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89-449: The stream was named for Richard Gwinn, who opened a trading post along it in 1669. Gwynns Falls does not actually have a waterfall , but its rough, rocky nature caused John Smith to comment on how the stream tumbled over "felles". This confusing local practice of using "falls" in the name of rocky streams was also applied to Baltimore's Jones Falls and Gunpowder Falls , which do not have waterfalls. During periods of high water conditions

178-424: A cirque landform (alternatively known as a corrie or as a cwm ) – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by arêtes ) – which collects and compresses through gravity the snow that falls into it. This snow accumulates and the weight of the snow falling above compacts it, forming névé (granular snow). Further crushing of the individual snowflakes and squeezing

267-467: A portage around the pipe crossing. 39°16′42″N 76°39′40″W  /  39.27833°N 76.66111°W  / 39.27833; -76.66111 This Baltimore location article is a stub . You can help Misplaced Pages by expanding it . This article about a location in Baltimore County, Maryland is a stub . You can help Misplaced Pages by expanding it . This article related to

356-549: A falls, so almost anything is possible given the right geological and hydrological setting. 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. In addition to gradual processes such as erosion, earth movement caused by earthquakes or landslides or volcanoes can lead to

445-522: A glacier is usually assessed by determining the glacier mass balance or observing terminus behavior. Healthy glaciers have large accumulation zones, more than 60% of their area is snow-covered at the end of the melt season, and they have a terminus with a vigorous flow. Following the Little Ice Age 's end around 1850, glaciers around the Earth have retreated substantially . A slight cooling led to

534-601: A glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water. On Earth, 99% of glacial ice is contained within vast ice sheets (also known as "continental glaciers") in the polar regions , but glaciers may be found in mountain ranges on every continent other than the Australian mainland, including Oceania's high-latitude oceanic island countries such as New Zealand . Between latitudes 35°N and 35°S, glaciers occur only in

623-411: A glacier via moulins . Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface. Most of the important processes controlling glacial motion occur in the ice-bed contact—even though it is only a few meters thick. The bed's temperature, roughness and softness define basal shear stress, which in turn defines whether movement of

712-408: A kilometer per year. Eventually, the ice will be surging fast enough that it begins to thin, as accumulation cannot keep up with the transport. This thinning will increase the conductive heat loss, slowing the glacier and causing freezing. This freezing will slow the glacier further, often until it is stationary, whence the cycle can begin again. The flow of water under the glacial surface can have

801-404: A large effect on the motion of the glacier itself. Subglacial lakes contain significant amounts of water, which can move fast: cubic kilometers can be transported between lakes over the course of a couple of years. This motion is thought to occur in two main modes: pipe flow involves liquid water moving through pipe-like conduits, like a sub-glacial river; sheet flow involves motion of water in

890-460: A lower heat conductance, meaning that the basal temperature is also likely to be higher. Bed temperature tends to vary in a cyclic fashion. A cool bed has a high strength, reducing the speed of the glacier. This increases the rate of accumulation, since newly fallen snow is not transported away. Consequently, the glacier thickens, with three consequences: firstly, the bed is better insulated, allowing greater retention of geothermal heat. Secondly,

979-465: A pioneering work on waterfalls. In 1942 Oscar von Engeln wrote of the lack of research on waterfalls: Waterfall sites more than any other geomorphic feature attract and hold the interest of the general public. Because they have such a popular approval waterfalls are not given serious attention by some students of systematic geomorphology. This attitude is not to be commended. Waterfalls are significant items for geomorphic investigation. As late as 1985

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1068-457: A river in Maryland is a stub . You can help Misplaced Pages by expanding it . Waterfall 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

1157-795: A role in many cultures, as religious sites and subjects of art and music. Many artists have painted waterfalls and they are referenced in many songs, such as those of the Kaluli people in Papua New Guinea . Michael Harner titled his study of the Jivaroan peoples of Ecuador The Jivaro: People of the Sacred Waterfalls. 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

1246-603: A scholar felt that "waterfalls remain a very much neglected aspect of river studies". Studies of waterfalls increased dramatically in the second half of the 20th century. Numerous waterfall guidebooks exist, and the World Waterfall Database is a website cataloging thousands of waterfalls. Many explorers have visited waterfalls. European explorers recorded waterfalls they came across. In 1493, Christopher Columbus noted Carbet Falls in Guadeloupe , which

1335-506: 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. When warm and cold water meets by a gorge in the ocean, large underwater waterfalls can form as

1424-488: A thin layer. A switch between the two flow conditions may be associated with surging behavior. Indeed, the loss of sub-glacial water supply has been linked with the shut-down of ice movement in the Kamb ice stream. The subglacial motion of water is expressed in the surface topography of ice sheets, which slump down into vacated subglacial lakes. The speed of glacial displacement is partly determined by friction . Friction makes

1513-410: A tremendous impact as the iceberg strikes the water. Tidewater glaciers undergo centuries-long cycles of advance and retreat that are much less affected by climate change than other glaciers. Thermally, a temperate glacier is at a melting point throughout the year, from its surface to its base. The ice of a polar glacier is always below the freezing threshold from the surface to its base, although

1602-468: Is a persistent body of dense ice that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries . It acquires distinguishing features, such as crevasses and seracs , as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques , moraines , or fjords . Although

1691-408: 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". 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

1780-456: Is above or at freezing at the interface and is able to slide at this contact. This contrast is thought to a large extent to govern the ability of a glacier to effectively erode its bed , as sliding ice promotes plucking at rock from the surface below. Glaciers which are partly cold-based and partly warm-based are known as polythermal . Glaciers form where the accumulation of snow and ice exceeds ablation . A glacier usually originates from

1869-407: Is affected by factors such as slope, ice thickness, snowfall, longitudinal confinement, basal temperature, meltwater production, and bed hardness. A few glaciers have periods of very rapid advancement called surges . These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous movement state. These surges may be caused by the failure of the underlying bedrock,

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1958-824: 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, and the Inga Falls on the Congo River are the biggest by flow rate , while the Dry Falls in Washington are the largest confirmed waterfalls ever. The highest known subterranean waterfall

2047-411: Is because these peaks are located near or in the hyperarid Atacama Desert . Glaciers erode terrain through two principal processes: plucking and abrasion . As glaciers flow over bedrock, they soften and lift blocks of rock into the ice. This process, called plucking, is caused by subglacial water that penetrates fractures in the bedrock and subsequently freezes and expands. This expansion causes

2136-406: Is by basal sliding, where meltwater forms between the ice and the bed itself. Whether a bed is hard or soft depends on the porosity and pore pressure; higher porosity decreases the sediment strength (thus increases the shear stress τ B ). Porosity may vary through a range of methods. Bed softness may vary in space or time, and changes dramatically from glacier to glacier. An important factor

2225-434: Is called glaciology . Glaciers are important components of the global cryosphere . Glaciers are categorized by their morphology, thermal characteristics, and behavior. Alpine glaciers form on the crests and slopes of mountains. A glacier that fills a valley is called a valley glacier , or alternatively, an alpine glacier or mountain glacier . A large body of glacial ice astride a mountain, mountain range, or volcano

2314-416: Is called rock flour and is made up of rock grains between 0.002 and 0.00625 mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides, which add even more material to the glacier. Glacial abrasion is commonly characterized by glacial striations . Glaciers produce these when they contain large boulders that carve long scratches in

2403-466: 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

2492-647: Is higher, and the mountains above 5,000 m (16,400 ft) usually have permanent snow. Even at high latitudes, glacier formation is not inevitable. Areas of the Arctic , such as Banks Island , and the McMurdo Dry Valleys in Antarctica are considered polar deserts where glaciers cannot form because they receive little snowfall despite the bitter cold. Cold air, unlike warm air, is unable to transport much water vapor. Even during glacial periods of

2581-685: 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. Artificial waterfalls are water features or fountains that imitate a natural waterfall. The Cascata delle Marmore is the tallest artificially built waterfall at 541 feet (165 m). Glacier A glacier ( US : / ˈ ɡ l eɪ ʃ ər / ; UK : / ˈ ɡ l æ s i ər , ˈ ɡ l eɪ s i ər / )

2670-897: Is termed an ice cap or ice field . Ice caps have an area less than 50,000 km (19,000 sq mi) by definition. Glacial bodies larger than 50,000 km (19,000 sq mi) are called ice sheets or continental glaciers . Several kilometers deep, they obscure the underlying topography. Only nunataks protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland. They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over 70 m (230 ft). Portions of an ice sheet or cap that extend into water are called ice shelves ; they tend to be thin with limited slopes and reduced velocities. Narrow, fast-moving sections of an ice sheet are called ice streams . In Antarctica, many ice streams drain into large ice shelves . Some drain directly into

2759-490: 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

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2848-413: Is the region where there is a net loss in glacier mass. The upper part of a glacier, where accumulation exceeds ablation, is called the accumulation zone . The equilibrium line separates the ablation zone and the accumulation zone; it is the contour where the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. In general, the accumulation zone accounts for 60–70% of

2937-402: Is the underlying geology; glacial speeds tend to differ more when they change bedrock than when the gradient changes. Further, bed roughness can also act to slow glacial motion. The roughness of the bed is a measure of how many boulders and obstacles protrude into the overlying ice. Ice flows around these obstacles by melting under the high pressure on their stoss side ; the resultant meltwater

3026-552: Is then forced into the cavity arising in their lee side , where it re-freezes. As well as affecting the sediment stress, fluid pressure (p w ) can affect the friction between the glacier and the bed. High fluid pressure provides a buoyancy force upwards on the glacier, reducing the friction at its base. The fluid pressure is compared to the ice overburden pressure, p i , given by ρgh. Under fast-flowing ice streams, these two pressures will be approximately equal, with an effective pressure (p i – p w ) of 30 kPa; i.e. all of

3115-804: Is undoubtedly presented by the continent of Africa, the 'darkness' of which is almost entirely due to this cause." 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

3204-993: The Andes , the Himalayas , the Rocky Mountains , the Caucasus , Scandinavian Mountains , and the Alps . Snezhnika glacier in Pirin Mountain, Bulgaria with a latitude of 41°46′09″ N is the southernmost glacial mass in Europe. Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period . In New Guinea, small, rapidly diminishing, glaciers are located on Puncak Jaya . Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya , and in

3293-644: The Faroe and Crozet Islands were completely glaciated. The permanent snow cover necessary for glacier formation is affected by factors such as the degree of slope on the land, amount of snowfall and the winds. Glaciers can be found in all latitudes except from 20° to 27° north and south of the equator where the presence of the descending limb of the Hadley circulation lowers precipitation so much that with high insolation snow lines reach above 6,500 m (21,330 ft). Between 19˚N and 19˚S, however, precipitation

3382-477: The Gocta Cataracts were first announced to the world in 2006. 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 . 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

3471-760: The Himalayas , Andes , and a few high mountains in East Africa, Mexico, New Guinea and on Zard-Kuh in Iran. With more than 7,000 known glaciers, Pakistan has more glacial ice than any other country outside the polar regions. Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly 13 million km (5 million sq mi) or about 98% of Antarctica 's 13.2 million km (5.1 million sq mi), with an average thickness of ice 2,100 m (7,000 ft). Greenland and Patagonia also have huge expanses of continental glaciers. The volume of glaciers, not including

3560-562: The Quaternary , Manchuria , lowland Siberia , and central and northern Alaska , though extraordinarily cold, had such light snowfall that glaciers could not form. In addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina are high (4,500 to 6,900 m or 14,800 to 22,600 ft) and cold, but the relative lack of precipitation prevents snow from accumulating into glaciers. This

3649-535: The Rwenzori Mountains . Oceanic islands with glaciers include Iceland, several of the islands off the coast of Norway including Svalbard and Jan Mayen to the far north, New Zealand and the subantarctic islands of Marion , Heard , Grande Terre (Kerguelen) and Bouvet . During glacial periods of the Quaternary, Taiwan , Hawaii on Mauna Kea and Tenerife also had large alpine glaciers, while

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3738-541: 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. 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 and in oceans. The geographer Andrew Goudie wrote in 2020 that waterfalls have received "surprisingly limited research." Alexander von Humboldt wrote about them in

3827-400: The 1820s. There is no name for the specific field of researching waterfalls, and in the published literature been described as "scattered", though it is popular to describe studying waterfalls as "waterfallology". An early paper written on waterfalls was published in 1884 by William Morris Davis , a geologist known as the "father of American geography". In the 1930s Edward Rashleigh published

3916-444: 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. In 2000 Mabin specified that "The horizontal distance between the positions of the lip and plunge pool should be no more than c 25% of

4005-448: The 1990s and 2000s. In a study using data from January 1993 through October 2005, more events were detected every year since 2002, and twice as many events were recorded in 2005 as there were in any other year. Ogives or Forbes bands are alternating wave crests and valleys that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals

4094-703: The 19th century. One of the versions of the Shinto purification ceremony of misogi involves standing underneath a waterfall in ritual clothing. 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

4183-412: The advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous. Glaciers move downhill by the force of gravity and the internal deformation of ice. At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the amount of strain (deformation) is proportional to

4272-520: The air from the snow turns it into "glacial ice". This glacial ice will fill the cirque until it "overflows" through a geological weakness or vacancy, such as a gap between two mountains. When the mass of snow and ice reaches sufficient thickness, it begins to move by a combination of surface slope, gravity, and pressure. On steeper slopes, this can occur with as little as 15 m (49 ft) of snow-ice. In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice called firn . Under

4361-430: The amount of melting at surface of the glacier, the faster the ice will flow. Basal sliding is dominant in temperate or warm-based glaciers. The presence of basal meltwater depends on both bed temperature and other factors. For instance, the melting point of water decreases under pressure, meaning that water melts at a lower temperature under thicker glaciers. This acts as a "double whammy", because thicker glaciers have

4450-713: The annual movement of the glacier. Ogives are formed when ice from an icefall is severely broken up, increasing ablation surface area during summer. This creates a swale and space for snow accumulation in the winter, which in turn creates a ridge. Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives. Glaciers are present on every continent and in approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant subantarctic island territories. Extensive glaciers are found in Antarctica, Argentina, Chile, Canada, Pakistan, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in

4539-426: The base of the waterfall by abrasion , creating a deep plunge pool in the gorge downstream. Streams can become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep area just below the waterfall because of the kinetic energy of the water hitting the bottom. However, a study of waterfalls systematics reported that waterfalls can be wider or narrower above or below

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4628-441: The bed, drilling it out. Sand and stones carried by the watercourse therefore increase erosion capacity. This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it. The rate of retreat for a waterfall can be as high as one-and-a-half metres per year. Often,

4717-432: The bedrock has frequent fractures on the surface, glacial erosion rates tend to increase as plucking is the main erosive force on the surface; when the bedrock has wide gaps between sporadic fractures, however, abrasion tends to be the dominant erosive form and glacial erosion rates become slow. Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching

4806-445: The bedrock. By mapping the direction of the striations, researchers can determine the direction of the glacier's movement. Similar to striations are chatter marks , lines of crescent-shape depressions in the rock underlying a glacier. They are formed by abrasion when boulders in the glacier are repeatedly caught and released as they are dragged along the bedrock. The rate of glacier erosion varies. Six factors control erosion rate: When

4895-401: The cold water rushes to the bottom. 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

4984-571: The created ice's density. The word glacier is a loanword from French and goes back, via Franco-Provençal , to the Vulgar Latin glaciārium , derived from the Late Latin glacia , and ultimately Latin glaciēs , meaning "ice". The processes and features caused by or related to glaciers are referred to as glacial. The process of glacier establishment, growth and flow is called glaciation . The corresponding area of study

5073-467: The deep profile of fjords , which can reach a kilometer in depth as ice is topographically steered into them. The extension of fjords inland increases the rate of ice sheet thinning since they are the principal conduits for draining ice sheets. It also makes the ice sheets more sensitive to changes in climate and the ocean. Although evidence in favor of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as

5162-483: The deformation to become a plastic flow rather than elastic. Then, the glacier will begin to deform under its own weight and flow across the landscape. According to the Glen–Nye flow law , the relationship between stress and strain, and thus the rate of internal flow, can be modeled as follows: where: The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing

5251-678: The environment of the waterfall itself. 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

5340-418: The essentially correct explanation in the 1840s, although it was several decades before it was fully accepted. The top 50 m (160 ft) of a glacier are rigid because they are under low pressure . This upper section is known as the fracture zone and moves mostly as a single unit over the plastic-flowing lower section. When a glacier moves through irregular terrain, cracks called crevasses develop in

5429-415: The formation of waterfalls. 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. Certain aquatic plants and insects also specialize in

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5518-475: The fracture zone. Crevasses form because of differences in glacier velocity. If two rigid sections of a glacier move at different speeds or directions, shear forces cause them to break apart, opening a crevasse. Crevasses are seldom more than 46 m (150 ft) deep but, in some cases, can be at least 300 m (1,000 ft) deep. Beneath this point, the plasticity of the ice prevents the formation of cracks. Intersecting crevasses can create isolated peaks in

5607-453: The glacier to melt, creating a water source that is especially important for plants, animals and human uses when other sources may be scant. However, within high-altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater. Since glacial mass is affected by long-term climatic changes, e.g., precipitation , mean temperature , and cloud cover , glacial mass changes are considered among

5696-428: The glacier will be accommodated by motion in the sediments, or if it'll be able to slide. A soft bed, with high porosity and low pore fluid pressure, allows the glacier to move by sediment sliding: the base of the glacier may even remain frozen to the bed, where the underlying sediment slips underneath it like a tube of toothpaste. A hard bed cannot deform in this way; therefore the only way for hard-based glaciers to move

5785-510: The glacier's surface area, more if the glacier calves icebergs. Ice in the accumulation zone is deep enough to exert a downward force that erodes underlying rock. After a glacier melts, it often leaves behind a bowl- or amphitheater-shaped depression that ranges in size from large basins like the Great Lakes to smaller mountain depressions known as cirques . The accumulation zone can be subdivided based on its melt conditions. The health of

5874-619: The ice at the bottom of the glacier move more slowly than ice at the top. In alpine glaciers, friction is also generated at the valley's sidewalls, which slows the edges relative to the center. Mean glacial speed varies greatly but is typically around 1 m (3 ft) per day. There may be no motion in stagnant areas; for example, in parts of Alaska, trees can establish themselves on surface sediment deposits. In other cases, glaciers can move as fast as 20–30 m (70–100 ft) per day, such as in Greenland's Jakobshavn Isbræ . Glacial speed

5963-420: The ice sheets of Antarctica and Greenland, has been estimated at 170,000 km . Glacial ice is the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the world's freshwater. Many glaciers from temperate , alpine and seasonal polar climates store water as ice during the colder seasons and release it later in the form of meltwater as warmer summer temperatures cause

6052-556: The ice to act as a lever that loosens the rock by lifting it. Thus, sediments of all sizes become part of the glacier's load. If a retreating glacier gains enough debris, it may become a rock glacier , like the Timpanogos Glacier in Utah. Abrasion occurs when the ice and its load of rock fragments slide over bedrock and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces

6141-488: The ice, called seracs . Crevasses can form in several different ways. Transverse crevasses are transverse to flow and form where steeper slopes cause a glacier to accelerate. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form near the edge of the glacier, caused by the reduction in speed caused by friction of the valley walls. Marginal crevasses are largely transverse to flow. Moving glacier ice can sometimes separate from

6230-411: The idea that meltwater, refreezing inside glaciers, caused the glacier to dilate and extend its length. As it became clear that glaciers behaved to some degree as if the ice were a viscous fluid, it was argued that "regelation", or the melting and refreezing of ice at a temperature lowered by the pressure on the ice inside the glacier, was what allowed the ice to deform and flow. James Forbes came up with

6319-418: The increased pressure can facilitate melting. Most importantly, τ D is increased. These factors will combine to accelerate the glacier. As friction increases with the square of velocity, faster motion will greatly increase frictional heating, with ensuing melting – which causes a positive feedback, increasing ice speed to a faster flow rate still: west Antarctic glaciers are known to reach velocities of up to

6408-423: The infrared OH stretching mode of the water molecule. (Liquid water appears blue for the same reason. The blue of glacier ice is sometimes misattributed to Rayleigh scattering of bubbles in the ice.) A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or terminus . Glaciers are broken into zones based on surface snowpack and melt conditions. The ablation zone

6497-679: The most deformation. Velocity increases inward toward the center line and upward, as the amount of deformation decreases. The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below. Because ice can flow faster where it is thicker, the rate of glacier-induced erosion is directly proportional to the thickness of overlying ice. Consequently, pre-glacial low hollows will be deepened and pre-existing topography will be amplified by glacial action, while nunataks , which protrude above ice sheets, barely erode at all – erosion has been estimated as 5 m per 1.2 million years. This explains, for example,

6586-445: The most sensitive indicators of climate change and are a major source of variations in sea level . A large piece of compressed ice, or a glacier, appears blue , as large quantities of water appear blue , because water molecules absorb other colors more efficiently than blue. The other reason for the blue color of glaciers is the lack of air bubbles. Air bubbles, which give a white color to ice, are squeezed out by pressure increasing

6675-661: The native peoples and got their names from gods in the local religion. "In Chinese tradition, the waterfall represents" the season of autumn , yin , and the Chinese dragon 's power over water that comes from the former two. 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

6764-725: The pooling of meltwater at the base of the glacier  — perhaps delivered from a supraglacial lake  — or the simple accumulation of mass beyond a critical "tipping point". Temporary rates up to 90 m (300 ft) per day have occurred when increased temperature or overlying pressure caused bottom ice to melt and water to accumulate beneath a glacier. In glaciated areas where the glacier moves faster than one km per year, glacial earthquakes occur. These are large scale earthquakes that have seismic magnitudes as high as 6.1. The number of glacial earthquakes in Greenland peaks every year in July, August, and September and increased rapidly in

6853-410: The pressure of the layers of ice and snow above it, this granular ice fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. Glacier ice is slightly more dense than ice formed from frozen water because glacier ice contains fewer trapped air bubbles. Glacial ice has a distinctive blue tint because it absorbs some red light due to an overtone of

6942-728: The rise of Romanticism , and increased importance of hydropower with the Industrial Revolution . European explorers often preferred to give waterfalls names in their own language; for instance, David Livingstone named Victoria Falls after Queen Victoria , though it was known by local peoples as Mosi-oa-Tunya. Many waterfalls have descriptive names which can come from the river they are on, places they are near, their features, or events that happened near them. Some countries that were colonized by European nations have taken steps to return names to waterfalls previously renamed by European explorers. Exploration of waterfalls continues;

7031-504: The rock stratum just below the more resistant shelf will be of a softer type, meaning that undercutting due to splashback will occur here to form a shallow cave-like formation known as a rock shelter under and behind the waterfall. Eventually, the outcropping , more resistant cap rock will collapse under pressure to add blocks of rock to the base of the waterfall. These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode

7120-567: The sea, often with an ice tongue , like Mertz Glacier . Tidewater glaciers are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, Baffin , Devon , and Ellesmere Islands in Canada, Southeast Alaska , and the Northern and Southern Patagonian Ice Fields . As the ice reaches the sea, pieces break off or calve, forming icebergs . Most tidewater glaciers calve above sea level, which often results in

7209-409: The stagnant ice above, forming a bergschrund . Bergschrunds resemble crevasses but are singular features at a glacier's margins. Crevasses make travel over glaciers hazardous, especially when they are hidden by fragile snow bridges . Below the equilibrium line, glacial meltwater is concentrated in stream channels. Meltwater can pool in proglacial lakes on top of a glacier or descend into the depths of

7298-562: The stream can be whitewater kayaked , with the most challenging section running from the dam in Dickeyville to the Washington Boulevard bridge crossing. This section is entirely within the city limits of Baltimore and runs under the historic Carrollton Viaduct . An unusual feature of the run is a point near U.S. Route 40 where a city water pipe main crosses about a foot above the water level; kayaking normally requires

7387-423: The stress being applied, ice will act as an elastic solid. Ice needs to be at least 30 m (98 ft) thick to even start flowing, but once its thickness exceeds about 50 m (160 ft) (160 ft), stress on the layer above will exceeds the inter-layer binding strength, and then it'll move faster than the layer below. This means that small amounts of stress can result in a large amount of strain, causing

7476-438: The surface snowpack may experience seasonal melting. A subpolar glacier includes both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier. In a similar way, the thermal regime of a glacier is often described by its basal temperature. A cold-based glacier is below freezing at the ice-ground interface and is thus frozen to the underlying substrate. A warm-based glacier

7565-546: 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. Human development has also threatened many waterfalls. For instance, the Guaíra Falls , once one of the most powerful waterfalls in

7654-423: 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

7743-417: The weight of the ice is supported by the underlying water, and the glacier is afloat. Glaciers may also move by basal sliding , where the base of the glacier is lubricated by the presence of liquid water, reducing basal shear stress and allowing the glacier to slide over the terrain on which it sits. Meltwater may be produced by pressure-induced melting, friction or geothermal heat . The more variable

7832-677: The world, were submerged in 1982 by a human-made dam, as were the Ripon Falls in 1952. Conversely, other waterfalls have seen significantly lower water levels as a result of diversion for hydroelectricity , such as the Tyssestrengene in Norway. Development of the areas around falls as tourist attractions has also destroyed the natural scene around many of them. Waterfalls are included on thirty-eight World Heritage Sites and many others are protected by governments. Waterfalls play

7921-551: Was likely the first waterfall Europeans recorded seeing in the Americas. In the late 1600s, Louis Hennepin visited North America, providing early descriptions of Niagara Falls and the Saint Anthony Falls . The geographer Brian J. Hudson argues that it was uncommon to specifically name waterfalls until the 1700s. The trend of Europeans specifically naming falls was in tandem with increased scientific focus on nature,

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