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A waterfall is usually a geological formation resulting from a stream flowing over an erosion-resistant rock formation that forms a sudden break in elevation. Waterfalls may also be artificial, and they are sometimes used for garden and landscape ornament. 

Some waterfalls form in montane environments where erosion is rapid and stream courses may be subject to sudden and catastrophic change. In such cases, the waterfall may not be the end product of many years of water action over a region, but rather the result of relatively sudden geological processes such as thrust faults or volcanic action. 

Most waterfalls are the result of many years' of action of water on the underlying strata. Typically, a stream will flow across an area of formations, and more resistant rock strata will form shelves across the streamway, elevated above the further stream bed when the less erosion-resistant rock around it disappears. Over a period of years, the edges of this shelf will gradually break away and the waterfall will steadily move upstream. Often, the rock strata just below the more resistant shelf will be of a softer type, and will erode out to form a shallow cave-like formation known as a rock shelter (also known as a rock house) under and behind the waterfall. 

Streams often become wider and more shallow just above waterfalls due to flowing over the rock shelf, and there is usually a deep pool just below the waterfall due to the kinetic energy of the water hitting the bottom. 

Waterfalls are a hindrance to river transportation. The Welland Canal was built in 1829 to allow ships to pass Niagara Falls in the Great Lakes. 

Notable waterfalls include: 

Angel Falls, the world's highest at 1000 m (3281 feet), in Venezuela in South America 
Victoria Falls, the world's largest, in Africa, on the Zambezi River on the border between Zambia and Zimbabwe 
Boyoma Falls, the world's highest volume, 17,000,000 liters per second (600,000 cubic feet per second), Congo River, Congo 
Yosemite Falls, the tallest in North America in Yosemite National Park in California, United States. 
Niagara Falls, the best-known in North America, located on the Niagara River on the border between New York, United States, and Ontario, Canada 
Iguaš˙ Falls, in South America, on the Iguašu River on the border between Brazil and Argentina 
Cumberland Falls, a North American waterfall advertised to have a "moonbow"; located in southeast Kentucky 
Rhine Falls, Europe's largest, located in Switzerland 
Reichenbach Falls, Switzerland, total drop 250 m (656 feet), at 90 m (300 feet) the Upper Reichenbach Falls is one of the highest cataracts in the Alps. Scene of the final fictional meeting between Sherlock Holmes and Professor Moriarty. 
The Falls of Saint Anthony, the highest on the Mississippi. 

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