Bodensee, Lake Constance or Lake of Constance (German and Alemannic Bodensee) is a large lake on the Rhine between Germany, Switzerland, and Austria. It lies approximately at 47°39′N, 9°19′E.
Lake Constance was first mentioned by the Roman geographer Pomponius Mela about AD 43. He noted that the Rhine flows through two lakes, and gave them the Latin names Lacus Venetus (today Obersee) and Lacus Acronius (today Untersee). Pliny the Elder used the name Lacus Brigantinus, after the Roman city of Brigantium (today Bregenz). The lake is colloquially also known as the Swabian Sea (das Schwäbische Meer).
The freshwater lake sits at 395 m above sea level and is Central Europe's third largest, after Lake Balaton and Lake Geneva. It is 63 km long, and at its widest point, nearly 14 km. It covers approximately 539 km² (208 mi²) of total area. The greatest depth is 252 m in the middle of the eastern part (Obersee). Its volume is approximately 55 km³. The lake has four parts: Obersee (main, 476 km²), Überlinger See (north, 61 km²), Untersee (west, 63 km²), and the Zeller See and Gnadensee (northwest). 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.
Approximately 1000 tons of fish were caught by 150 professional fishermen in 2001 which was below the previous ten year average of 1200 tons per year. The Lake Constance trout (Salmo trutta) was almost extinct in the 1980s due to pollution, but thanks to protective measures has made a significant return.
The lake itself is an important drinking water source for south-west Germany called Bodenseewasserversorgung