Map of the rivers Eider and Treene, Rheider Au and Schlie.

The Rheider Au (Danish: Rejde Å) is a tributary of the Treene. Its source is on the Geest near Schleswig. In the Viking period the route Eider - Treene - Rheider Au - Schlei served as a navigation way and/or transport or trade route between places to the north and the Baltic Sea, as commercial centres functioned (see Dorestad, Haithabu).

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Coordinates: 54°27′N 9°20′E / 54.45, 9.333