The Alps are the watershed of Western Europe. The Rhine, together with its tributaries the Aare and the Thur drain about two thirds of the water into the North Sea. The Rhône and the Ticino drain about 18% of the water into the Mediterranean Sea. The Inn which flows into the Danube outside of Switzerland drains about 4.4% of the water into the Black Sea. The Swiss Alps also contain many of Central Europe's glaciers. There are about 1,800 glaciers which cover of the total glaciated area of the Alps.