Lake Constance (German: Bodensee, pronounced [ˈboːdn̩zeː] ) refers to three bodies of water on the Rhine at the northern foot of the Alps: Upper Lake Constance (Obersee), Lower Lake Constance (Untersee), and a connecting stretch of the Rhine, called the Seerhein (lit. 'Rhine of the lake(s)'). These waterbodies lie within the Lake Constance Basin (Bodenseebecken) in the Alpine Foreland through which the Rhine flows. The nearby Mindelsee is not considered part of Lake Constance.The lake is situated where Germany, Switzerland, and Austria meet. Its shorelines lie in the German states of Baden-Württemberg and Bavaria; the Swiss cantons of St. Gallen, Thurgau, and Schaffhausen; and the Austrian state of Vorarlberg. The actual locations of the country borders within the lake are disputed, with Austria, Germany and Switzerland all holding different opinions on the matter.
The Alpine Rhine forms, in its original course (Alter Rhein), the Austro-Swiss border and flows into the lake from the south. The High Rhine flows westbound out of the lake and forms (with the exception of the Canton of Schaffhausen, Rafzerfeld and Basel-Stadt) the German-Swiss border as far as to the city of Basel. The Leiblach forms the Austria–Germany border east of the lake.