Editing Silt

{{Short description|Classification of soil or sediment}}
{{Other uses|Silt (disambiguation)}}
[[File:Windrow of windblown glacial silt, NWT.jpg|thumb|Windrow of windblown silt, [[Northwest Territories]], Canada|right]]
'''Silt''' is [[granular material]] of a size between [[sand]] and [[clay]] and composed mostly of broken grains of [[quartz]].<ref>{{cite journal |last1=Assallay |first1=A. |last2=Rogers |first2=C.D.F. |last3=Smalley |first3=I.J. |last4=Jefferson |first4=I.F. |title=Silt: 2–62 μm, 9–4φ |journal=[[Earth-Science Reviews]] |date=November 1998 |volume=45 |issue=1–2 |pages=61–88 |doi=10.1016/S0012-8252(98)00035-X |bibcode=1998ESRv...45...61A}}</ref> Silt may occur as a [[soil]] (often mixed with sand or clay) or as [[sediment]] mixed in [[suspension (chemistry)|suspension]] with water. Silt usually has a floury feel when dry, and lacks [[Plasticity (physics)|plasticity]] when wet. Silt can also be felt by the tongue as granular when placed on the front teeth (even when mixed with clay particles).

Silt is a common material, making up 45% of average modern [[mud]]. It is found in many river deltas and as wind-deposited accumulations, particularly in central Asia, north China, and North America. It is produced in both very hot climates (through such processes as collisions of quartz grains in [[dust storms]]) and very cold climates (through such processes as glacial grinding of quartz grains.)

[[Loess]] is soil rich in silt which makes up some of the most fertile agricultural land on Earth. However, silt is very vulnerable to erosion, and it has poor mechanical properties, making construction on silty soil problematic. The failure of the [[Teton Dam]] in 1976 has been attributed to the use of unsuitable loess in the dam core, and [[liquefication]] of silty soil is a significant earthquake hazard. Windblown and waterborne silt are significant forms of environmental pollution, often exacerbated by poor farming practices.

==Description==
Silt is [[Detritus (geology)|detritus]] (fragments of [[Weathering|weathered]] and [[eroded]] rock) with properties intermediate between [[sand]] and [[clay]]. A more precise definition of silt used by geologists is that it is detrital particles with sizes between 1/256 and 1/16&nbsp;mm (about 4 to 63 microns).<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |page=381}}</ref> This corresponds to particles between 8 and 4 phi units on the [[Krumbein phi scale]].<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=silt [sed]}}</ref><ref>{{cite book |last1=Potter |first1=Paul Edwin |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |title=Sedimentology of shale : study guide and reference source |date=1980 |publisher=Springer-Verlag |location=New York |isbn=0387904301 |page=15}}</ref> Other geologists define silt as detrital particles between 2 and 63 microns or 9 to 4 phi units.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} A third definition is that silt is fine-grained detrital material composed of [[quartz]] rather than [[clay minerals]].{{sfn|Potter|Maynard|Pryor|1980|p=13}} Since most clay mineral particles are smaller than 2 microns,{{sfn|Potter|Maynard|Pryor|1980|p=15}} while most detrital particles between 2 and 63 microns in size are composed of broken quartz grains, there is good agreement between these definitions in practice.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

The upper size limit of 1/16&nbsp;mm or 63 microns corresponds to the smallest particles that can be discerned with the unaided eye.{{sfn|Jackson|1997|loc="silt [sed]"}} It also corresponds to a ''Tanner gap'' in the distribution of particle sizes in [[sediments]]: Particles between 120 and 30 microns in size are scarce in most sediments, suggesting that the distinction between sand and silt has physical significance.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} As noted above, the lower limit of 2 to 4 microns corresponds to the transition from particles that are predominantly broken quartz grains to particles that are predominantly clay mineral particles.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Assallay and coinvestigators further divide silt into three size ranges: C (2–5 microns), which represents post-glacial clays and desert dust; D1 (20–30 microns) representing "traditional" [[loess]]; and D2 (60 microns) representing the very coarse North African loess.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Silt can be distinguished from clay in the [[Field work|field]] by its lack of plasticity or cohesiveness and by its grain size. Silt grains are large enough to give silt a gritty feel,{{sfn|Potter|Maynard|Pryor|1980|p=15}} particularly if a sample is placed between the teeth. Clay-size particles feel smooth between the teeth.<ref>{{cite book |last1=Tucker |first1=Maurice E. |title=Sedimentary rocks in the field : a practical guide |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex |isbn=9780470689165 |page=160 |edition=4th}}</ref> The proportions of coarse and fine silt in a sediment sample are determined more precisely in the laboratory using the pipette method, which is based on settling rate via [[Stokes' law]] and gives the particle size distribution accordingly.{{sfn|Blatt|Middleton|Murray|1980|p=63}} The mineral composition of silt particles can be determined with a [[petrographic microscope]] for grain sizes as low as 10 microns.{{sfn|Blatt|Middleton|Murray|1980|p=305}}

''Vadose silt'' is silt-sized [[calcite]] crystals found in pore spaces and vugs in [[limestone]]. This is emplaced as sediment is carried through the [[vadose zone]] to be deposited in pore space.{{sfn|Blatt|Middleton|Murray|1980|pp=492-493}}

=== Definitions ===
'''ASTM''' American Standard of Testing Materials: 200 sieve – 0.005&nbsp;mm.

'''USDA''' United States Department of Agriculture 0.05–0.002&nbsp;mm.

'''ISSS''' International Society of Soil Science  0.02–0.002&nbsp;mm.

Civil engineers in the United States define silt as material made of particles that pass a number 200 sieve (0.074&nbsp;mm or less) but show little plasticity when wet and little cohesion when air-dried.{{sfn|Jackson|1997|loc="silt [eng]"}} The International Society of Soil Science (ISSS) defines silt as soil containing 80% or more of particles between 0.002&nbsp;mm to 0.02&nbsp;mm in size{{sfn|Jackson|1997|loc="silt [soil]"}} while the U.S. Department of Agriculture puts the cutoff at 0.05mm.<ref>{{cite web |url=http://soils.usda.gov/technical/handbook/contents/part618.html#43 |title=Particle Size (618.43) |work=National Soil Survey Handbook Part 618 (42-55) Soil Properties and Qualities |publisher= - Natural Resource Conservation Service |access-date=2006-05-31 |url-status=dead |archive-url=https://web.archive.org/web/20060527111746/http://soils.usda.gov/technical/handbook/contents/part618.html#43 |archive-date=2006-05-27 }}</ref> The term ''silt'' is also used informally for material containing much sand and clay as well as silt-sized particles, or for mud suspended in water.{{sfn|Jackson|1997|loc="silt [sed]"}}

==Occurrence==
Silt is a very common material, and it has been estimated that there are a billion trillion trillion (10<sup>33</sup>) silt grains worldwide. Silt is abundant in [[Aeolian processes|eolian]] and [[alluvial]] deposits, including [[river delta]]s, such as the [[Nile]] and [[Niger River]] deltas. [[Bangladesh]] is largely underlain by silt deposits of the [[Ganges]] delta. Silt is also abundant in northern China, central Asia, and North America.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} However, silt is relatively uncommon in the tropical regions of the world.<ref name=Chesworth1982>{{cite journal |last1=Chesworth |first1=W. |year=1982 |title=Late Cenozoic geology and the second oldest profession |journal=Geoscience Canada |volume=9 |number=1 |url=https://journals.lib.unb.ca/index.php/GC/article/view/3288 |access-date=12 October 2021}}</ref>

Silt is commonly found in suspension in river water, and it makes up over 0.2% of river sand. It is abundant in the matrix between the larger sand grains of [[graywackes]]. Modern [[mud]] has an average silt content of 45%.{{sfn|Blatt|Middleton|Murray|1980|pp=73, 374, 381}} Silt is often found in [[mudrock]] as thin [[Lamination (geology)|laminae]], as clumps, or dispersed throughout the rock. Laminae suggest deposition in a weak current that [[Winnowing (sedimentology)|winnows]] the silt of clay, while clumps suggest an origin as [[fecal pellets]]. Where silt is dispersed throughout the mudrock, it likely was deposited by rapid processes, such as [[flocculation]].{{sfn|Potter|Maynard|Pryor|1980|pp=108-109}} [[Sedimentary rock]] composed mainly of silt is known as [[siltstone]].{{sfn|Blatt|Middleton|Murray|1980|pp=381-382}}

Silt is common throughout the [[geologic record]], but it seems to be particularly common in [[Quaternary]] formations. This may be because deposition of silt is favored by the [[glaciation]] and [[arctic]] conditions characteristic of the Quaternary.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} Silt is sometimes known as ''[[rock flour]]'' or ''glacier meal'', especially when produced by glacial action.{{sfn|Jackson|1997|loc="rock flour"}} Silt suspended in water draining from glaciers is sometimes known as ''rock milk'' or ''moonmilk''.{{sfn|Jackson|1997|loc="rock milk"}}

== Sources ==
[[File:Sunlight on silty stream at Myrstigen 5.jpg|thumb|A stream carrying silt from fields in [[Brastad]], Sweden]]
A simple explanation for silt formation is that it is a straightforward continuation to a smaller scale of the disintegration of rock into [[gravel]] and sand.<ref>{{cite journal |last1=Schubert |first1=C. |year=1964 |title=Size-frequency distribution of sand-sized grains in an abrasion mill |journal=Sedimentology |volume=3 |number=4 |pages=288–295|doi=10.1111/j.1365-3091.1964.tb00643.x |bibcode=1964Sedim...3..288S }}</ref> However, the presence of a Tanner gap between sand and silt (a scarcity of particles with sizes between 30 and 120 microns) suggests that different physical processes produce sand and silt.<ref>{{cite journal |last1=Rogers |first1=J.J.W. |last2=Krueger |first2=W.C. |last3=Krog |first3=M. |title=Sizes of Naturally Abraded Materials |journal=SEPM Journal of Sedimentary Research |date=1963 |volume=33 |pages=628–632 |doi=10.1306/74D70ED9-2B21-11D7-8648000102C1865D}}</ref> The mechanisms of silt formation have been studied extensively in the laboratory<ref name=WrightEtal1998>{{cite journal |last1=Wright |first1=J. |last2=Smith |first2=B. |last3=Whalley |first3=B. |title=Mechanisms of loess-sized quartz silt production and their relative effectiveness: laboratory simulations |journal=[[Geomorphology (journal)|Geomorphology]] |date=May 1998 |volume=23 |issue=1 |pages=15–34 |doi=10.1016/S0169-555X(97)00084-6 |bibcode=1998Geomo..23...15W}}</ref> and compared with field observations. These show that silt formation requires high-energy processes acting over long periods of time, but such processes are present in diverse geologic settings.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Quartz silt grains are usually found to have a platy or bladed shape.<ref>{{cite journal |last1=Krinsley |first1=D. H. |last2=Smalley |first2=I. J. |title=Shape and Nature of Small Sedimentary Quartz Particles |journal=Science |date=22 June 1973 |volume=180 |issue=4092 |pages=1277–1279 |doi=10.1126/science.180.4092.1277|pmid=17759122 |bibcode=1973Sci...180.1277K |s2cid=11606901 }}</ref> This may be characteristic of how larger grains abrade, or reflect the shape of small quartz grains in foliated [[metamorphic rock]], or arise from [[authigenic]] growth of quartz grains parallel to bedding in [[sedimentary rock]].<ref>{{cite journal |last1=Blatt |first1=H. |title=Perspectives; Oxygen isotopes and the origin of quartz |journal=Journal of Sedimentary Research |date=1 March 1987 |volume=57 |issue=2 |pages=373–377 |doi=10.1306/212F8B34-2B24-11D7-8648000102C1865D|bibcode=1987JSedR..57..373B }}</ref> Theoretically, particles formed by random fracturing of an isotropic material, such as quartz, naturally tend to be blade-shaped.<ref>{{cite journal |last1=Rogers |first1=C.F. |last2=Smalley |first2=I.J. |year=1993 |title=The shape of loess particles |journal=Naturwissenschaften |volume=80 |number=10 |pages=461–462|doi=10.1007/BF01136036 |bibcode=1993NW.....80..461R |s2cid=44606484 }}</ref> The size of silt grains produced by abrasion or shattering of larger grains may reflect defects in the crystal structure of the quartz, known as ''Moss defects.''<ref>{{cite journal |last1=Moss |first1=A. J. |last2=Green |first2=Patricia |title=Sand and silt grains: Predetermination of their formation and properties by microfractures in quartz |journal=[[Journal of the Geological Society of Australia]] |date=1975 |volume=22 |issue=4 |pages=485–495 |doi=10.1080/00167617508728913 |bibcode=1975AuJES..22..485M}}</ref> Such defects are produced by tectonic deformation of the parent rock, and also arise from the high-low transition of quartz: Quartz experiences a sharp decrease in volume when it cools below a temperature of about {{convert|573|C||sp=us}},<ref>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |page=68}}</ref> which creates strain and crystal defects in the quartz grains in a cooling body of granite.<ref>{{cite journal |last1=Smalley |first1=I.J. |year=1966 |title=Formation of quartz sand |journal=Nature |volume=211 |number=5048 |pages=476–479|doi=10.1038/211476a0 |bibcode=1966Natur.211..476S |s2cid=4258725 }}</ref>

Mechanisms for silt production include:{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}
* Erosion of initially silt-sized grains from low-grade metamorphic rock.
* Production of silt-sized grains from fracture of larger grains during initial rock weathering and [[soil formation]],<ref>{{cite journal |last1=Nahon |first1=D. |last2=Trompette |first2=R. |title=Origin of siltstones: glacial grinding versus weathering |journal=[[Sedimentology (journal)|Sedimentology]] |date=February 1982 |volume=29 |issue=1 |pages=25–35 |doi=10.1111/j.1365-3091.1982.tb01706.x |bibcode=1982Sedim..29...25N}}</ref> through processes such as [[frost shattering]]<ref>{{cite journal |last1=Lautridou |first1=J. P. |last2=Ozouf |first2=J. C. |title=Experimental frost shattering |journal=[[Progress in Physical Geography]] |date=19 August 2016 |volume=6 |issue=2 |pages=215–232 |doi=10.1177/030913338200600202|s2cid=140197148 }}</ref> and [[haloclasty]].<ref>{{cite journal |last1=Goudie |first1=A. S. |last2=Watson |first2=A. |title=Rock block monitoring of rapid salt weathering in southern Tunisia |journal=[[Earth Surface Processes and Landforms]] |date=January 1984 |volume=9 |issue=1 |pages=95–98 |doi=10.1002/esp.3290090112 |bibcode=1984ESPL....9...95G}}</ref> This produces silt particles whose size of 10–30 microns is determined by Moss defects.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}
* Production of silt-sized grains from grain-to-grain impact during transport of coarser sediments.
* Formation of authigenic quartz during weathering to clay.
* Crystallization of the tests of siliceous organisms deposited in mudrock.

Laboratory experiments have produced contradictory results regarding the effectiveness of various silt production mechanisms. This may be due to the use of vein or pegmatite quartz in some of the experiments. Both materials form under conditions promoting ideal crystal growth, and may lack the Moss defects of quartz grains in granites. Thus production of silt from vein quartz is very difficult by any mechanism, whereas production of silt from granite quartz proceeds readily by any of a number of mechanisms.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} However, the main process is likely [[Abrasion (geology)|abrasion]] through transport, including [[fluvial]] [[comminution]], [[Aeolian processes|aeolian]] [[Attrition (weathering)|attrition]] and [[glacial]] grinding.<ref name=WrightEtal1998/>

Because silt deposits (such as ''[[loess]]'', a soil composed mostly of silt<ref name="Frechen 2011">{{cite journal | last1 = Frechen | first1 = M | year = 2011 | title = Loess in Europe: Guest Editorial| journal = E&G Quaternary Science Journal| volume = 60 | issue = 1| pages = 3–5 | doi = 10.3285/eg.60.1.00 | doi-access = free }}</ref>) seem to be associated with glaciated or mountainous regions in Asia and North America, much emphasis has been placed on glacial grinding as a source of silt. High Asia has been identified as a major generator of silt, which accumulated to form the fertile soils of north India and Bangladesh, and the loess of central Asia and north China.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} Loess has long been thought to be absent or rare in deserts lacking nearby mountains (Sahara, Australia).<ref>{{cite journal |last1=Smalley |first1=Ian J. |last2=Krinsley |first2=David H. |title=Loess deposits associated with deserts |journal=CATENA |date=April 1978 |volume=5 |issue=1 |pages=53–66 |doi=10.1016/S0341-8162(78)80006-X|bibcode=1978Caten...5...53S }}</ref> However, laboratory experiments show eolian and fluvial processes can be quite efficient at producing silt,<ref name=WrightEtal1998/> as can weathering in tropical climates.<ref>{{cite journal |last1=Pye |first1=Kenneth |title=Formation of quartz silt during humid tropical weathering of dune sands |journal=Sedimentary Geology |date=April 1983 |volume=34 |issue=4 |pages=267–282 |doi=10.1016/0037-0738(83)90050-7|bibcode=1983SedG...34..267P }}</ref> Silt seems to be produced in great quantities in dust storms, and silt deposits found in Israel, Tunisia, Nigeria, and Saudi Arabia cannot be attributed to glaciation. Furthermore, desert source areas in Asia may be more important for loess formation than previously thought. Part of the problem may be the conflation of high rates of production with environments conducive to deposition and preservation, which favors glacial climates more than deserts.<ref>{{cite journal |last1=Wright |first1=Janet S |title="Desert" loess versus "glacial" loess: quartz silt formation, source areas and sediment pathways in the formation of loess deposits |journal=Geomorphology |date=February 2001 |volume=36 |issue=3–4 |pages=231–256 |doi=10.1016/S0169-555X(00)00060-X|bibcode=2001Geomo..36..231W }}</ref>

Loess associated with glaciation and cold weathering may be distinguishable from loess associated with hot regions by the size distribution. Glacial loess has a typical particle size of about 25 microns. Desert loess contains either larger or smaller particles, with the fine silt produced in dust storms and the coarse silt fraction possibly representing the fine particle tail of sand production.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

==Human impact==
Loess underlies some of the most productive agricultural land worldwide. However, it is very susceptible to erosion.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} The quartz particles in silt do not themselves provide nutrients, but they promote excellent [[soil structure]], and silt-sized particles of other minerals, present in smaller amounts, provide the necessary nutrients.<ref name=Chesworth1982/> Silt, deposited by annual floods along the [[Nile River]], created the rich, fertile soil that sustained the [[Ancient Egypt]]ian civilization. The closure of the [[Aswan High Dam]] has cut off this source of silt, and the fertility of the Nile delta is deteriorating.<ref>{{cite journal |last1=Stanley |first1=D. J. |last2=Warne |first2=A. G. |title=Nile Delta: Recent Geological Evolution and Human Impact |journal=Science |date=30 April 1993 |volume=260 |issue=5108 |pages=628–634 |doi=10.1126/science.260.5108.628|pmid=17812219 |bibcode=1993Sci...260..628S |s2cid=31544176 }}</ref>

Loess tends to lose strength when wetted, and this can lead to failure of building foundations.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} The silty material has an open structure that collapses when wet.<ref>{{cite journal |last1=Rogers |first1=C.D.F. |last2=Dijkstra |first2=T.A. |last3=Smalley |first3=I.J. |title=Hydroconsolidation and subsidence of loess: Studies from China, Russia, North America and Europe |journal=Engineering Geology |date=June 1994 |volume=37 |issue=2 |pages=83–113 |doi=10.1016/0013-7952(94)90045-0}}</ref> [[Quick clay]] (a combination of very fine silt and clay-sized particles from glacial grinding) is a particular challenge for [[civil engineering]].<ref>{{cite journal |last1=Cabrera |first1=J.G. |last2=Smalley |first2=I.J. |title=Quickclays as products of glacial action: a new approach to their nature, geology, distribution and geotechnical properties |journal=Engineering Geology |date=October 1973 |volume=7 |issue=2 |pages=115–133 |doi=10.1016/0013-7952(73)90041-0|bibcode=1973EngGe...7..115C }}</ref>

The failure of the [[Teton Dam]] has been attributed to the use of loess from the Snake River floodplain in the core of the dam.<ref>{{cite journal |last1=Smalley |first1=I.J. |last2=Dijkstra |first2=T.A. |title=The Teton Dam (Idaho, U.S.A.) failure: problems with the use of loess material in earth dam structures |journal=Engineering Geology |date=October 1991 |volume=31 |issue=2 |pages=197–203 |doi=10.1016/0013-7952(91)90006-7|bibcode=1991EngGe..31..197S }}</ref> Loess lacks the necessary plasticity for use in a dam core, but its properties were poorly understood, even by the [[U.S. Bureau of Reclamation]], with its wealth of experience building [[earthen dam]]s.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Silt is susceptible to [[liquefaction]] during strong earthquakes due to its lack of plasticity. This has raised concerns about the earthquake damage potential in the silty soil of the central United States in the event of a major earthquake in the [[New Madrid Seismic Zone]].<ref>{{cite journal |last1=Guo |first1=Tianqiang |last2=Prakash |first2=Shamsher |title=Liquefaction of Silts and Silt-Clay Mixtures |journal=Journal of Geotechnical and Geoenvironmental Engineering |date=August 1999 |volume=125 |issue=8 |pages=706–710 |doi=10.1061/(ASCE)1090-0241(1999)125:8(706)}}</ref><ref>{{cite journal |last1=Omermeier |first1=S.F. |last2=Jacobson |first2=R.B. |last3=Smoot |first3=J.P. |last4=Weems |first4=R.E. |last5=Gohn |first5=G.S. |last6=Monroe |first6=J.E. |last7=Powars |first7=D.S. |title=Earthquake-induced liquefaction features in the coastal setting of South Carolina and in the fluvial setting of the New Madrid seismic zone |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |date=1990 |volume=1504 |doi=10.3133/pp1504|doi-access=free }}</ref>

== Environmental impacts ==
[[File:Burgwall Jatzke3.jpg|thumb|right|A silted lake located in [[Eichhorst]], Germany]]
Silt is easily transported in [[water]]<ref>{{cite journal |last1=Chengrui |first1=Mei |last2=Dregne |first2=Harold E. |title=Review article: Silt and the future development of China's Yellow River |journal=The Geographical Journal |date=March 2001 |volume=167 |issue=1 |pages=7–22 |doi=10.1111/1475-4959.00002|bibcode=2001GeogJ.167....7C }}</ref> and is fine enough to be carried long distances by air in the form of [[dust]].<ref>{{cite journal |last1=Evans |first1=R.D. |last2=Jefferson |first2=I.F. |last3=Kumar |first3=R. |last4=O’Hara-Dhand |first4=K. |last5=Smalley |first5=I.J. |title=The nature and early history of airborne dust from North Africa; in particular the Lake Chad basin |journal=Journal of African Earth Sciences |date=May 2004 |volume=39 |issue=1–2 |pages=81–87 |doi=10.1016/j.jafrearsci.2004.06.001}}</ref> While the coarsest silt particles (60 micron) settle out of a meter of still water in just five minutes, the finest silt grains (2 microns) can take several days to settle out of still water.{{sfn|Potter|Maynard|Pryor|1980|pp=8-9}} When silt appears as a pollutant in water the phenomenon is known as [[siltation]].<ref>{{cite journal |last1=Berkman |first1=Hilary E. |last2=Rabeni |first2=Charles F. |title=Effect of siltation on stream fish communities |journal=Environmental Biology of Fishes |date=December 1987 |volume=18 |issue=4 |pages=285–294 |doi=10.1007/BF00004881|bibcode=1987EnvBF..18..285B |s2cid=1616346 }}</ref>

Silt deposited by the [[Mississippi River]] throughout the 20th&nbsp;century has decreased due to a system of [[levees]], contributing to the disappearance of protective [[wetlands]] and [[barrier islands]] in the [[Mississippi River Delta|delta]] region surrounding [[New Orleans]].<ref>{{cite web
 | title=Mississippi River
 | work=USGS Biological Resources
 | url=http://biology.usgs.gov/s+t/SNT/noframe/ms137.htm
 | access-date=2006-03-08 |archive-url = https://web.archive.org/web/20051028140409/http://biology.usgs.gov/s+t/SNT/noframe/ms137.htm <!-- Bot retrieved archive --> |archive-date = 2005-10-28}}
</ref>

In southeast Bangladesh, in the [[Noakhali district]], cross dams were built in the 1960s whereby silt gradually started forming new land called "chars". The district of Noakhali has gained more than {{convert|73|km2|sqmi|0}} of land in the past 50&nbsp;years. With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, and the effort has since become a multi-agency operation building roads, [[culvert]]s, embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some {{convert|100|km2|acre|sigfig=1}} to 21,000&nbsp;families.<ref>{{cite web|url=http://pulitzercenter.org/openitem.cfm?id%3D1973 |title= Bangladesh fights for survival against climate change|access-date=October 22, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20100212215115/http://www.pulitzercenter.org/openitem.cfm?id=1973 |archive-date=February 12, 2010}}</ref>

A main source of silt in urban rivers is disturbance of soil by construction activity.<ref>{{Cite book|url=https://books.google.com/books?id=6OtWpor6LnsC&q=main+source+of+silt+in+urban+rivers+is+disturbance+of+soil+by+construction+activity|title=Planning for Urban Fishing and Waterfront Recreation|last1=Leedy|first1=Daniel L.|last2=Franklin|first2=Thomas M.|last3=Maestro|first3=Robert M.|date=1981|publisher=U.S. Department of the Interior, Fish and Wildlife Service, Eastern Energy and Land Use Team|language=en|url-status=live|archive-url=https://web.archive.org/web/20171224224857/https://books.google.com/books?id=6OtWpor6LnsC&q=main+source+of+silt+in+urban+rivers+is+disturbance+of+soil+by+construction+activity&dq=main+source+of+silt+in+urban+rivers+is+disturbance+of+soil+by+construction+activity&hl=en&sa=X&ved=0ahUKEwjAuKX726PYAhXLyFQKHcaSA_oQ6AEILzAB|archive-date=2017-12-24}}</ref> A main source in rural rivers is [[erosion]] from plowing of farm fields,<ref>{{cite journal |last1=Grisseur |first1=D.H. |last2=Mubeteneh |first2=T.C. |last3=Aurore |first3=D. |title=Pollution and Siltation of Rivers in the Western Highlands of Cameroon: a Consequence of Farmland Erosion by Runoff |journal=21st Century Watershed Technology Conference and Workshop Improving Water Quality and the Environment |date=3 November 2014 |pages=1–8 |doi=10.13031/wtcw.2014-012|hdl=2268/173760 |isbn=9781940956268 |hdl-access=free }}</ref> [[clearcutting]] or [[slash and burn]] treatment of [[forest]]s.<ref>{{cite journal |last1=Sanchez |first1=P. A. |title=Alternatives to Slash and Burn Agriculture |journal=ASA Special Publications |date=26 October 2015 |pages=33–39 |doi=10.2134/asaspecpub56.c4|isbn=9780891183228 }}</ref>

== Culture ==
The fertile black silt of the [[Nile river]]'s banks is a symbol of rebirth, associated with the [[Egyptian pantheon|Egyptian god]] [[Anubis]].<ref>{{cite book|last=Hart|first=George|year=1986|title=A Dictionary of Egyptian Gods and Goddesses|location=London|publisher=Routledge & Kegan Paul|isbn=978-0-415-34495-1|page=22}}</ref><ref>{{cite book|last=Freeman|first=Charles|author-link = Charles Freeman (historian) |year=1997|title=The Legacy of Ancient Egypt|location=New York|publisher=Facts on File|isbn=978-0-816-03656-1|page=91}}</ref>

==See also==
{{Commons category|Silt}}
*[[Aleurite]]
*[[Erosion control]]
*[[Nonpoint source pollution]]
*[[Sediment control]]
*[[Silt fence]]

==References==
{{Reflist|30em}}

{{Geotechnical engineering|state=collapsed}}{{Soil type}}{{Authority control}}

[[Category:Soil]]
[[Category:Sedimentology]]
[[Category:Sediments]]