Alps rivers such as the Danube have major tributaries flowing into them that originate in the Alps. The Rhône is second to the Nile as a freshwater source to the Mediterranean Sea; the river begins as glacial meltwater, flows into Lake Geneva, and from there to France where one of its uses is to cool nuclear power plants. The Rhine originates in a 30 square kilometre area in Switzerland and represents almost 60 percent of water exported from the country. Tributary valleys, some of which are complicated, channel water to the main valleys which can experience flooding during the snow melt
Swiss Alps on the shores of Lake Geneva to Rorschach on the shores of Lake Constance, passing close to the cities of Thun and Lucerne. The not well defined regions in Switzerland that lie on the margin of the Alps, especially those on the north side, are called the "Swiss Prealps" ("Préalpes" in French, "Voralpen" in German, "Prealpi" in Italian). The Swiss Prealps are mainly made of limestone and they generally do not exceed . The Alpine cantons (from highest to lowest) are Valais, Bern, Graubünden, Uri, Glarus, Ticino, St. Gallen, Vaud, Obwalden, Nidwalden, Schwyz, Appenzell Innerrhoden, Appenzell Ausserrhoden, Fribourg, Lucerne and