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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Introduction to How Waterfalls Work

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.

How did something like Victoria Falls in Zimbabwe actually form?

How did something like Victoria Falls in Zimbabwe actually form?

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Waterfall Formation

In all their majesty and seeming calm, waterfalls might look like a permanent fixture on the side of a mountain -- as long as the river's always there, the waterfall will be there, too, right? As it turns out, waterfalls are actually formed very slowly over the course of several thousand years. You would hardly notice any changes in one during a lifetime.

Imagine a simple river flowing along bedrock, the harder rock that lays underneath loose earth like soil and sand. It's moving along pretty quickly and at a fairly steep incline. The bedrock over which the water is flowing has varying degrees of density and strength -- some layers are soft, while others are much harder. When water flows over a layer of hard rock, it erodes the softer rock beyond it. The bed of the river gets steeper as the water carries the softer rock downstream, and eventually the flow of water at this point becomes steep enough to be considered a waterfall.

Water continues to fall against a back wall, which also continues to wear away. Soon, the soft rock underneath the hard rock falls back, and a plunge pool is created where the water collects. Enough water moving over the hard rock will undercut it and break it away, and big pieces of rock will collapse and fall into the plunge pool, which makes it even bigger and deeper than before. The soft rock below the hard rock is receding so much that the hard rock becomes an overhang.

A view of water rushing over Victoria Falls.

A view of water rushing over Victoria Falls.

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Although the waterfalls we see today will be around for a long time, they'll eventually recede and disappear. As hard rock is slowly eroded by the constant flow of water, it falls into the plunge pool and creates a large gorge. The waterfall is actually retreating backwards. This happens very slowly -- just as it takes thousands of years for a waterfall to form, it takes just as long for it to disintegrate. Niagara Falls, for instance, is retreating at the rate of 3.3 feet (one meter) per year.

This formation can lead to a wide variety of waterfall types. To learn about the different kinds of waterfalls, see the next page.

An example of a cascade.

Examples of a cascade (left) and a cataract (right)

Mathieu Ricard/Getty Images Steve Allen/Getty Images

Types of Waterfalls


Examples of a cascade (left) and a cataract (right)

Mathieu Ricard/Getty Images Steve Allen/Getty Images

You may have heard two other terms used to define a waterfall: cascade and cataract. Although they both describe waterfalls, they mean slightly different things. A cascade is the most common term and usually describes a waterfall with any kind of irregular surface underneath the water. It flows down in a fairly low volume, and several stages can make up one large waterfall. A cataract, on the other hand, is a waterfall with larger, more powerful volumes of water and is typically accompanied by rapids.

Now that we've explained the terminology, let's examine some different types of waterfalls. The most basic and recognizable type of waterfall is the plunge waterfall. This happens simply when a river spills out water over a ledge, and the water descends vertically without coming into contact with any of the rock on the way down -- it just crashes right into the plunge pool. This type of waterfall would take longer to retreat, since the hard rock over which the water is flowing is more resistant to erosion.

Igazu Falls, a block waterfall in Brazil.

Igazu Falls, a block waterfall in Brazil.

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A block or sheet waterfall is formed from a wide river -- when the water spills over the edge, it looks like a big sheet, especially if the flow isn't broken by any stray rocks protruding from the back-wall. A block waterfall is usually wider than it is high. Similar in nature is the curtain waterfall, which is simply taller than it is wide, but still looks like a long sheet.

Horsetail waterfalls are in constant or semi-constant contact with rocks, which may erode faster than other types because of constant runoff.

Punchbowl Falls at Eagle Creek, Ore.

Punchbowl Falls at Eagle Creek, Ore.

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Because the geography of the land is never limited, a waterfall can be one of these things or have any combination of them. For instance, a punchbowl waterfall might descend into a small plunge pool, but the plunge pool might quickly lead to another ledge where the water descends as a plunge waterfall. These waterfalls are generally called tiered. There are seemingly endless possibilities, which is probably the biggest reason people look for and are interested in new waterfalls.

A punchbowl waterfall happens when water from a wider river flows through a narrower area and out into the plunge pool -- the water appears to punch through the thin space and into the open.

For lots more information about waterfalls and other geological formations, see the next page.

Lots More Information

Related HowStuffWorks ArticlesMore Great LinksSources
  • Gross, Michael. "Chasing the ultimate waterfall." New York Times. March 18, 2007.
  • Hamson, Rob. "Waterfalls." Royal Melbourne University of Technology.
  • "Observe river erosion creating waterfalls and chasms." Exploring Earth. McDougal Littell.
  • "Waterfalls." Kent County Council.
  • "Waterfalls." School Portal. UniServity.
  • "What do you consider a waterfall?" World Waterfall Database.