The official head of the Isar is located in the Hinterau valley east of the village Scharnitz in the Karwendel mountains in Tyrol at a height above sea level of . In 2003 the Isar spring was developed, however, this move has been discussed due to environmental reasons. The Lavatschbach, which is the Isar's longest headstream, has its source farther to the southeast near a mountain cabin named Hallangerhaus. It is also sometimes called the Isar's head. Further on the young Isar is supplied water by the Birkkarbach, which has its source at the foot of the highest mountain of the Karwendel, the Birkkarspitze and the Moserkarbach.