The Adriatic Sea is a body of water separating the Italian Peninsula from the Balkan peninsula. The Adriatic is the northernmost arm of the Mediterranean Sea, extending from the Strait of Otranto to the northwest and the Po Valley. The countries with coasts on the Adriatic are Albania, Bosnia and Herzegovina, Croatia, Italy, Montenegro and Slovenia. The Adriatic contains over 1,300 islands, mostly located along its eastern, Croatian coast. It is divided into three basins, the northern being the shallowest and the southern being the deepest, with a maximum depth of 1,233 metres (4,045 ft). The Otranto Sill, an underwater ridge, is located at the border between the Adriatic and Ionian Seas. The prevailing currents flow counterclockwise from the Strait of Otranto, along the eastern coast and back to the strait along the western (Italian) coast. Tidal movements in the Adriatic are slight, although larger amplitudes are known to occur occasionally. The Adriatic's salinity is lower than the Mediterranean's because the Adriatic collects a third of the fresh water flowing into the Mediterranean, acting as a dilution basin. The surface water temperatures generally range from 30 °C (86 °F) in summer to 12 °C (54 °F) in winter, significantly moderating the Adriatic Basin's climate.