The Baltic is a shallow, virtually enclosed inland sea with little tide. It does not benefit from the warmth of the North Atlantic Drift, and its northern branches, the Gulf of Bothnia and the Gulf of Finland, ice over in the winter. A large influx of river water gives the Baltic a low salinity—it is the largest area of brackish water in the world. Its only outflow is to the North Sea via the Danish Straits (three channels linking the Baltic to the Kattegat), Kattegat Bay, and the Skagerrak Strait. There is a weak influx of dense salt water at depth that isolates the basin floor from the surface waters, producing an oxygen-depleted dead zone. Without significant out- flows, the Baltic Sea is vulnerable to pollution carried in by rivers and from large population centers on its coasts. Although there is a sea route to the North Sea, there is also a shorter, more sheltered route through the Kiel Canal.