History, creeks training
Dug in a resistant limestone creeks consist of many landscapes of the Mediterranean coasts. We can bring them in form, rias or Breton estuaries or fjords.
The sea water is often fresher because the cliffs fall right under the sea without real coastal plateau and because many sources of fresh water flows into the sea, under its surface through an underground network that collects mountain runoff (Marseilleveyre) and trays (Carpiagne).
All limestone coasts have experienced this phenomenon when the sea is down to 135 m  before recovering drowning sources and streams. In the Mediterranean Messinian between -7 and -5 million years, sea, virtually disappearing, its level is « descended » from 1000 to 1500 m.
Thus, an underground river of high flow (5 to 7m³ / sec) results in the cove of Port-Miou, almost at sea level; it seems to come from the Sainte Baume and would be fed by an aquifer 1,000 km² in Provence. Such a flow would provide drinking water to more than a million people. Since ancient times, this source is known (Pytheas mentions it). However according to the dives, it is a brackish water as it undergoes sea water lift by faults and, moreover, it is contaminated by the discharge of red mud (from the processing of bauxite) rejected 300m off the Cassis bay.