The Alter Rhein (German for Old Rhine) is the old river bed of the Alpine Rhine in St. Gallen and Vorarlberg in the Rhine Valley, which was cut off when the Rhine was straightened during the 20th century. These cut-off arms have become valuable recreational areas and a nature reserve.
In 1900, the short-cut at Fussach was completed. The old river course, from Höchst and St. Margrethen to the village of Altenrhein in Tahl on the shore of Lake Constance forms the border between Austria
. In 1906, the Rheintaler Binnenkanal ("Rhine Valley Inland Canal") was dug, connecting Sennwald to St. Margrethen. In 1923, another short-cut was completed at Diepoldsau, creating another dead branch, also named Alter Rhein.