In a karstic country cut by a canyon in which runs an allogenic river, there are several base levels (karstic, fluvial...); chiefly if the limestones develop greatly under the bottom of the gorge. The difficulty of characterizing them appears in all the definitions given. From the example of the three most important canyons of the Eastern Bas Languedoc (South-East of France) and with the different geological controls (s.l.) one ascertains that at present: a) When an allogenic and surface river is perennial without important loss there is a total connection between fluvial level and karstic base level. b) When an allogenic and temporary surface river doesn’t flow and has losses, there is no connection between the major fluvial and base level and the karstic base level.