GeezerVison

In geology and geomorphology a base level is the lower limit for the vertical position of 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 horizontal projection of the sea level under landmasses (the geoid).^[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 bodies of water 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, closed off from the global ocean, saw a significant proportion of its water dry up. As a result, its base level dropped 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]