Channel (geography)

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Vivari Channel in Albania links Lake Butrint with the Straits of Corfu. Vivari Channel.jpg
Vivari Channel in Albania links Lake Butrint with the Straits of Corfu.

In physical geography, a channel is a type of landform consisting of the outline of a path of relatively shallow and narrow body of fluid, most commonly the confine of a river, river delta or strait. The word is cognate to canal, and sometimes takes this form, e.g. the Hood Canal.

Contents

Formation

Channel initiation refers to the site on a mountain slope where water begins to flow between identifiable banks. [1] This site is referred to as the channel head and it marks an important boundary between hillslope processes and fluvial processes. [1] The channel head is the most upslope part of a channel network and is defined by flowing water between defined identifiable banks. [1] A channel head forms as overland flow and/or subsurface flow accumulate to a point where shear stress can overcome erosion resistance of the ground surface. [1] Channel heads are often associated with colluvium, hollowss and landslides. [1]

Overland flow is a primary factor in channel initiation where saturation overland flow deepens to increase shear stress and begin channel incision. [1] Overland flows converge in topographical depressions where channel initiation begins. Soil composition, vegetation, precipitation, and topography dictate the amount and rate of overland flow. The composition of a soil determines how quickly saturation occurs and cohesive strength retards the entrainment of material from overland flows. [1] Vegetation slows infiltration rates during precipitation events and plant roots anchor soil on hillslopes. [1]

Subsurface flow destabilizes soil and resurfaces on hillslopes where channel heads are often formed. This often results in abrupt channel heads and landslides. Hollows form due to concentrated subsurface flows where concentrations of colluvium are in a constant flux. [1] Channel heads associated with hollows in steep terrain frequently migrate up and down hillslopes depending on sediment supply and precipitation.

Natural channels

Natural channels are formed by fluvial process and are found across the Earth. These are mostly formed by flowing water from the hydrological cycle, though can also be formed by other fluids such as flowing lava can form lava channels. Channels also describe the deeper course through a reef, sand bar, bay, or any shallow body of water. An example of a river running through a sand bar is the Columbia Bar the mouth of the Columbia river.

A stream channel is the physical confine of a stream (river) consisting of a bed and stream banks. Stream channels exist in a variety of geometries. Stream channel development is controlled by both water and sediment movement. There is a difference between low gradient streams (less than a couple of percent in gradient or slightly sloped) and high gradient streams (steeply sloped). A wide variety of stream channel types can be distinguished (e.g. braided rivers, wandering rivers, single-thread sinuous rivers etc.). During floods, water flow may exceed the capacity of the channel and flood waters will spill out of the channel and across the valley bottom, floodplain or drainage area.

Examples of rivers that are trapped in their channels: Grand Canyon and Black Canyon of the Gunnison.

In a larger nautical context, as a geographical place name, the term channel is another word for strait, which is defined as a relatively narrow body of water that connects two larger bodies of water. In this nautical context, the terms strait, channel, sound, and passage are synonymous and usually interchangeable. For example, in an archipelago, the water between islands are typically called channels or passage. The English Channel is the strait between England and France.

Waterflow channels

The channel form is described in terms of geometry (plan, cross-sections, profile) enclosed by the materials of its bed and banks. This form is under influence of two major forces: water discharge and sediment supply. For erodible channels the mutual dependence of its parameters may be qualitatively described by Lane's Principle (also known as Lane's relationship): [2] the product of the sediment load and bed Bukhara size is proportional to the product of discharge and channel slope. [3]

Nautical channels

Wooden pilings mark the navigable channel for vessels entering Lake George from the St. Johns River in Florida. Lake George Channel.jpg
Wooden pilings mark the navigable channel for vessels entering Lake George from the St. Johns River in Florida.

It is especially used as a nautical term to mean the dredged and marked lane of safe travel which a cognizant governmental entity guarantees to have a minimum depth across its specified minimum width to all vessels transiting a body of water (see Buoy). The term not only includes the deep-dredged   ship-navigable parts of an estuary or river leading to port facilities, but also to lesser channels accessing boat port-facilities such as marinas. When dredged channels traverse bay mud or sandy bottoms, repeated dredging is often necessary because of the unstable subsequent movement of benthic soils. [4]

Responsibility for monitoring navigability conditions of navigation channels to various port facilities varies, and the actual maintenance work is frequently performed by a third party. Storms, sea-states, flooding, and seasonal sedimentation adversely affect navigability. In the U.S., navigation channels are monitored and maintained by the United States Army Corps of Engineers (USACE), although dredging operations are often carried out by private contractors (under USACE supervision). USACE also monitors water quality and some remediation. This was first established under the Rivers and Harbors Act of 1899 and modified under acts of 1913, 1935, and 1938. For example, the USACE developed the Intracoastal Waterway, and has the Mississippi Valley Division responsible for the Mississippi River from the Gulf to Cairo, Illinois, the North Atlantic Division for New York Harbor and Port of Boston, and the South Pacific Division for Port of Los Angeles and Port of Long Beach. Waterways policing as well as some emergency spill response falls under United States Coast Guard jurisdiction, including inland channels serving ports like Saint Louis hundreds of miles from any coast. The various state or local governments maintain lesser channels, for example former Erie Canal.

Extraterrestrial channels

Extraterrestrial natural channels are found elsewhere in the Solar System than the Earth and the longest and widest of which are the outflow channels on Mars and the channels of Venus many of which are tens of kilometres wide (the network of channels flowing from Argyre Planitia on Mars for example is 8000 km in length and the Baltis Vallis Venus is 7000 km compared to the 6,650 km Nile, the largest active channel on Earth). The exact formation of these large ancient channels is unknown although it is theorised that those on Mars may have been formed due to catastrophic flooding and on Venus by lava flow. In planetary science the term "rille" is sometimes used for similar formations found on The Moon and Mercury that are of inconclusive origin. Channels have also been recently discovered on Titan. The Saturnian moon has the only known liquid-filled channels in the Solar System other than Earth, the largest of which (Vid Flumina) is 400 km in length. [5] [6] These are believed to be formed from flowing hydrocarbons in the hypothesized methanological cycle. [7]

See also

Related Research Articles

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.

Geomorphology Scientific study of landforms and the processes that shape them

Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth's surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform and terrain history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling. Geomorphologists work within disciplines such as physical geography, geology, geodesy, engineering geology, archaeology, climatology and geotechnical engineering. This broad base of interests contributes to many research styles and interests within the field.

Alluvial fan Fan- or cone-shaped deposit of sediment crossed and built up by streams

An alluvial fan is an accumulation of sediments shaped like a section of a shallow cone, with its apex at a point source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but are also found in more humid environments subject to intense rainfall and in areas of modern glaciation. They range in area from less than 1 square kilometre (0.39 sq mi) to almost 20,000 square kilometres (7,700 sq mi).

Fluvial processes Processes associated with rivers and streams

In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.

Landforms are categorized by characteristic physical attributes such as their creating process, shape, elevation, slope, orientation, rock exposure, and soil type.

Gully Landform created by running water and/or mass movement eroding sharply into soil

A gully is a landform created by running water, mass movement, or commonly a combination of both eroding sharply into soil or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble large ditches or small valleys, but are metres to tens of metres in depth and width and are characterised by a distinct 'headscarp' or 'headwall' and progress by headward erosion. Gullies are commonly related to intermittent or ephemeral water flow usually associated with localised intense or protracted rainfall events, or snowmelt. Gullies can be formed and accelerated by cultivation practices on hillslopes in farmland, and they can develop rapidly in rangelands from existing natural erosion forms subject to vegetative cover removal and livestock activity.

Thalweg Line of lowest elevation in a watercourse or valley

In geography and fluvial geomorphology, a thalweg or talweg is the line of lowest elevation within a valley or watercourse.

Colluvium

Colluvium is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combination of these processes. Colluvium is typically composed of a heterogeneous range of rock types and sediments ranging from silt to rock fragments of various sizes. This term is also used to specifically refer to sediment deposited at the base of a hillslope by unconcentrated surface runoff or sheet erosion.

Drainage density is a quantity used to describe physical parameters of a drainage basin. First described by Robert E. Horton, drainage density is defined as the total length of channel in a drainage basin divided by the total area, represented by the following equation:

Meander Sinuous bend in a series in the channel of a river

A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank and deposits sediments on an inner, convex bank which is typically a point bar. The result of this coupled erosion and sedimentation is the formation of a sinuous course as the channel migrates back and forth across the axis of a floodplain.

River engineering Study of human intervention in the course, characteristics, or flow of rivers

River engineering is a discipline of civil engineering which studies human intervention in the course, characteristics, or flow of a river with the intention of producing some defined benefit. People have intervened in the natural course and behaviour of rivers since before recorded history—to manage the water resources, to protect against flooding, or to make passage along or across rivers easier. Since the Yuan Dynasty and Ancient Roman times, rivers have been used as a source of hydropower. From the late 20th century, river engineering has had environmental concerns broader than immediate human benefit and some river engineering projects have been concerned exclusively with the restoration or protection of natural characteristics and habitats.

Sediment transport Movement of solid particles, typically by gravity and fluid entrainment

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

Perennial stream type of river

A perennial stream or perennial river is a stream or river (channel) which has constant stream throughout the year through parts of its stream bed during years of normal rainfall. In the absence of irregular, prolonged, or extreme drought, a perennial stream is a watercourse, or segment, element, or emerging body of water which continually delivers groundwater. For example, an artificial disruption of stream, variability in flow or stream selection associated with the activity in hydropower installations, shall not affect the measurement. Perennial streams do not consist of stagnant water for the wetlands, reservoirs, and ponds that occur all the period. All other streams, or portions thereof, should be considered seasonal rivers or lakes. The stream can cycle from broken to perpetual through multiple iterations, to intermittent through its mechanism.

Point bar Depositional feature of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope

A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend, being very similar to, though often smaller than, towheads, or river islands.

River Natural flowing watercourse

A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of water. Small rivers can be referred to using names such as stream, creek, brook, rivulet, and rill. There are no official definitions for the generic term river as applied to geographic features, although in some countries or communities a stream is defined by its size. Many names for small rivers are specific to geographic location; examples are "run" in some parts of the United States, "burn" in Scotland and northeast England, and "beck" in northern England. Sometimes a river is defined as being larger than a creek, but not always: the language is vague.

An alluvial river is one in which the bed and banks are made up of mobile sediment and/or soil. Alluvial rivers are self-formed, meaning that their channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment. For this reason, alluvial rivers can assume a number of forms based on the properties of their banks; the flows they experience; the local riparian ecology; and the amount, size, and type of sediment that they carry.

Bank erosion marginal wear of a watercourse

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from erosion of the bed of the watercourse, which is referred to as scour.

Meander cutoff

A meander cutoff is a natural form of a cutting or cut in a river occurs when a pronounced meander (hook) in a river is breached by a flow that connects the two closest parts of the hook to form a new channel, a full loop. The steeper drop in gradient (slope) causes the river flow gradually to abandon the meander which will silt up with sediment from deposition. Cutoffs are a natural part of the evolution of a meandering river. Rivers form meanders as they flow laterally downstream, see sinuosity.

The exposed strata at the surface in and around Wichita Falls are the products of one ancient period of deposition with a modest amount of recent and modern alteration. In all cases, the strata are products of terrigenous (non-marine) environments dominated by fluvial depositional and erosional systems.

Legacy sediment (LS) is depositional bodies of sediment inherited from the increase of human activities since the Neolithic. These include a broad range of land use and land cover changes, such as agricultural clearance, lumbering and clearance of native vegetation, mining, road building, urbanization, as well as alterations brought to river systems in the form of dams and other engineering structures meant to control and regulate natural fluvial processes. The concept of LS is used in geomorphology, ecology, as well as in water quality and toxicological studies.

References

  1. 1 2 3 4 5 6 7 8 9 Bierman, R. B, David R. Montgomery (2014). Key Concepts in Geomorphology. W. H. Freeman and Company Publishers. United States.
  2. Lane, E.W. "The importance of fluvial morphology in hydraulic engineering", Proc. American Society of Civil Engineers, 1955, vol. 81, paper 745, pp. 533–551.
  3. Edward Beighley, R.; Killgore, Mark W. (23 May 2011). World Environmental and Water Resources Congress 2011. ISBN   9780784476628. Archived from the original on 24 June 2016. Retrieved 26 November 2015.
  4. History of the Waterways of the Atlantic Coast of the United States Archived January 3, 2007, at the Wayback Machine , USACE, January 1983
  5. O'Neill, Ian. Titan's 'Nile River' Discovered Archived 2012-12-14 at the Wayback Machine Dec 12, 2012
  6. Poggiali, V.; Mastrogiuseppe, M.; Hayes, A.; Seu, R.; Birch, S. P.; Hofgartner, J. D.; Flamini, E.; Lorenz, R. D.; Grima, C.; Kargel, J. S.; Mullen, J. (December 2015). "Liquid-Filled Channels On Titan". AGU Fall Meeting Abstracts. 2015: P12B–02. Bibcode:2015AGUFM.P12B..02P.
  7. pg 71. Large Rivers: Geomorphology and Management. Avijit Gupta. John Wiley & Sons, 2007