In geology and geomorphology a base level is the lower limit for an erosion process.<ref name=Goudie2004>Template:Cite encyclopedia</ref> The modern term was introduced by John Wesley Powell in 1875.<ref name=Goudie2004/> The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory.<ref name=Goudie2004/><ref name=Orme2007>Template:Cite journal</ref> The "ultimate base level" is the surface that results from projection of the sea level under landmasses.<ref name=Goudie2004/> 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.<ref name="Phillips">Phillips, Jonathan D. (2002), "Erosion, isostatic response, and the missing peneplains", Geomorphology, Vol. 45, No. 3-4. Elsevier Template:Webarchive, 15 June 2002, pp. 225-241. {{#invoke:doi|main}}.</ref><ref>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.</ref><ref name=Greenetal2013>Template:Cite journal</ref><ref name=Karnaetal2014>Template:Cite journal</ref>
There are also lesser structural base levels where erosion is delayed by resistant rocks.<ref name=Goudie2004/> Examples of this include karst regions underlain by insoluble rock.<ref name=Ford2004>Template:Cite encyclopedia</ref> Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins.<ref name=Goudie2004/> 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.<ref>Template:Cite journal</ref><ref name=Goudie2005>Template:Cite journal</ref>
The height of a base level also influences the position of deltas and river terraces.<ref name=Goudie2004/> Together with river discharge and sediment flux the position of the base level influences the gradient, width and bed conditions in rivers.<ref name=Whipple2004>Template:Cite encyclopedia</ref> A relative drop in base level can trigger re-adjustments in river profiles including knickpoint migration and abandonment of terraces leaving them "hanging".<ref name=Spotila2004>Template:Cite encyclopedia</ref> Base level fall is also known to result in progradation of deltas and river sediment at lakes or sea.<ref name=Kossetal1994>Template:Cite journal</ref> 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.<ref name=Kossetal1994/>
When base levels are stable or rising rivers may aggrade.<ref name=Kossetal1994/> 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.<ref name=Goudie2005/>
Base level change may be related to the following factors:
- Sea level change<ref name=Goudie2004/>
- Tectonic movement<ref name=Goudie2004/>
- River capture<ref name=Goudie2004/>
- Extensive sedimentation<ref>Template:Cite journal</ref>