The Obersee ("Upper Lake") the larger of the two parts of Lake Constance.It has an area of 473 km² in size and extends over 63 km between Bregenz and Bodman-Ludwigshafen. Its maximum width is 14 km. The Romans called it Lacus Venetus, Lacus Brigantinus and Lacus Constantinus. In the Middle Ages the dominant term was Lacus Bodamicus, or in German Bodensee. Gradually, this name began to include the Untersee (Lacus Acronius), so the term "Upper Lake" was introduced for the larger lake.
The Upper Lake is drained by the Seerhein in Constance, which flows into the Lower Lake. Its main tributary is the Alpine Rhine.Parts of the Upper Lake are the Lake Überlingen, the Bregenz Bay and the Constance Hopper. Lake Überlingen is sometimes considered to be a separate lake, in which case the name "Obersee" only refers to the section between Bregenz and Constance.
The lake is bordered by the Swiss cantons of Thurgau and St. Gallen, by the Austrian state of Vorarlberg and the German states of Baden-Württemberg and Bavaria. The borders between the countries bordering on the south-eastern part of the Obersee have never been established (further explanations in Lake_Constance#International_borders), only the smaller northwestern Lake Überlingen is completely German territory.[source]