Base level explained

In geology and geomorphology a base level is the lower limit for an erosion process.^[1] The modern term was introduced by John Wesley Powell in 1875.^[1] The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory.^{[1] [2]} The "ultimate base level" is the surface that results from projection of the sea level under landmasses.^[1] 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.^{[3] [4] [5] [6]}

There are also lesser structural base levels where erosion is delayed by resistant rocks.^[1] Examples of this include karst regions underlain by insoluble rock.^[7] Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins.^[1] 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.^{[8] [9]}

The height of a base level also influences the position of deltas and river terraces.^[1] Together with river discharge and sediment flux the position of the base level influences the gradient, width and bed conditions in rivers.^[10] A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging".^[11] Base level fall is also known to result in progradation of deltas and river sediment at lakes or sea.^[12] 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.^[12]

When base levels are stable or rising rivers may aggrade.^[12] 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.^[9]

Base level change may be related to the following factors:

1. Sea level change^[1]
2. Tectonic movement^[1]
3. River capture^[1]
4. Extensive sedimentation^[13]

Notes and References

1. Encyclopedia: Goudie. A.S.. Andrew Goudie (geographer). Goudie. A.S.. Encyclopedia of Geomorphology. Base level. 2004. 62. Routledge.
2. Orme . Anthony R. . 2007 . The Rise and Fall of the Davisian Cycle of Erosion: Prelude, Fugue, Coda, and Sequel . Physical Geography . 28 . 6 . 474–506 . 10.2747/0272-3646.28.6.474. 2007PhGeo..28..474O . 128907423 .
3. Phillips, Jonathan D. (2002), "Erosion, isostatic response, and the missing peneplains", Geomorphology, Vol. 45, No. 3-4. Elsevier, 15 June 2002, pp. 225-241. .
4. 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.
5. Green . Paul F. . Lidmar-Bergström . Karna . Bonow . Johan M. . Japsen . Peter . Chalmers . James A. . Karna Lidmar-Bergström . 2013 . Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins . . 30 . 18 . 10.34194/geusb.v30.4673 . free .
6. Lidmar-Bergström . Karna . Bonow . Johan M. . Japsen . Peter . Karna Lidmar-Bergström. 2013 . Stratigraphic Landscape Analysis and geomorphological paradigms: Scandinavia as an example of Phanerozoic uplift and subsidence . . 100 . 153–171 . 10.1016/j.gloplacha.2012.10.015. 2013GPC...100..153L .
7. Encyclopedia: Ford. Derek C.. Andrew Goudie (geographer). Goudie. A.S.. Encyclopedia of Geomorphology. Cave . 2004 . 124–128 . Routledge.
8. Fairbridge . Rhodes W. . Finkl Jr. . Charles W.. Rhodes Fairbridge . 1980 . Cratonic erosion unconformities and peneplains . . 88 . 1 . 69–86 . 10.1086/628474. 1980JG.....88...69F . 129231129 .
9. Goudie . A.S.. Andrew Goudie (geographer) . 2005 . The drainage of Africa since the Cretaceous . . 67 . 3–4. 437–456 . 10.1016/j.geomorph.2004.11.008. 2005Geomo..67..437G.
10. Encyclopedia: Whipple. Kelin X.. Andrew Goudie (geographer). Goudie. A.S.. Encyclopedia of Geomorphology. Bedrock channel. 2004. 81–82. Routledge.
11. Encyclopedia: Spotila. James A.. Andrew Goudie (geographer). Goudie. A.S.. Encyclopedia of Geomorphology. Crustal deformation. 2004. 201–203. Routledge.
12. Koss . John E.. Ethridge . Frank G.. Schumm . S.A. . 1994 . An Experimental Study of the Effects of Base-Level Change on Fluvial, Coastal Plain and Shelf Systems . Journal of Sedimentary Research . 64B . 2 . 90–98 . 10.1306/D4267F64-2B26-11D7-8648000102C1865D.
13. Babault . Julien . Van Den Driessche . Jean. Bonnet . Stephanie. Castelltort . Sébastien. Crave . Alain . 2005 . Origin of the highly elevated Pyrenean peneplain . Tectonics . 24 . 2. n/a . 10.1029/2004TC001697 . 2005Tecto..24.2010B . free .