The correct answer is rapids and waterfalls.
Rapids refers to the segments of streams with extensively vigorous currents, various obstacles, and steps in their streambeds. A waterfall refers to a vertical drop in a stream bed. Both are the locations of vigorous erosion.
The rapids are generally produced where the resilient bedrock limits a stream to a narrow channel and forces an enhancement in the velocity of water.
On the other hand, waterfalls are produced where the fast-flowing water negotiates a geologic contact amid the more resilient and less resistant layers of rock, or through a fault, which has analogized distinct kinds of rocks. Or it can be said that waterfalls usually produce at the end of a series of rapids.