Waterfall

Since the 18th century, they have received increased attention as tourist destinations, sources of hydropower, and—particularly since the mid-20th century—as subjects of research.

A waterfall is generally defined as a point in a river where water flows over a steep drop that is close to or directly vertical.

[8][9] Waterfalls are commonly formed in the upper course of a river where lakes flow into valleys in steep mountains.

[10][13] As the watercourse increases its velocity at the edge of the waterfall, it may pluck material from the riverbed, if the bed is fractured or otherwise more erodible.

[15] A process known as "potholing" involves local erosion of a potentially deep hole in bedrock due to turbulent whirlpools spinning stones around on the bed, drilling it out.

These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool in the gorge downstream.

After a long period of being fully formed, the water falling off the ledge will retreat, causing a horizontal pit parallel to the waterfall wall.

Eventually, as the pit grows deeper, the waterfall collapses to be replaced by a steeply sloping stretch of river bed.

[10] In addition to gradual processes such as erosion, earth movement caused by earthquakes or landslides or volcanoes can lead to the formation of waterfalls.

[17] Waterfalls are an important factor in determining the distribution of lotic organisms such as fish and aquatic invertebrates, as they may restrict dispersal along streams.

The presence or absence of certain species can have cascading ecological effects, and thus cause differences in trophic regimes above and below waterfalls.

[19][20] A 2012 study of the Agbokim Waterfalls, has suggested that they hold biodiversity to a much higher extent than previously thought.

They create a small microclimate in their immediate vicinity characterized by cooler temperatures and higher humidity than the surrounding region, which may support diverse communities of mosses and liverworts.

These species preferentially nest in the space behind the falling water, which is thought to be a strategy to avoid predation.

[34] An early paper written on waterfalls was published in 1884 by William Morris Davis, a geologist known as the "father of American geography".

[17][38] In 1885, the geographer George Chisholm wrote that, "The most signal example of the effect of waterfalls and rapids in retarding the development of civilisation is undoubtedly presented by the continent of Africa, the 'darkness' of which is almost entirely due to this cause.

Hudson theorizes that they make good tourism sites because they are generally considered beautiful and are relatively uncommon.

However, many waterfalls were essentially inaccessible due to the treacherous terrain surrounding them until improvements began to be made such as paths to the falls, becoming common across the United Kingdom and America in the 1800s and continuing through the 1900s and into the 21st century.

Conversely, other waterfalls have seen significantly lower water levels as a result of diversion for hydroelectricity, such as the Tyssestrengene in Norway.

[43] Artists such as those of the Hudson River School and J. M. W. Turner and John Sell Cotman painted particularly notable pictures of waterfalls in the 19th century.

[44] One of the versions of the Shinto purification ceremony of misogi involves standing underneath a waterfall in ritual clothing.