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. Having entered Upper Bavaria after ca. just north of Scharnitz, the Isar is dammed for the first time between Mittenwald and Krün. Some of its waters are led via the Rißbach, whose flowing direction was reversed for this purpose, into the Walchensee. Here it is fed into the giant downpipes of the Walchensee power plant. 200 metres lower the plant releases this part of the water into the Kochelsee. From here it flows into the river Loisach, Isar's second most important tributary. Not far down the river a large reservoir called Sylvensteinsee was created between 1954 and 1959 to make more energy generation possible and also to avoid flooding. It is located some south of Lenggries. It has prevented some bad floods in Munich, for example in 1979, 1999 and most important, in 2005. In that year for the first time ever water had to be released from the lake because its capacity was exhausted. Flooding was barely avoided in Munich, literally 'by the width of one hand'. At Bad Tölz the Isar leaves the Bavarian Alps. This town marks the river's transit from its upper to its middle course. It now flows through a sub-alpine, glacial morainic landscape towards Wolfratshausen. Just north of the town it absorbs the Loisach, including the water previously drained off for the power plant. The Isar continues on to and through the Munich gravel plain, into which it cuts a narrow, steeply sloped valley. The latter finally opens up and ends within the city limits of Munich. Further downstream the river passes Freising and then absorbs the waters of the Amper, its most important tributary, at Moosburg.