The Ambracian Gulf is the second largest gulf in Greece and covers approximately 450 square kilometers. The gulf formed initially two million years ago as a result of a regional tectonic depression. Ten thousand years ago the sea level rose and flooded the depression, already partially shaped by the action of the rivers flowing through it, thus forming today’s coastline The entrance to the gulf is through a 600-meter wide channel in Preveza to the west. The narrow entrance, and the abundant fresh water from the rivers that drain into it, result in a lower level of salinity compared to the open sea, especially near the river estuaries.