth, it often carries fine sediment rather than dropping over rock steps. This is why waterfalls are typically found in youthful or upper-course river landscapes, not in the lower course.