Primary watersheds, also known as main European watersheds, are the boundaries between the catchment areas of rivers that flow into the seas around the Continent of Europe. Most of Switzerland (from the crest of the Alps to the north) drains into the Rhine (27,823 km²); the Rhone mainly collects water from the Canton of Valais, the Lake Geneva area and the Ajoie (7,691 km²); the Po collects water from the Ticino (3,827 km²); the Engadine is part of the Danube river basin (1,818 km²); and the Müstair valley is part of the Etsch/Adige basin (126 km²). The St. Gothard is the most important centre for sources of rivers in Switzerland and indeed in Western Europe, and it is the point where the catchment areas of the Rhine, Rhone, and Po meet up.
Secondary watersheds separate the primary catchment areas into river basins. For instance a secondary watershed delimits the catchment area of the River Aare within the river basin of the Rhine, and tertiary watersheds divide the Aare river basin into the basins of the rivers Reuss and Limmat.