From the Nature Park Info Center Scharnitz (parking lot P2) on the Hinterautal road always straight ahead in the direction of Hinterautal. After about 2.5 km the road becomes a forest path. Follow this path until you reach the so-called Gleirschhöhe (lookout point with cross). Then turn left and continue hiking in the direction of Isarursprung/Kastenalm. After a short downhill stretch, you reach the actual valley floor and now hike mostly along the crystal-clear Isar to the source of the Isar. From there it is another 2 km to the Kastenalm, which is open during the summer months.