thumb|[[Dynjandi, a series of waterfalls located in the Westfjords (Vestfirðir), Iceland.]]
A waterfall is any point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge
of a tabular iceberg or ice shelf. Waterfalls can be formed in several ways, but the most common method of formation is that a river courses over a top layer of resistant bedrock before falling onto softer rock, which erodes faster, leading to an increasingly high fall. Waterfalls have been studied for their impact on species living in and around them.
Humans have had a distinct relationship with waterfalls since prehistory, travelling to see them, exploring and naming them. They can present formidable barriers to navigation along rivers. Waterfalls are religious sites in many cultures. Since the 18th century, they have received increased attention as tourist destinations, sources of hydropower, andparticularly since the mid-20th centuryas subjects of research.
Definition and terminology
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. In 2000 Mabin specified that "The horizontal distance between the positions of the lip and plunge pool should be no more than 25% of the waterfall height." There are various types and methods to classify waterfalls. Some scholars have included rapids as a subsection. What actually constitutes a waterfall continues to be debated.
Waterfalls are sometimes interchangeably referred to as "cascades" and "cataracts", though some sources specify a cataract as being a larger and more powerful waterfall and a cascade as being smaller. A plunge pool is a type of stream pool formed at the bottom of a waterfall. A waterfall may also be referred to as a "foss" or "force".
Formation
thumb|upright=1.5|Formation of a waterfall
Natural waterfalls are commonly formed in the upper course of a river where lakes flow into valleys in steep mountains.
A river sometimes flows over a large step in the rocks that may have been formed by a fault line. Waterfalls can occur along the edge of a glacial trough, where 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, which is referred to as a hanging valley. Another reason hanging valleys may form is where two rivers join and one is flowing faster than the other.
A waterfall is usually defined as being at least five feet high, although most are higher. Mill Pond Falls, in Newington, Connecticut, high, is claimed to be the smallest natural waterfall in the United States. Some sources use a minimum height of . A glossary of waterfall terms was published in 1996. There are many different types of waterfalls.
- Combination:
- Cascade:
- Cataract
- Ledge
- Block / Sheet
- Classical
- Curtain
- Overhanging Ledge
- Plunge / Vertical
- Punchbowl
- Chute
- Fan
- Horsetail
- Parallel / Twin
- Ribbon
- Scree / Talus
- Segmented
- Slide
- Slot / Keyhole
- Tiered / Staircase / Multi-Stepped
- Veil
- Frozen
- Dry / Historic
Caprock model
The caprock model of waterfall formation states that the river courses over resistant bedrock, erosion happens slowly and is dominated by impacts of water-borne sediment on the rock, while downstream the erosion occurs more rapidly. 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. Hydraulic jets and hydraulic jumps at the toe of a falls can generate large forces to erode the bed, especially when forces are amplified by water-borne sediment. Horseshoe-shaped falls focus the erosion to a central point, also enhancing riverbed change below a waterfall.
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. Sand and stones carried by the watercourse therefore increase erosion capacity. The rate of retreat for a waterfall can be as high as one-and-a-half metres per year. Waterfalls normally form in a rocky area due to erosion. 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. A 2012 study of the Agbokim Waterfalls, has suggested that they hold biodiversity to a much higher extent than previously thought.
Waterfalls also affect terrestrial species. 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. Species of these plants may have disjunct populations at waterfall zones far from their core range.
Waterfalls provide nesting cover for several species of bird, such as the black swift and white-throated dipper. These species preferentially nest in the space behind the falling water, which is thought to be a strategy to avoid predation.
Types
{| class="wikitable"
|+
!Type
!Image
!Notes
|-
|Ledge waterfall
|frameless|100x100px
|Water descends vertically over a vertical cliff, maintaining partial contact with the bedrock. May be several types:
- Block/Sheet: Water descends from a relatively wide stream or river.
- Fan: Water spreads horizontally as it falls while remaining in contact with bedrock.
|}
thumb|An example of an ephemeral waterfall. This one, when flowing, feeds into the [[Chagrin River.]]
Some waterfalls are also distinct in that they do not flow continuously. Ephemeral waterfalls only flow after a rain or a significant snowmelt.
Waterfalls can also be found underground.
Waterfalls may also be found under the ocean, where cold, denser water overflows a cataract and sinks below the adjacent warmer water. Beneath the Denmark Strait, an underwater fall flows from a point 2000 feet beneath the strait's surface to the ocean floor about 11,500 feet deep.
Waterfalls can pose major barriers to travel. Canals are sometimes built as a method to go around them, other times things must be physically carried around or a railway built.
Development and tourism
Waterfalls are often visited by people simply to see them. Hudson theorizes that they make good tourism sites because they are generally considered beautiful and are relatively uncommon. Activities at waterfalls can include bathing, swimming, photography, rafting, canyoning, abseiling, rock climbing, and ice climbing. Waterfalls can also be sites for generating hydroelectric power and can hold good fishing opportunities. Wealthy people were known to visit areas with features such as waterfalls at least as early as in Ancient Rome and China. 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. Remote waterfalls are now often visited by air travel. In Japan the Nachi Falls are a site of pilgrimage, as are falls near Tirupati, India, and the Saut-d'Eau, Haiti. The Otavalos use Piguchi waterfall as part of the Churru ritual which serves as a coming of age ceremony. Many waterfalls in Africa were places of worship for the native peoples and got their names from gods in the local religion.
List
thumb|260px|[[Shaki Waterfall in Armenia]]
There are thousands of waterfalls in the world, though no exact number has been calculated. The World Waterfall Database lists 7,827 as of 2013, but this is likely incomplete; as noted by Hudson, over 90% of their listings are in North America. Many guidebooks to local waterfalls have been published. There is also no agreement how to measure the height of a waterfall, or even what constitutes one. Angel Falls in Venezuela is the tallest waterfall in the world, the Khone Phapheng Falls in Laos are the widest, while the Dry Falls in Washington are the largest confirmed waterfalls ever. The highest known subterranean waterfall is in Vrtoglavica Cave in Slovenia. The Denmark Strait cataract is an undersea overflow which could be considered a "waterfall" under a very broad usage of that term; if so included, it is the largest known waterfall. The Cascata delle Marmore is the tallest artificially built waterfall at .
See also
- Reverse waterfall
- Subterranean waterfall