American Meteorological Society - Glossary of Meteorology

base level of erosion

  1. An imaginary surface of irregular shape, inclined toward the lower end of the principal, or trunk, stream of a basin, below which the stream and its tributaries were presumed to be unable to erode.
  2. More generally applied to the critical plane of erosion, represented approximately by sea level on coasts, which would be the lowest point toward which running water would usually erode.