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91-617: The river flows through the large river valley of Aarhus Ådal. The valley itself, stretches from Silkeborg to the coastal city of Aarhus , but the Aarhus River only runs through the easternmost parts. The river drains a basin of 324 square kilometres (125 sq mi) in the eastern part of Søhøjlandet and discharges into the Bay of Aarhus which is an embayment of the Kattegat . It originates 54 metres (177 ft) above sea level in

182-429: A glacial armor . Ice can not only erode mountains but also protect them from erosion. Depending on glacier regime, even steep alpine lands can be preserved through time with the help of ice. Scientists have proved this theory by sampling eight summits of northwestern Svalbard using Be10 and Al26, showing that northwestern Svalbard transformed from a glacier-erosion state under relatively mild glacial maxima temperature, to

273-414: A broader floodplain may result. Deposition dominates over erosion. A typical river basin or drainage basin will incorporate each of these different types of valleys. Some sections of a stream or river valleys may have vertically incised their course to such an extent that the valley they occupy is best described as a gorge , ravine , or canyon . Rapid down-cutting may result from localized uplift of

364-533: A cirque glacier. During glacial periods, for example, the Pleistocene ice ages, it is in these locations that glaciers initially form and then, as the ice age proceeds, extend downhill through valleys that have previously been shaped by water rather than ice. Abrasion by rock material embedded within the moving glacial ice causes the widening and deepening of the valley to produce the characteristic U or trough shape with relatively steep, even vertical sides and

455-426: A considerable depth. A gully is distinguished from a rill based on a critical cross-sectional area of at least one square foot, i.e. the size of a channel that can no longer be erased via normal tillage operations. Extreme gully erosion can progress to formation of badlands . These form under conditions of high relief on easily eroded bedrock in climates favorable to erosion. Conditions or disturbances that limit

546-408: A fall in sea level, can produce a distinctive landform called a raised beach . Chemical erosion is the loss of matter in a landscape in the form of solutes . Chemical erosion is usually calculated from the solutes found in streams. Anders Rapp pioneered the study of chemical erosion in his work about Kärkevagge published in 1960. Formation of sinkholes and other features of karst topography

637-485: A glacier-armor state occupied by cold-based, protective ice during much colder glacial maxima temperatures as the Quaternary ice age progressed. These processes, combined with erosion and transport by the water network beneath the glacier, leave behind glacial landforms such as moraines , drumlins , ground moraine (till), glaciokarst , kames, kame deltas, moulins, and glacial erratics in their wake, typically at

728-464: A homogeneous bedrock erosion pattern, curved channel cross-section beneath the ice is created. Though the glacier continues to incise vertically, the shape of the channel beneath the ice eventually remain constant, reaching a U-shaped parabolic steady-state shape as we now see in glaciated valleys . Scientists also provide a numerical estimate of the time required for the ultimate formation of a steady-shaped U-shaped valley —approximately 100,000 years. In

819-423: A large river can remove enough sediments to produce a river anticline , as isostatic rebound raises rock beds unburdened by erosion of overlying beds. Shoreline erosion, which occurs on both exposed and sheltered coasts, primarily occurs through the action of currents and waves but sea level (tidal) change can also play a role. Hydraulic action takes place when the air in a joint is suddenly compressed by

910-492: A mountain mass similar to the Himalaya into an almost-flat peneplain if there are no significant sea-level changes . Erosion of mountains massifs can create a pattern of equally high summits called summit accordance . It has been argued that extension during post-orogenic collapse is a more effective mechanism of lowering the height of orogenic mountains than erosion. Examples of heavily eroded mountain ranges include

1001-594: A narrow valley with steep sides. Gill is used to describe a ravine containing a mountain stream in Cumbria and the Pennines . The term combe (also encountered as coombe ) is widespread in southern England and describes a short valley set into a hillside. Other terms for small valleys such as hope, dean, slade, slack and bottom are commonly encountered in place-names in various parts of England but are no longer in general use as synonyms for valley . The term vale

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1092-432: A relatively flat bottom. Interlocking spurs associated with the development of river valleys are preferentially eroded to produce truncated spurs , typical of glaciated mountain landscapes. The upper end of the trough below the ice-contributing cirques may be a trough-end . Valley steps (or 'rock steps') can result from differing erosion rates due to both the nature of the bedrock (hardness and jointing for example) and

1183-465: A slower rate than that of the main valley floor; thus the difference in the two valleys' depth increases over time. The tributary valley, composed of more resistant rock, then hangs over the main valley. Trough-shaped valleys also form in regions of heavy topographic denudation . By contrast with glacial U-shaped valleys, there is less downward and sideways erosion. The severe downslope denudation results in gently sloping valley sides; their transition to

1274-427: A valley between its sides is referred to as the valley floor. The valley floor is typically formed by river sediments and may have fluvial terraces . The development of a river valley is affected by the character of the bedrock over which the river or stream flows, the elevational difference between its top and bottom, and indeed the climate. Typically the flow will increase downstream and the gradient will decrease. In

1365-510: A variety of transitional forms between V-, U- and plain valleys can form. The floor or bottom of these valleys can be broad or narrow, but all valleys have a shoulder. The broader a mountain valley, the lower its shoulders are located in most cases. An important exception is canyons where the shoulder almost is near the top of the valley's slope. In the Alps – e.g. the Tyrolean Inn valley –

1456-670: A watercourse only rarely. In areas of limestone bedrock , dry valleys may also result from drainage now taking place underground rather than at the surface. Rift valleys arise principally from earth movements , rather than erosion. Many different types of valleys are described by geographers, using terms that may be global in use or else applied only locally. Valleys may arise through several different processes. Most commonly, they arise from erosion over long periods by moving water and are known as river valleys. Typically small valleys containing streams feed into larger valleys which in turn feed into larger valleys again, eventually reaching

1547-501: A wave closing the entrance of the joint. This then cracks it. Wave pounding is when the sheer energy of the wave hitting the cliff or rock breaks pieces off. Abrasion or corrasion is caused by waves launching sea load at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with corrosion ). Corrosion is the dissolving of rock by carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind of erosion. Attrition

1638-412: A weak bedrock (containing material more erodible than the surrounding rocks) erosion pattern, on the contrary, the amount of over deepening is limited because ice velocities and erosion rates are reduced. Glaciers can also cause pieces of bedrock to crack off in the process of plucking. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at

1729-488: Is deposited . Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution . Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres. Agents of erosion include rainfall ; bedrock wear in rivers ; coastal erosion by

1820-405: Is a tributary valley that is higher than the main valley. They are most commonly associated with U-shaped valleys, where a tributary glacier flows into a glacier of larger volume. The main glacier erodes a deep U-shaped valley with nearly vertical sides, while the tributary glacier, with a smaller volume of ice, makes a shallower U-shaped valley. Since the surfaces of the glaciers were originally at

1911-494: Is also more prone to mudslides, landslides, and other forms of gravitational erosion processes. Tectonic processes control rates and distributions of erosion at the Earth's surface. If the tectonic action causes part of the Earth's surface (e.g., a mountain range) to be raised or lowered relative to surrounding areas, this must necessarily change the gradient of the land surface. Because erosion rates are almost always sensitive to

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2002-625: Is an elongated low area often running between hills or mountains and typically containing a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over a very long period. Some valleys are formed through erosion by glacial ice . These glaciers may remain present in valleys in high mountains or polar areas. At lower latitudes and altitudes, these glacially formed valleys may have been created or enlarged during ice ages but now are ice-free and occupied by streams or rivers. In desert areas, valleys may be entirely dry or carry

2093-484: Is an example of extreme chemical erosion. Glaciers erode predominantly by three different processes: abrasion/scouring, plucking , and ice thrusting. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood. Scientists have shown that, in addition to the role of temperature played in valley-deepening, other glaciological processes, such as erosion also control cross-valley variations. In

2184-409: Is distinguished from changes on the bed of the watercourse, which is referred to as scour . Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times. Thermal erosion is the result of melting and weakening permafrost due to moving water. It can occur both along rivers and at

2275-405: Is of two primary varieties: deflation , where the wind picks up and carries away loose particles; and abrasion , where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) surface creep , where larger, heavier particles slide or roll along the ground; (2) saltation , where particles are lifted a short height into

2366-501: Is only 50 meters (160 ft) deep while the main fjord is around 1,300 meters (4,300 ft) at the same point. Glaciated terrain is not the only site of hanging streams and valleys. Hanging valleys are also simply the product of varying rates of erosion of the main valley and the tributary valleys. The varying rates of erosion are associated with the composition of the adjacent rocks in the different valley locations. The tributary valleys are eroded and deepened by glaciers or erosion at

2457-404: Is sparse and soil is dry (and so is more erodible). Other climatic factors such as average temperature and temperature range may also affect erosion, via their effects on vegetation and soil properties. In general, given similar vegetation and ecosystems, areas with more precipitation (especially high-intensity rainfall), more wind, or more storms are expected to have more erosion. In some areas of

2548-457: Is the main climatic factor governing soil erosion by water. The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by vegetation . This might be during periods when agricultural activities leave the soil bare, or in semi-arid regions where vegetation is naturally sparse. Wind erosion requires strong winds, particularly during times of drought when vegetation

2639-488: Is the word cwm borrowed from Welsh . The word dale occurs widely in place names in the north of England and, to a lesser extent, in southern Scotland. As a generic name for a type of valley, the term typically refers to a wide valley, though there are many much smaller stream valleys within the Yorkshire Dales which are named "(specific name) Dale". Clough is a word in common use in northern England for

2730-629: Is used in England and Wales to describe a wide river valley, usually with a particularly wide flood plain or flat valley bottom. In Southern England, vales commonly occur between the outcrops of different relatively erosion-resistant rock formations, where less resistant rock, often claystone has been eroded. An example is the Vale of White Horse in Oxfordshire. Some of the first human complex societies originated in river valleys, such as that of

2821-400: Is where particles/sea load carried by the waves are worn down as they hit each other and the cliffs. This then makes the material easier to wash away. The material ends up as shingle and sand. Another significant source of erosion, particularly on carbonate coastlines, is boring, scraping and grinding of organisms, a process termed bioerosion . Sediment is transported along the coast in

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2912-699: The Appalachian Mountains , intensive farming practices have caused erosion at up to 100 times the natural rate of erosion in the region. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes ) ecological collapse , both because of loss of the nutrient-rich upper soil layers . In some cases, this leads to desertification . Off-site effects include sedimentation of waterways and eutrophication of water bodies , as well as sediment-related damage to roads and houses. Water and wind erosion are

3003-664: The Great Plains , it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years. Mass wasting or mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity . Mass wasting is an important part of the erosional process and is often the first stage in the breakdown and transport of weathered materials in mountainous areas. It moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up

3094-604: The Latin terms for 'valley, 'gorge' and 'ditch' respectively. The German term ' rille ' or Latin term 'rima' (signifying 'cleft') is used for certain other elongate depressions on the Moon. See also: Erosion Erosion is the action of surface processes (such as water flow or wind ) that removes soil , rock , or dissolved material from one location on the Earth's crust and then transports it to another location where it

3185-504: The Nile , Tigris-Euphrates , Indus , Ganges , Yangtze , Yellow River , Mississippi , and arguably the Amazon . In prehistory , the rivers were used as a source of fresh water and food (fish and game), as well as a place to wash and a sewer. The proximity of water moderated temperature extremes and provided a source for irrigation , stimulating the development of agriculture . Most of

3276-737: The Timanides of Northern Russia. Erosion of this orogen has produced sediments that are now found in the East European Platform , including the Cambrian Sablya Formation near Lake Ladoga . Studies of these sediments indicate that it is likely that the erosion of the orogen began in the Cambrian and then intensified in the Ordovician . If the erosion rate exceeds soil formation , erosion destroys

3367-416: The accumulation zone above the glacial equilibrium line altitude), which causes increased rates of erosion of the mountain, decreasing mass faster than isostatic rebound can add to the mountain. This provides a good example of a negative feedback loop . Ongoing research is showing that while glaciers tend to decrease mountain size, in some areas, glaciers can actually reduce the rate of erosion, acting as

3458-668: The first civilizations developed from these river valley communities. Siting of settlements within valleys is influenced by many factors, including the need to avoid flooding and the location of river crossing points. Numerous elongate depressions have been identified on the surface of Mars , Venus , the Moon , and other planets and their satellites and are known as valles (singular: 'vallis'). Deeper valleys with steeper sides (akin to canyons) on certain of these bodies are known as chasmata (singular: 'chasma'). Long narrow depressions are referred to as fossae (singular: 'fossa'). These are

3549-405: The impact of a falling raindrop creates a small crater in the soil , ejecting soil particles. The distance these soil particles travel can be as much as 0.6 m (2.0 ft) vertically and 1.5 m (4.9 ft) horizontally on level ground. If the soil is saturated , or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs. If

3640-455: The lower crust and mantle . Because tectonic processes are driven by gradients in the stress field developed in the crust, this unloading can in turn cause tectonic or isostatic uplift in the region. In some cases, it has been hypothesised that these twin feedbacks can act to localize and enhance zones of very rapid exhumation of deep crustal rocks beneath places on the Earth's surface with extremely high erosion rates, for example, beneath

3731-407: The surface runoff which may result from rainfall, produces four main types of soil erosion : splash erosion , sheet erosion , rill erosion , and gully erosion . Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four). In splash erosion ,

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3822-499: The 21st century, a strong link has been drawn between the increase in storm frequency with an increase in sediment load in rivers and reservoirs, highlighting the impacts climate change can have on erosion. Vegetation acts as an interface between the atmosphere and the soil. It increases the permeability of the soil to rainwater, thus decreasing runoff. It shelters the soil from winds, which results in decreased wind erosion, as well as advantageous changes in microclimate. The roots of

3913-466: The Brabrand Lake as the most productive fishing spot. As seen the world over, eel went into a rapid decline and with it the local fishing industry in the Aarhus River dried up. Through the 20th century, the river and its ecosystem went into a serious decline due to nutrient pollution from household wastewater and farmland run-off. Today, after some costly efforts to restore the damaging effects of

4004-524: The Scandinavian ice sheet during the various ice ages advanced slightly uphill against the lie of the land. As a result, its meltwaters flowed parallel to the ice margin to reach the North Sea basin, forming huge, flat valleys known as Urstromtäler . Unlike the other forms of glacial valleys, these were formed by glacial meltwaters. Depending on the topography , the rock types , and the climate ,

4095-511: The actual valley bottom is unclear. Trough-shaped valleys occur mainly in periglacial regions and in tropical regions of variable wetness. Both climates are dominated by heavy denudation. Box valleys have wide, relatively level floors and steep sides. They are common in periglacial areas and occur in mid-latitudes, but also occur in tropical and arid regions. Rift valleys, such as the Albertine Rift and Gregory Rift are formed by

4186-433: The air, and bounce and saltate across the surface of the soil; and (3) suspension , where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%). Wind erosion is much more severe in arid areas and during times of drought. For example, in

4277-413: The amount of eroded material that is already carried by, for example, a river or glacier. The transport of eroded materials from their original location is followed by deposition, which is arrival and emplacement of material at a new location. While erosion is a natural process, human activities have increased by 10–40 times the rate at which soil erosion is occurring globally. At agriculture sites in

4368-470: The base along with the glacier. This method produced some of the many thousands of lake basins that dot the edge of the Canadian Shield . Differences in the height of mountain ranges are not only being the result tectonic forces, such as rock uplift, but also local climate variations. Scientists use global analysis of topography to show that glacial erosion controls the maximum height of mountains, as

4459-402: The city of Aarhus since its foundation in the early Viking Age , and archaeological and historical research suggests that it played a crucial role in the origins of the city. There used to be a sizeable catch of eel , and to a lesser degree pike , in the Aarhus River up until the later part of the 20th century. Several families, often based at the rivers' watermills, made a living from it with

4550-522: The coast. Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion, as these portions of the banks are composed of permafrost-cemented non-cohesive materials. Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the Arctic coast , where wave action and near-shore temperatures combine to undercut permafrost bluffs along

4641-409: The coastline. Where there is a bend in the coastline, quite often a buildup of eroded material occurs forming a long narrow bank (a spit ). Armoured beaches and submerged offshore sandbanks may also protect parts of a coastline from erosion. Over the years, as the shoals gradually shift, the erosion may be redirected to attack different parts of the shore. Erosion of a coastal surface, followed by

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4732-433: The destructive eutrofication , the ecosystem and biodiversity of the Aarhus River is recovering and now includes a wide range of fish species once again. Two former fish farms along the Aarhus River at Pinds Mølle and Gammel Harlev has been re-purposed as put-and-take spots with trout for sports fishing. With the advance of the automobile, the river's run through the city of Aarhus was paved over and covered by roads in

4823-411: The direction of the prevailing current ( longshore drift ). When the upcurrent supply of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form as a result of deposition . These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of

4914-509: The expansion of the Earth 's crust due to tectonic activity beneath the Earth's surface. There are many terms used for different sorts of valleys. They include: Similar geographical features such as gullies , chines , and kloofs , are not usually referred to as valleys. The terms corrie , glen , and strath are all Anglicisations of Gaelic terms and are commonly encountered in place-names in Scotland and other areas where Gaelic

5005-405: The extremely steep terrain of Nanga Parbat in the western Himalayas . Such a place has been called a " tectonic aneurysm ". Human land development, in forms including agricultural and urban development, is considered a significant factor in erosion and sediment transport , which aggravate food insecurity . In Taiwan, increases in sediment load in the northern, central, and southern regions of

5096-582: The flood regions result from glacial Lake Missoula , which created the channeled scablands in the Columbia Basin region of eastern Washington . Wind erosion is a major geomorphological force, especially in arid and semi-arid regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as deforestation , urbanization , and agriculture . Wind erosion

5187-443: The flow slower and both erosion and deposition may take place. More lateral erosion takes place in the middle section of a river's course, as strong currents on the outside of its curve erode the bank. Conversely, deposition may take place on the inside of curves where the current is much slacker, the process leading to the river assuming a meandering character. In the lower valley, gradients are lowest, meanders may be much broader and

5278-417: The growth of protective vegetation ( rhexistasy ) are a key element of badland formation. Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward , deepening the valley , and headward , extending the valley into the hillside, creating head cuts and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical,

5369-411: The island can be tracked with the timeline of development for each region throughout the 20th century. The intentional removal of soil and rock by humans is a form of erosion that has been named lisasion . Mountain ranges take millions of years to erode to the degree they effectively cease to exist. Scholars Pitman and Golovchenko estimate that it takes probably more than 450 million years to erode

5460-427: The land surface or rejuvenation of the watercourse as a result for example of a reduction in the base level to which the river is eroded, e.g. lowered global sea level during an ice age . Such rejuvenation may also result in the production of river terraces . There are various forms of valleys associated with glaciation. True glacial valleys are those that have been cut by a glacier which may or may not still occupy

5551-409: The local slope (see above), this will change the rates of erosion in the uplifted area. Active tectonics also brings fresh, unweathered rock towards the surface, where it is exposed to the action of erosion. However, erosion can also affect tectonic processes. The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on

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5642-418: The material and move it to even lower elevations. Mass-wasting processes are always occurring continuously on all slopes; some mass-wasting processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide . However, landslides can be classified in a much more detailed way that reflects

5733-407: The material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence. Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe

5824-438: The mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a scree slope. Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show a spoon-shaped isostatic depression , in which

5915-484: The morphologic impact of glaciations on active orogens, by both influencing their height, and by altering the patterns of erosion during subsequent glacial periods via a link between rock uplift and valley cross-sectional shape. At extremely high flows, kolks , or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called rock-cut basins . Examples can be seen in

6006-404: The most erosion occurs during times of flood when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles , and boulders can also act erosively as they traverse a surface, in a process known as traction . Bank erosion is the wearing away of the banks of a stream or river. This

6097-434: The nutrient-rich upper soil layers . In some cases, the eventual result is desertification . Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation ; combined, they are responsible for about 84% of the global extent of degraded land , making excessive erosion one of

6188-463: The ocean or perhaps an internal drainage basin . In polar areas and at high altitudes, valleys may be eroded by glaciers ; these typically have a U-shaped profile in cross-section, in contrast to river valleys, which tend to have a V-shaped profile. Other valleys may arise principally through tectonic processes such as rifting . All three processes can contribute to the development of a valley over geological time. The flat (or relatively flat) portion of

6279-411: The order of a few centimetres (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers. Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to

6370-556: The plants bind the soil together, and interweave with other roots, forming a more solid mass that is less susceptible to both water and wind erosion. The removal of vegetation increases the rate of surface erosion. The topography of the land determines the velocity at which surface runoff will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes (especially those without adequate vegetative cover) are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes. Steeper terrain

6461-436: The power of the moving ice. In places, a rock basin may be excavated which may later be filled with water to form a ribbon lake or else by sediments. Such features are found in coastal areas as fjords . The shape of the valley which results from all of these influences may only become visible upon the recession of the glacier that forms it. A river or stream may remain in the valley; if it is smaller than one would expect given

6552-413: The relief between mountain peaks and the snow line are generally confined to altitudes less than 1500 m. The erosion caused by glaciers worldwide erodes mountains so effectively that the term glacial buzzsaw has become widely used, which describes the limiting effect of glaciers on the height of mountain ranges. As mountains grow higher, they generally allow for more glacial activity (especially in

6643-828: The rolling of dislodged soil particles 0.5 to 1.0 mm (0.02 to 0.04 in) in diameter by wind along the soil surface. On the continental slope , erosion of the ocean floor to create channels and submarine canyons can result from the rapid downslope flow of sediment gravity flows , bodies of sediment-laden water that move rapidly downslope as turbidity currents . Where erosion by turbidity currents creates oversteepened slopes it can also trigger underwater landslides and debris flows . Turbidity currents can erode channels and canyons into substrates ranging from recently deposited unconsolidated sediments to hard crystalline bedrock. Almost all continental slopes and deep ocean basins display such channels and canyons resulting from sediment gravity flows and submarine canyons act as conduits for

6734-515: The runoff has sufficient flow energy , it will transport loosened soil particles ( sediment ) down the slope. Sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of

6825-446: The same elevation , the shallower valley appears to be 'hanging' above the main valley. Often, waterfalls form at or near the outlet of the upper valley. Hanging valleys also occur in fjord systems underwater. The branches of Sognefjord are much shallower than the main fjord. The mouth of Fjærlandsfjord is about 400 meters (1,300 ft) deep while the main fjord nearby is 1,200 meters (3,900 ft) deep. The mouth of Ikjefjord

6916-666: The sea and waves ; glacial plucking , abrasion , and scour; areal flooding; wind abrasion; groundwater processes; and mass movement processes in steep landscapes like landslides and debris flows . The rates at which such processes act control how fast a surface is eroded. Typically, physical erosion proceeds the fastest on steeply sloping surfaces, and rates may also be sensitive to some climatically controlled properties including amounts of water supplied (e.g., by rain), storminess, wind speed, wave fetch , or atmospheric temperature (especially for some ice-related processes). Feedbacks are also possible between rates of erosion and

7007-538: The shoreline and cause them to fail. Annual erosion rates along a 100-kilometre (62-mile) segment of the Beaufort Sea shoreline averaged 5.6 metres (18 feet) per year from 1955 to 2002. Most river erosion happens nearer to the mouth of a river. On a river bend, the longest least sharp side has slower moving water. Here deposits build up. On the narrowest sharpest side of the bend, there is faster moving water so this side tends to erode away mostly. Rapid erosion by

7098-550: The shoulders are quite low (100–200 meters above the bottom). Many villages are located here (esp. on the sunny side) because the climate is very mild: even in winter when the valley's floor is filled with fog, these villages are in sunshine . In some stress-tectonic regions of the Rocky Mountains or the Alps (e.g. Salzburg ), the side valleys are parallel to each other, and are hanging . Smaller streams flow into rivers as deep canyons or waterfalls . A hanging valley

7189-849: The size of its valley, it can be considered an example of a misfit stream . Other interesting glacially carved valleys include: A tunnel valley is a large, long, U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. Such valleys can be up to 100 km (62 mi) long, 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep (its depth may vary along its length). Tunnel valleys were formed by subglacial water erosion . They once served as subglacial drainage pathways carrying large volumes of meltwater. Their cross-sections exhibit steep-sided flanks similar to fjord walls, and their flat bottoms are typical of subglacial glacial erosion. In northern Central Europe,

7280-593: The soil. Lower rates of erosion can prevent the formation of soil features that take time to develop. Inceptisols develop on eroded landscapes that, if stable, would have supported the formation of more developed Alfisols . While erosion of soils is a natural process, human activities have increased by 10-40 times the rate at which erosion occurs globally. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes ) ecological collapse , both because of loss of

7371-402: The swampy bogland of Astrup Mose close to Stilling-Solbjerg Lake , southwest of the city of Aarhus (56°6' northern latitude, 10°9' eastern longitude) and exits into Aarhus Harbour at 56°9' northern latitude, 10°13' eastern longitude. The river defines the border between Skanderborg and Aarhus Municipality during its first few kilometres. Aarhus River has been important for the development of

7462-433: The terminus or during glacier retreat . The best-developed glacial valley morphology appears to be restricted to landscapes with low rock uplift rates (less than or equal to 2mm per year) and high relief, leading to long-turnover times. Where rock uplift rates exceed 2mm per year, glacial valley morphology has generally been significantly modified in postglacial time. Interplay of glacial erosion and tectonic forcing governs

7553-409: The transfer of sediment from the continents and shallow marine environments to the deep sea. Turbidites , which are the sedimentary deposits resulting from turbidity currents, comprise some of the thickest and largest sedimentary sequences on Earth, indicating that the associated erosional processes must also have played a prominent role in Earth's history. The amount and intensity of precipitation

7644-563: The two primary causes of land degradation ; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of the most significant environmental problems worldwide. Intensive agriculture , deforestation , roads , anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall , and

7735-407: The upper valley, the stream will most effectively erode its bed through corrasion to produce a steep-sided V-shaped valley. The presence of more resistant rock bands, of geological faults , fractures , and folds may determine the course of the stream and result in a twisting course with interlocking spurs . In the middle valley, as numerous streams have coalesced, the valley is typically wider,

7826-407: The valley at the present day. Such valleys may also be known as glacial troughs. They typically have a U-shaped cross-section and are characteristic landforms of mountain areas where glaciation has occurred or continues to take place. The uppermost part of a glacial valley frequently consists of one or more 'armchair-shaped' hollows, or ' cirques ', excavated by the rotational movement downslope of

7917-427: The valleys have a typical V-shaped cross-section and the stream gradient is relatively steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far

8008-554: The world (e.g. western Europe ), runoff and erosion result from relatively low intensities of stratiform rainfall falling onto the previously saturated soil. In such situations, rainfall amount rather than intensity is the main factor determining the severity of soil erosion by water. According to the climate change projections, erosivity will increase significantly in Europe and soil erosion may increase by 13–22.5% by 2050 In Taiwan , where typhoon frequency increased significantly in

8099-491: The world (e.g. the mid-western US ), rainfall intensity is the primary determinant of erosivity (for a definition of erosivity check, ) with higher intensity rainfall generally resulting in more soil erosion by water. The size and velocity of rain drops is also an important factor. Larger and higher-velocity rain drops have greater kinetic energy , and thus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops. In other regions of

8190-639: The years 1930-1958. In the process, the last long stretch was named Åboulevarden and created a new transport route to the thriving industrial harbour. In 1989, it was politically decided that Aarhus River should be opened again, and the project commenced in September 2005. In 2008, the reopened section of the river at Åboulevarden was inaugurated, and work on the last section at the harbor front began, finishing in 2015. 56°09′16″N 10°12′40″E  /  56.15444°N 10.21111°E  / 56.15444; 10.21111 River valley A valley

8281-414: Was once widespread. Strath signifies a wide valley between hills, the floor of which is either level or slopes gently. A glen is a river valley which is steeper and narrower than a strath. A corrie is a basin-shaped hollow in a mountain. Each of these terms also occurs in parts of the world formerly colonized by Britain . Corrie is used more widely by geographers as a synonym for (glacial) cirque , as

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