The Großache is a river, 79 kilometres (49 mi) long, in the east of the Austrian state of Tyrol and the Bavarian region of Chiemgau.

It rises on the Thurn Pass on the territory of Salzburg state, reaches the Tyrolean border after just 400 metres (1,300 ft) and the German border after 55 kilometres (34 mi), discharging 24 kilometres (15 mi) later near Grabenstätt into the Chiemsee lake.

The nominal source of the Jochberger Ache lies at 1,270 m (4,170 ft) above sea level on the Thurn Pass.

The most important is the Trattenbach which starts as a trickle from a high cirque at about 1,900 m (6,200 ft) above sea level between the mountains of the Zweitausender and the Rossgruberkogel south of the Kleiner Rettenstein, and is fed by several other springs before it joins the Jochberger Ache.

Shortly after St. Johann, still in the valley of Leukental, lie the villages of Kirchdorf in Tirol and Erpfendorf.

This was achieved by deepening the river bed and widening the banks to produce greater drainage volume.