The Breg is a river in Baden-Württemberg, Germany. It is currently the primary headstream of the Danube. It begins at above sea level in the Black Forest, near Furtwangen im Schwarzwald, from the European Watershed. It flows through Furtwangen and Vöhrenbach. After , it joins the Brigach Stream, currently the Danube's other headstream, in the city of Donaueschingen and is henceforth called Danube. Further downstream the Danube passes over a sinkhole called the Danube Sinkhole, where most of its water sinks into the ground, flowing through a cavern system and resurfaces near Lake Constance, feeding the Rhine. Bellow the sinkhole, for about 155 days out of the year, the Danube runs dry for several miles downstream. Every year the sinkhole and underground cavern system grow larger and sometimes new sinkholes form in the riverbed, and since the sinkhole first formed in 1874, the average number of days the Danube runs dry has sharply increased (when the sinkhole first formed the Danube only ran dry for an average of 80 days per year). Because of this it is inevitable that the upper Danube will someday disappear entirely in favor of the Rhine, and the new sources will be the currently minor tributaries Krähenbach and Elta.
Breg may refer to: