The source of the Iller is located in Oberstdorf, where it is formed through the confluence of the Trettach, Stillach and Breitach rivers. As it starts to head north, the Iller winds its way through the Allgäu past the towns of Sonthofen, Immenstadt and Kempten. On the second half of its journey, the Iller flows through Upper Swabia, roughly marking the state border between Baden-Württemberg and Bavaria, before joining the Danube in Ulm and continuing to the Black Sea. The Breitach’s source is in the Kleinwalsertal valley, while the sources of the two other tributary rivers, the Trettach and Stillach, are located in Bavaria. The Iller’s source is within easy walking distance of Oberstdorf, just by the "in form park" fitness centre to the north of the village.