St. Margrethen in the context of Austria–Switzerland border

The Alter Rhein (German pronunciation: [ˈaltɐ ˈʁaɪ̯n] ; German for Old Rhine) is the old river bed of the Alpine Rhine in St. Gallen and Vorarlberg in the Alpine 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, linking the natural course of the Rhine just upstream of Höchst and St. Margrethen directly to a new outflow into Lake Constance. The old river course downstream of this point, via Rheineck to Altenrhein on the shore of Lake Constance, became known as the Alter Rhein and continues to form the border between Austria and Switzerland. The arm is navigable from the lake to Rheineck, and is used by the Swiss shipping services (Schweizerische Bodensee-Schifffahrt [de]) on the lake to reach landing stages adjacent to Rheineck railway station.