Lake Constance (German: Bodensee) is a lake on the Rhine at the northern foot of the Alps, and consists of three bodies of water: the Obersee ("upper lake"), the Untersee ("lower lake"), and a connecting stretch of the Rhine, called the Seerhein.
The lake is situated in Germany, Switzerland and Austria near the Alps. Specifically, its shorelines lie in the German federal-states of Bavaria and Baden-Württemberg, the Austrian federal-state of Vorarlberg, and the Swiss cantons of Thurgau and St. Gallen. The Rhine flows into it from the south following the Austro-Swiss frontier.
Lake Constance was first mentioned by the Roman geographer Pomponius Mela about 43AD. He noted that the Rhine flows through two lakes, and gave them the Latin names Lacus Venetus (Obersee) and Lacus Acronius (Untersee). Pliny the Elder used the name Lacus Brigantinus, after the Roman city of Brigantium (today Bregenz). The lake is also colloquially known as the Swabian Sea (das schwäbische Meer).
The freshwater lake sits at 395 m (1,296 ft) above sea level and is Central Europe's third largest, after Lake Balaton and Lake Geneva. It is 63 km (39 mi) long, and at its widest point, nearly 14 km (8.7 mi). It covers approximately 571 km2 (220 sq mi) of total area. The greatest depth is 252 metres (827 ft) in the middle of the eastern part (Obersee). Its volume is approximately 1e10 m3 (1.3×1010 cu yd). The lake has four parts: the main section, called Obersee, 476 km (0.184 sq mi); the north section, Überlinger See, 61 km2 (24 sq mi); the west section, Untersee, 63 km2 (24 sq mi); and the northwest section, the Zeller See and Gnadensee. The regulated Rhine flows into the lake in the southeast, through the Obersee, the city of Konstanz and the Untersee and flows out near Stein am Rhein. Lake Constance provides fresh water to many cities in south Germany.
Lake Constance was formed by the Rhine Glacier during the ice age. The Rhine, the Bregenzer Ache, and the Dornbirner Ache carry sediments from the Alps to the lake, thus gradually decreasing the depth of the lake in the southeast.
The lake itself is an important drinking water source for southwestern Germany, called Bodensee-Wasserversorgung ("Lake Constance Water Supply").
Car ferries link Romanshorn, Switzerland to Friedrichshafen, and Konstanz to Meersburg.