North Sea tides reached as far as Hollingstedt, so larger vessels were able to navigate the river, until a dam at Koldenbüttel closed off the Treene in 1570.
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).