What is a waterfall?

A waterfall is where a river falls steeply over a band of hard rock. It is formed through a thousand-year process in which a river flows over hard rock followed by soft rock, leaving a small step of hard rock sticking up into the river. Over thousands of years, the soft rock is worn away until the river falls from a great height. Not all waterfalls are created by rivers alone. Some are made when movements in the Earth crust create a fault; others are formed by glaciers.

What are rapids?

Rapids are stretches of fast-flowing water tumbling over a rocky-shallow riverbed. They are caused by different resistance among various rock. They are formed when the water goes from one hard rock that resists the water's erosion to a softer rock that is easier eroded. The rocks break up the flow of the flow, but are not big enough to form a waterfall. Over time, rapids are formed.