A waterfall is usually a geological formation resulting from water, often in the form of 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 created as garden and landscape ornament.

Some waterfalls form in mountain 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.

Typically, a stream flow across an area of formations 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.

Waterfalls can also form due to glaciation, whereby a stream or river flowing into a glacier continues to flow into a valley after the glacier has receded or melted. The large waterfalls in Yosemite Valley are examples of this phenomenon.

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.

Block

Water descends from a relatively wide stream or river.

Cascade

Water descends a series of rock steps.

Cataract

A large waterfall.

Fan

Water spreads horizontally as it descends while remaining in contact with bedrock.

Horsetail

Descending water maintains some contact with bedrock.

Plunge

Water descends vertically, losing contact with the bedrock surface.

Punchbowl

Water descends in a constricted form, then spreads out in a wider pool.

Segmented

Distinctly separate flows of water form as it descends.

Tiered

Water drops in a series of distinct steps or falls.

Multi step

A series of waterfalls one after another of roughly the same size each with its own sunken plunge pool.

Famous Waterfalls

Explaining the Waterfall Illusion New Scientist - August 30, 2003

It is an illusion that has bedazzled people since Aristotle described it 2000 years ago. If you look at a waterfall for a short time, then look at the bank beside it, the bank will appear to drift upwards. Now an experiment that monitors brain activity has explained how the "waterfall effect" arises.

It confirms a hypothesis proposed in the 19th century by the German psychologist Sigmund Exner. He said the waterfall illusion was caused by neurons tuned to opposite directions of motion.

While watching a waterfall, the brain cells that detect downward motion become tired. When the eyes look away, the cells that detect upward motion are more active and a stationary object appears to be moving up.

Neuroscientists at New York University used a more precise version of the waterfall scene to confirm this. Adam Kohn and Tony Movshon played a series of moving images to monkeys while recording the activity of neurons in their brains' motion processing centres.

Less responsiveWhen the monkeys were shown an image of a series of lines moving down, the cells attuned to register downward motion became less responsive over time, while cells attuned to upward motion were unaffected.

The researchers then superimposed two moving images - one of lines moving up and another of lines moving down. Much as predicted by Exner, the neurons sensing upward motion in the monkeys' brains were more active than those sensing downward motion, which were by now fatigued. "This imbalance could give rise to illusory upward motion," Kohn says.

This illusion confirms the waterfall effect. If you look at the superimposed image after looking at just the downward-moving image, the lines moving down disappear and the image seems to drift up.

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