{{short description|Sediment processes associated with rivers and streams}} [[File:Matanuska River 8727.JPG|right|thumb|Deep, eroding [[glaciofluvial deposit]]s alongside the [[Matanuska River]], [[Alaska]]]]
In [[geography]] and [[geology]], '''fluvial sediment processes''' or '''fluvial sediment transport''' are associated with [[river]]s and [[stream]]s and the [[Deposition (geology)|deposits]] and [[landform]]s created by [[sediment]]s. It can result in the formation of [[ripple marks|ripple]]s and [[dune]]s, in [[fractal]]-shaped patterns of erosion, in complex patterns of natural river systems, and in the development of [[floodplain]]s and the occurrence of [[flash floods]]. Sediment moved by water can be larger than sediment moved by air because water has both a higher [[density]] and [[viscosity]]. In typical rivers the largest carried sediment is of [[sand]] and [[gravel]] size, but larger floods can carry [[Cobble (geology)|cobbles]] and even [[boulders]]. When the stream or rivers are associated with [[glacier]]s, [[ice sheet]]s, or [[ice cap]]s, the term ''[[glaciofluvial]]'' or [[Fluvioglacial landform|''fluvioglacial'']] is used, as in [[periglacial]] flows and [[glacial lake outburst flood]]s.<ref>{{cite book |url=https://store.americangeosciences.org/glossary-of-geology-fifth-edition-revised.html |editor-first1=Klaus K.E. |editor-last1=Neuendorf |editor-first2=James P. Jr |editor-last2=Mehl |editor-first3=Julia A. |editor-last3=Jackson |year=2011 |title=Glossary of Geology |publisher=American Geological Institute |location=Alexandria, Virginia |page=800 |edition=5th revised |isbn=978-3-642-06621-4 |oclc=751527782}}</ref><ref>[https://books.google.com/books?id=if-PaNVS7cAC&pg=PA84&dq=fluvioglacial+definition&lr=&ei=KaQrSpnHMIncygTF_fScBw Wilson, W.E. & Moore, J.E. 2003. Glossary of Hydrology, American Geological Institute, Springer, 248pp.]</ref> Fluvial sediment processes include the [[sediment transport|motion of sediment]] and [[erosion]] or [[deposition (geology)|deposition]] on the [[river bed]].<ref>{{cite book|last=Charlton|first=Ro|title=Fundamentals of fluvial geomorphology|url=https://archive.org/details/fundamentalsfluv00char|url-access=limited|year=2008|publisher=Rutledge|location=London|isbn=978-0-415-33454-9|page=[https://archive.org/details/fundamentalsfluv00char/page/n274 234]}}</ref><ref>{{cite book|last=Wohl|first=Ellen|title=Rivers in the Landscape: Science and Management|year=2014|publisher=Wiley-Blackwell|isbn=978-1-118-41489-7|page=330}}</ref>
==Principles== [[File:ISS013-E-46370.jpg|thumb|The White River is so named due to the clay it picks up in the Badlands of South Dakota. Here it flows into the Missouri River and colors it with clay.]]
The movement of water across the [[stream bed]] exerts a [[shear stress]] directly onto the bed. If the [[Cohesion (geology)|cohesive]] strength of the substrate is lower than the shear exerted, or the bed is composed of loose sediment which can be mobilized by such stresses, then the bed will be lowered purely by clearwater flow. In addition, if the river carries significant quantities of [[sediment]], this material can act as tools to enhance wear of the bed ([[abrasion (geology)|abrasion]]). At the same time the fragments themselves are ground down, becoming smaller and more rounded ([[Attrition (erosion)|attrition]]).
Sediment in rivers is transported as either [[bedload]] (the coarser fragments which move close to the bed) or [[suspended load]] (finer fragments carried in the water). There is also a component carried as dissolved material.
For each grain size there is a specific [[flow velocity]] at which the grains start to move, called ''entrainment velocity''. However the grains will continue to be transported even if the velocity falls below the entrainment velocity due to the reduced (or removed) [[friction]] between the grains and the river bed. Eventually the velocity will fall low enough for the grains to be deposited. This is shown by the [[Hjulström curve]].
A river is continually picking up and dropping solid particles of rock and soil from its bed throughout its length. Where the river flow is fast, more particles are picked up than dropped. Where the river flow is slow, more particles are dropped than picked up. Areas where more particles are dropped are called [[alluvial]] or flood plains, and the dropped particles are called [[alluvium]].
Even small streams make alluvial deposits, but it is in [[floodplain]]s and [[River delta|deltas]] of large rivers that large, geologically-significant alluvial deposits are found.
The amount of matter carried by a large river is enormous. It has been estimated that the [[Mississippi River]] annually carries 406 million tons of sediment to the sea,<ref>Mathur, Anuradha; Dilip da Cunha (2001). Mississippi Floods: Designing a Shifting Landscape. New Haven, CT: Yale University Press. {{ISBN|0-300-08430-7}}</ref> the [[Yellow River]] 796 million tons, and the [[Po River]] in [[Italy]] 67 million tons.<ref>Dill, William A. (1990). Inland fisheries of Europe. Rome, Italy: UN Food and Agriculture Organization. {{ISBN|92-5-102999-7}}. http://www.fao.org/docrep/009/t0377e/t0377e00.htm {{Webarchive|url=https://web.archive.org/web/20180301093624/http://www.fao.org/docrep/009/t0377e/t0377e00.htm |date=2018-03-01 }}</ref> The names of many rivers derive from the color that the transported matter gives the water. For example, the [[Yellow River|Yellow River (Huang He)]] in [[China]] is named after the hue of the sediment it carries,<ref>{{Cite book |last1=MOSTERN |first1=RUTH |title=The Yellow River: A Natural and Unnatural History |last2=HORNE |first2=RYAN M. |date=2021 |publisher=Yale University Press |isbn=978-0-300-23833-4 |page=33 |doi=10.2307/j.ctv1vbd1d8.7|jstor=j.ctv1vbd1d8 }}</ref> and the [[White Nile]] is named for the clay it carries.
==Types== The main kinds of fluvial processes are: {{div col}} *{{annotated link|Bradshaw model}} *{{annotated link|Corrosion}} (solution) *{{annotated link|Erosion}} **{{annotated link|Downcutting}} *{{annotated link|Saltation (geology)}} *{{annotated link|Suspension (chemistry)}} {{end div col}} *Transportation - The movement of sediment downstream via bedload, suspended load, or dissolved load. *Deposition - The settling of sediment when the river's energy decreases.
== Depositional environments == The major fluvial (river and stream) [[depositional environment]]s include: * [[River delta|Deltas]] (arguably an intermediate environment between fluvial and marine) * [[Point bar]]s * [[Alluvial fan]]s * [[Braided river]]s * [[Oxbow lake]]s * [[Levee]]s * [[Waterfall]]s * Crevasse splays * Natural levees * Backswamps
==Related concepts==
===Particle motion=== Rivers and streams carry sediment in their flows. This sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle (drag and lift forces), and the [[terminal velocity|settling velocity]] of the particle. These relationships are shown in the following table for the [[Rouse number]], which is a ratio of sediment settling velocity (fall velocity) to upwards velocity.<ref>{{cite journal |last1=Ali |first1=Sk Zeeshan |last2=Dey |first2=Subhasish |title=Mechanics of advection of suspended particles in turbulent flow |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |date=November 2016 |volume=472 |issue=2195 |article-number=20160749 |doi=10.1098/rspa.2016.0749|bibcode=2016RSPSA.47260749A |doi-access=free }}</ref><ref>{{cite journal |last1=Kumbhakar |first1=Manotosh |last2=Ghoshal |first2=Koeli |last3=Singh |first3=Vijay P. |title=Derivation of Rouse equation for sediment concentration using Shannon entropy |journal=Physica A: Statistical Mechanics and Its Applications |date=January 2017 |volume=465 |pages=494–499 |doi=10.1016/j.physa.2016.08.068|bibcode=2017PhyA..465..494K }}</ref>
<math display="block">\textbf{Rouse}=\frac{\text{Settling velocity}}{\text{Upwards velocity from lift and drag}}=\frac{w_s}{\kappa u_*}</math>
where * <math>w_s</math> is the settling velocity * <math>\kappa</math> is the [[von Kármán constant]] * <math>u_*</math> is the [[shear velocity]] [[File:Hjulströms diagram en.PNG|thumb|{{center|[[Hjulström curve]]: the velocities of currents required for erosion, transportation, and deposition (sedimentation) of sediment particles of different sizes}}]] {| class="wikitable" |- style="background:#efefef;" !Mode of transport !Rouse number |- |[[Bed load]] |>2.5 |- |[[Suspended load]]: 50% Suspended |>1.2, <2.5 |- |[[Suspended load]]: 100% Suspended |>0.8, <1.2 |- |[[Wash load]] |<0.8 |}
If the upwards velocity is approximately equal to the settling velocity, sediment will be transported downstream entirely as [[suspended load]]. If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move (see [[Sediment transport#Initiation of motion|Initiation of motion]]), it will move along the bed as [[bed load]] by rolling, sliding, and [[Saltation (geology)|saltating]] (jumping up into the flow, being transported a short distance then settling again). If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as [[wash load]].<ref>{{cite web |last1=Whipple |first1=K. X |year=2004 |title=12.163 Course Notes, MIT Open Courseware. |url=https://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-163-surface-processes-and-landscape-evolution-fall-2004/lecture-notes/4_sediment_transport_edited.pdf |access-date=23 September 2021}}</ref>
As there are generally a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions.
=== Fluvial bedforms === {{main|Bedform}} [[File:Ripples mcr1.JPG|thumb|Modern asymmetric ripples developed in sand on the floor of the Hunter River, New South Wales, Australia. Flow direction is from right to left.]] [[File:Sinuous dunes mcr1.JPG|thumb|Sinuous-crested dunes exposed at low tide in the Cornwallis River near Wolfville, Nova Scotia]] [[File:Channel-StellartonFm-CoalburnPit.JPG|thumb|Ancient channel deposit in the Stellarton Formation ([[Pennsylvanian (geology)|Pennsylvanian]]), Coalburn Pit, near Thorburn, Nova Scotia.]]
Sediment motion can create self-organized structures such as [[ripple marks|ripple]]s, [[dune]]s, or [[antidune]]s on the river or [[stream bed]]. These bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment.
=== Surface runoff === {{main|Surface runoff}} Overland flow can erode soil particles and transport them downslope. The erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions. * If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion. * If overland flow is directly responsible for sediment entrainment but does not form gullies, it is called "sheet erosion". * If the flow and the substrate permit channelization, gullies may form; this is termed "gully erosion".
==See also== * [[Body of water]] * [[Channel pattern]] * [[Sorting (sediment)|Sorting]] * [[List of fluvial landforms]]
==References== {{reflist}}
{{Sediment transport}}
[[Category:Hydrology]] [[Category:Fluvial landforms| ]] [[Category:Sedimentology]] [[Category:Geomorphology]]