base level

{{short description|Lowest limit for erosion processes}}

File:Desembocadura del Ebro.jpg river as it reaches the Mediterranean Sea by the Ebro Delta]]

In geology and geomorphology a base level is the lower limit for the vertical position of an erosion process.{{cite encyclopedia|last=Goudie|first=A.S.|author-link=Andrew Goudie (geographer)|editor-last=Goudie|editor-first=A.S.|encyclopedia=Encyclopedia of Geomorphology|title=Base level|year=2004|page=62|publisher=Routledge}} The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory.{{cite journal |last1=Orme |first1=Anthony R. |date=2007 |title=The Rise and Fall of the Davisian Cycle of Erosion: Prelude, Fugue, Coda, and Sequel |journal=Physical Geography |volume=28 |issue=6 |pages=474–506 |doi= 10.2747/0272-3646.28.6.474|bibcode=2007PhGeo..28..474O |s2cid=128907423 }} The "ultimate base level" is the surface that results from horizontal projection of the sea level under landmasses (the geoid). It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.Phillips, Jonathan D. (2002), [https://www.sciencedirect.com/science/article/pii/S0169555X01001568 "Erosion, isostatic response, and the missing peneplains"], Geomorphology, Vol. 45, No. 3-4. [http://www.elsevier.com/wps/find/homepage.cws_home Elsevier] {{Webarchive|url=https://web.archive.org/web/20100124190936/http://www.elsevier.com/wps/find/homepage.cws_home |date=2010-01-24 }}, 15 June 2002, pp. 225-241. {{doi|10.1016/S0169-555X(01)00156-8}}.Chorley, R.J. (1973). The History and Study of Landforms or The Development of Geomorphology. Vol. Two: The Life and Work of William Morris Davis, Methuen.{{cite journal |last1=Green |first1=Paul F. |last2=Lidmar-Bergström |first2=Karna |last4=Bonow |first4=Johan M. |last3=Japsen |first3=Peter |last5=Chalmers |first5=James A. |author-link2=Karna Lidmar-Bergström |date=2013 |title=Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins |journal=Geological Survey of Denmark and Greenland Bulletin |volume=30 |pages=18 |doi=10.34194/geusb.v30.4673 |doi-access=free }}{{cite journal |last1=Lidmar-Bergström |first1=Karna |last2=Bonow |first2=Johan M. |last3=Japsen |first3=Peter |author-link=Karna Lidmar-Bergström|date=2013 |title=Stratigraphic Landscape Analysis and geomorphological paradigms: Scandinavia as an example of Phanerozoic uplift and subsidence |journal=Global and Planetary Change |volume=100 |pages=153–171 |doi= 10.1016/j.gloplacha.2012.10.015|bibcode=2013GPC...100..153L }}

There are also lesser structural base levels where erosion is delayed by resistant rocks. Examples of this include karst regions underlain by insoluble rock.{{cite encyclopedia|last=Ford|first=Derek C.|editor-link=Andrew Goudie (geographer)|editor-last=Goudie|editor-first=A.S.|encyclopedia=Encyclopedia of Geomorphology|title=Cave |year=2004 |pages=124–128 |publisher=Routledge}} Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins. An example of this is the Messinian salinity crisis, in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level.{{cite journal |last1=Fairbridge |first1=Rhodes W. |last2=Finkl Jr. |first2=Charles W.|author-link=Rhodes Fairbridge |date=1980 |title=Cratonic erosion unconformities and peneplains |journal=The Journal of Geology |volume=88 |issue=1 |pages=69–86 |doi= 10.1086/628474|bibcode=1980JG.....88...69F |s2cid=129231129 }}{{cite journal |last1=Goudie |first1=A.S.|author-link=Andrew Goudie (geographer) |date=2005 |title=The drainage of Africa since the Cretaceous |journal=Geomorphology |volume=67 |issue= 3–4|pages=437–456 |doi= 10.1016/j.geomorph.2004.11.008|bibcode=2005Geomo..67..437G}}

The height of a base level also influences the position of deltas and river terraces. Together with river discharge and sediment flux the position of the base level influences the gradient, width and bed conditions in rivers.{{cite encyclopedia|last=Whipple|first=Kelin X.|editor-link=Andrew Goudie (geographer)|editor-last=Goudie|editor-first=A.S.|encyclopedia=Encyclopedia of Geomorphology|title=Bedrock channel|year=2004|pages=81–82|publisher=Routledge}} A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging".{{cite encyclopedia|last=Spotila|first=James A.|editor-link=Andrew Goudie (geographer)|editor-last=Goudie|editor-first=A.S.|encyclopedia=Encyclopedia of Geomorphology|title=Crustal deformation|year=2004|pages=201–203|publisher=Routledge}} Base level fall is also known to result in progradation of deltas and river sediment at lakes or sea.{{cite journal |last1=Koss |first1=John E.|last2=Ethridge |first2=Frank G.|last3=Schumm |first3=S.A. |date=1994 |title=An Experimental Study of the Effects of Base-Level Change on Fluvial, Coastal Plain and Shelf Systems |journal=Journal of Sedimentary Research |volume=64B |issue=2 |pages=90–98 |doi= 10.1306/D4267F64-2B26-11D7-8648000102C1865D}} If the base level falls below the continental shelf, rivers may form a plain of braided rivers until headward erosion penetrates enough inland from the shelfbreak.

When base levels are stable or rising rivers may aggrade. Rising base levels may also drown the lower courses of rivers creating rias. This happened in the Nile during the Zanclean flood when its lower course became, in a relatively short time, a large estuary extending up to 900 km inland from the Mediterranean coast.

Base level change may be related to the following factors:

1. Sea level change
2. Tectonic movement
3. River capture
4. Extensive sedimentation{{cite journal |last1=Babault |first1=Julien |last2=Van Den Driessche |first2=Jean|last3=Bonnet |first3=Stephanie|last4=Castelltort |first4=Sébastien|last5=Crave |first5=Alain |date=2005 |title=Origin of the highly elevated Pyrenean peneplain |url= https://archive-ouverte.unige.ch/unige:20401/ATTACHMENT01|journal=Tectonics |volume=24 |issue= 2|pages=n/a |doi=10.1029/2004TC001697 |bibcode=2005Tecto..24.2010B |doi-access=free }}