Tourists at Iguacu Falls in Brazil can walk out on a metal platform to see the view.
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Waterfalls are simultaneously the most peaceful and dynamic geological features on Earth. Here, tourists at Iguacu Falls in Brazil can walk out on a metal platform to see the view. See more pictures of waterfalls.

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Sailing lazily down the river in a small, wooden boat, you lie back, place a hat over your face to shield your eyes from the sun and start to doze off. The slow drift and quiet flow of the water is peaceful, and right now you don't have a care in the world. That is, of course, until you start to hear a low rumble. It's nothing alarming at first; just a calm frothing, maybe the product of some faster rapids. Upon sitting up, however, you realize it's much worse -- that rumbling sound is the crashing of a steep waterfall, the edge of which you're quickly approaching.

Waterfalls represent two completely different ideas to people. On one hand, they characterize peaceful, natural landscapes. When most people see a picture of a waterfall, they probably see a place of calm beauty that could be part of a paradise. You could even say they're romantic -- for instance, over 50,000 couples plan honeymoon visits to Niagara Falls, the famous waterfalls that separate Ontario, Canada and the state of New York [source: Niagara Falls Tourism].

On the other hand, waterfalls are actually chaotic in nature. If you ever fell over the edge of a tall waterfall, it wouldn't be the most pleasant experience. If the impact of hitting the water from great heights didn't harm you, the powerful churning at the bottom of a waterfall could cause you to drown.

Waterfalls are also strangely tricky to define. Geologists sometimes have a hard time defining where a waterfall truly ends and begins. So how do waterfalls work? Are they just there all of the time, continually spilling over the edges of mountains, or do they actually form and change over time? To learn about waterfalls, read the next page.