The Bondenau, as the main headstream of the Treene, rises 15.6 km (9.7 mi) east of the Tressee on the peninsula of Angeln, bounded by the Flensburger Förde and the Schlei.

Half a kilometre before the Treßsee (Lake Tress) it is joined by a stream named Kielstau or Kielsau, by some people considered another headstream of the Treene.

Between the Treßsee and Tüdal there is a natural reserve covering an area of 20 square kilometres (7.7 sq mi) called Obere Treenelandschaft that extends along the river.

From antiquity to High Middle Ages, the strip of dry land between Treene wetlands and Schlei had the function of an isthmus.

Whether thereby boats were pulled between Selker Noor (an extension of Schlei inlet) and the Rheider Au 16 km (9.9 mi) on primitive rollers over the 20 m high land ridges (usual opinion), or whether the Kograben, 1 to 2 km south of and parallel to the walls of Danevirke, also served as a shipping canal, has not yet finally been clarified.