Channel pattern

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Channel patterns are found in rivers, streams, and other bodies of water that transport water from one place to another. Systems of branching river channels dissect most of the sub-aerial landscape, each in a valley proportioned to its size. Whether formed by chance or necessity, by headward erosion or downslope convergence, whether inherited or newly formed. [1] Depending on different geological factors such as weathering, erosion, depositional environment, and sediment type, different types of channel patterns can form.

Contents

Bedrock vs. alluvial channels

There are two main types of channels, bedrock and alluvial, which are present no matter the sub-classification. Bedrock channels are composed entirely of compacted rock, with only patches of alluvium scattered throughout. Because the bedrock is constantly exposed it takes much less stream power to carve the channel. The hydraulic force of flowing water can push and pull detached joint blocks out of their initial position and roll or drag them downstream. Plucking is common in jointed rocks, where this occurs. [2]

Alluvial channels are much more common and can be large or small. All large rivers, and most small ones, have channels that are usually lined with alluvium, sediment that was carried to that channel reach by the river and that eventually will be carried farther downstream. [3] This lining of alluvium creates a protective shield over the bedrock, which means it takes a much greater stream power to carve the channel.

Braided channels

There are a few distinctly different channel types based on their geological structure and depositional environment. Braided rivers carry fairly coarse-grained sediment down a fairly steep gradient. They typically exhibit numerous channels that split off and rejoin each other to give a braided appearance. Additionally, the water discharge tends to be highly variable. Consequently, braided rivers usually exist near mountainous regions, especially those with glaciers. [4] These braided channels usually occur in tectonically active environments and have a larger sediment load due to varying water flow and discharge.

Anastomosed channels

Often confused with braided channels, anastomosing is reserved for a type of river with multiple, interconnected, coexisting channel belts on alluvial plains. Based on its geomorphology, saucer-shaped islands called flood-basins characterize anastomosing rivers. [5] These channels are classified as a composite form of which the individual channel belts may have braided, meandering or straight channels. Although similar to, and even encompass other channel types, anastomosed rivers are their own entity and have just begun to be studied by geologists, revealing that much is still unknown.

Fluvial processes form several channel patterns, including:

See also

Related Research Articles

Floodplain Land adjacent to a river which is flooded during periods of high discharge

A floodplain or flood plain or flood-plain is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge. The soils usually consist of clays, silts, sands, and gravels deposited during floods.

Geomorphology The 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.

Braided river A network of river channels separated by small, and often temporary, islands

A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, aits or eyots. Braided streams tend to occur in rivers with high sediment loads and/or coarse grain sizes, and in rivers with steeper slopes than typical rivers with straight or meandering channel patterns. They are also associated with rivers with rapid and frequent variation in the amount of water they carry, i.e., with "flashy" rivers, and with rivers with weak banks. Braided channels are found in a variety of environments all over the world, including gravelly mountain streams, sand bed rivers, on alluvial fans, on river deltas, and across depositional plains.

River delta Silt deposition landform at the mouth of a river

A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or another river that cannot carry away the supplied sediment. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position.

Alluvium Loose soil or sediment that is eroded and redeposited in a non-marine setting

Alluvium is loose, unconsolidated soil or sediment that has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it is called an alluvial deposit.

Alluvial fan A 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.

Alluvial plain Region on which rivers have deposited sediment

An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.

Fluvial terraces are elongated terraces that flank the sides of floodplains and fluvial valleys all over the world. They consist of a relatively level strip of land, called a “tread,” separated from either an adjacent floodplain, other fluvial terraces, or uplands by distinctly steeper strips of land called “risers.” These terraces lie parallel to and above the river channel and its floodplain. Because of the manner in which they form, fluvial terraces are underlain by fluvial sediments of highly variable thickness.

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

A meander is one of a series of regular sinuous curves, bends, loops, turns, or windings in the channel of a river, stream, or other watercourse. It is also a term for walking slowly. It is produced by a stream or river swinging from side to side as it flows across its floodplain or shifts its channel within a valley. A meander is produced by a stream or river as it erodes the sediments comprising an outer, concave bank and deposits this and other sediment downstream on an inner, convex bank which is typically a point bar. The result of sediments being eroded from the outside concave bank and their deposition on an inside convex bank is the formation of a sinuous course as a channel migrates back and forth across the down-valley axis of a floodplain. The zone within which a meandering stream shifts its channel across either its floodplain or valley floor from time to time is known as a meander belt. It typically ranges from 15 to 18 times the width of the channel. Over time, meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.

Terrace (geology) A step-like landform

In geology, a terrace is a step-like landform. A terrace consists of a flat or gently sloping geomorphic surface, called a tread, that is typically bounded on one side by a steeper ascending slope, which is called a "riser" or "scarp". The tread and the steeper descending slope together constitute the terrace. Terraces can also consist of a tread bounded on all sides by a descending riser or scarp. A narrow terrace is often called a bench.

Depositional environment The combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment

In geology, depositional environment or sedimentary environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited, the more likely that direct modern analogues are not available.

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.

Avulsion (river) The rapid abandonment of a river channel and formation of a new channel

In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course.

Bar (river morphology) An elevated region of sediment in a river that has been deposited by the flow

A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars, point bars, and mouth bars. The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions, and can show where sediment supply rate is greater than the transport capacity.

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.

Slip-off slope Depositional landform on the inside convex bank of a meandering river

A slip-off slope is a depositional landform that occurs on the inside convex bank of a meandering river. The term can refer to two different features: one in a freely meandering river with a floodplain and the other in an entrenched river.

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.

Glaciofluvial deposits

Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers. They are transported, sorted and deposited by streams of water. The deposits are formed beside, below or downstream from the ice. They include kames, kame terraces and eskers formed in ice contact and outwash fans and outwash plains below the ice margin. Typically the outwash sediment is carried by fast and turbulent fluvio-glacial meltwater streams, but occasionally it is carried by catastrophic outburst floods. Larger elements such as boulders and gravel are deposited nearer to the ice margin, while finer elements are carried farther, sometimes into lakes or the ocean. The sediments are sorted by fluvial processes. They differ from glacial till, which is moved and deposited by the ice of the glacier, and is unsorted.

References

  1. ↑ "The Stages of a River". www.primaryhomeworkhelp.co.uk. Retrieved 2017-05-16.
  2. ↑ Leroy., Bloom, Arthur (1998). Geomorphology : a systematic analysis of late Cenozoic landforms. Prentice-Hall. ISBN   0135054966. OCLC   299847567.
  3. ↑ Diplas, Panayiotis (1990). "Characteristics of Self‐Formed Straight Channels". Journal of Hydraulic Engineering. 116 (5): 707–728. doi:10.1061/(asce)0733-9429(1990)116:5(707).
  4. ↑ Stanley, Chernicoff; L., Whitney, Donna (2007). Geology : an introduction to physical geology. Pearson Prentice Hall. ISBN   978-0131474642. OCLC   489065099.
  5. ↑ Makaske, Bart (2001). "Anastomosing rivers: a review of their classification, origin and sedimentary products" (PDF). Earth-Science Reviews. 53 (3–4): 149–196. Bibcode:2001ESRv...53..149M. doi:10.1016/S0012-8252(00)00038-6. Archived from the original (PDF) on 2016-10-11. Retrieved 2017-05-16.
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