Editing Erosion (section)

===Rivers and streams===

{{further|topic=water's erosive ability|Hydraulic action}}
[[File: Dobbingstone Burn - geograph.org.uk - 1291882.jpg|thumb|Dobbingstone [[Burn (landform)|Burn]], Scotland, showing two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the stream's banks are [[glacial till]] that was left behind as ice age glaciers flowed over the terrain.]]
[[File:Tauglbach 4.JPG|thumb|Layers of [[chalk]] exposed by a river eroding through them]]
[[File:Green land soil erosion.jpg|alt=Green land erosion |thumb|Green land erosion]]
''Valley'' or ''stream erosion'' occurs with continued water flow along a [[Linear feature extraction|linear feature.]] The erosion is both [[Downcutting|downward]], deepening the [[valley]], and [[headward erosion|headward]], extending the valley into the hillside, creating [[Head cut (stream geomorphology)|head cuts]] and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, 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 [[meander]]s across the valley floor. In all stages of stream erosion, by far 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, [[pebble]]s, and [[boulder]]s can also act erosively as they traverse a surface, in a process known as ''traction''.<ref>Ritter, Michael E. (2006) [http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/geologic_work_of_streams.html "Geologic Work of Streams"] {{webarchive|url=https://web.archive.org/web/20120506040721/http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/geologic_work_of_streams.html |date=2012-05-06 }} ''The Physical Environment: an Introduction to Physical Geography'' University of Wisconsin, {{OCLC|79006225}}</ref>

''[[Bank erosion]]'' is the wearing away of the banks of a stream or river. This is distinguished from changes on the bed of the watercourse, which is referred to as ''scour''. Erosion and [[River bank failure|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.<ref>{{Cite book |chapter-url=https://books.google.com/books?id=_PJHw-hSKGgC&pg=PA113 |title=Stream hydrology: an introduction for ecologists |author=Nancy D. Gordon |chapter=Erosion and Scour |isbn=978-0-470-84357-4 |date=2004 |publisher=John Wiley and Sons }}</ref>

''Thermal erosion'' is the result of melting and weakening [[permafrost]] due to moving water.<ref name="nsidc_thermal">{{cite web|url=http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion |title=Thermal Erosion |work=NSIDC Glossary |publisher=[[National Snow and Ice Data Center]] |access-date=21 December 2009 |archive-url=https://web.archive.org/web/20101218124656/http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion |archive-date=2010-12-18 |url-status=live }}</ref> It can occur both along rivers and at 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.<ref name="lena">{{cite journal|doi=10.1002/esp.592|title=Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)|year=2003|last1=Costard|first1=F.|last2=Dupeyrat|first2=L.|last3=Gautier|first3=E.|last4=Carey-Gailhardis|first4=E.|journal=[[Earth Surface Processes and Landforms]]|volume=28|pages=1349–1359|bibcode = 2003ESPL...28.1349C|issue=12 |s2cid=131318239 }}</ref> Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the [[Arctic Ocean|Arctic coast]], where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a {{convert|100|km|mi|abbr=off|adj=on}} segment of the [[Beaufort Sea]] shoreline averaged {{convert|5.6|m|ft|abbr=off}} per year from 1955 to 2002.<ref name="jones_arctic">{{cite journal|last=Jones|first=B.M.|author2=Hinkel, K.M.|author3=Arp, C.D.|author4=Eisner, W.R.|year=2008|title=Modern Erosion Rates and Loss of Coastal Features and Sites, Beaufort Sea Coastline, Alaska|journal=Arctic|volume=61|issue=4|pages=361–372|url=http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/44/115|doi=10.14430/arctic44|url-status=dead|archive-url=https://web.archive.org/web/20130517101602/http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/44/115|archive-date=2013-05-17|hdl=10535/5534|hdl-access=free}}</ref>

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 a large river can remove enough sediments to produce a [[river anticline]],<ref name="mont">{{cite journal|last=Montgomery|first=David R.|author2=Stolar, Drew B. |title=Reconsidering Himalayan river anticlines|journal=Geomorphology|date=1 December 2006|volume=82|issue=1–2|pages=4–15|doi=10.1016/j.geomorph.2005.08.021|bibcode = 2006Geomo..82....4M }}</ref> as [[isostatic rebound]] raises rock beds unburdened by erosion of overlying beds.