The Amper (known in its upper reaches as the Ammer) is a river in southern Bavaria, Germany – the largest tributary of the Isar river. The Amper flows in a generally north-eastwards direction, reaching the Isar in Moosburg, 185 kilometres (115 mi) from its source in the Ammergau Alps, with a flow of 45 m³/s. It runs through the Ammersee; upstream of the Ammersee it is known as the Ammer.