Roaring like thunder, a wall of blue satin threaded with silver plunging down the rock face, or perhaps just a gentle tinkle of water splashing over stones. Mighty or small, waterfalls are always a joy to watch.

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A waterfall is an area of a river or stream where the water flows over a steep vertical drop, often landing in a plunge pool below. There are many types of waterfalls and it is possible for a waterfall to fit more than one category. They are also grouped into ten broad classes based on the average volume of water going over the falls. Niagara Falls, Khone Falls, and Inga Falls are in class 10, and Victoria Falls is in class 9.

Erosion plays an important part in the formation of waterfalls as do the waterfalls themselves. The velocity of the water increases as it nears a waterfall, increasing the amount of erosion. The plunge pool at the base of a waterfall gets bigger over time as rushing water and sediment erodes it, and the area behind the waterfall can also be worn away, creating cave-like shelters.

Of course, earthquakes, landslides, glaciers, and volcanoes can create cliffs, cracks, faults, and other changes in elevation that can lead to a waterfall forming.