base level A theoretical plane surface underlying a land mass, denoting the depth below which erosion would be unable to occur. Sea level provides a base level on a regional scale. Local base levels may be provided by the base of a hill-slope, lakes, or by the junction between a tributary and the main river.

base level A theoretical plane surface underlying a land mass, denoting the depth below which erosion would be unable to occur. Sea level provides a base level on a regional scale. Local base levels may be provided by the base of a hillslope, lakes, or by the junction between a tributary and the main river.