The Amper, called the Ammer upstream of the Ammersee, through which it runs, is the largest tributary of the Isar in southern Bavaria, Germany. It flows generally north-eastward, reaching the Isar in Moosburg, about 185 kilometres (115 mi) from its source in the Ammergau Alps, with a flow of 45 m³/s. Including its tributary, Linder, it is 209.5 km (130.2 mi) long. Major tributaries are the Glonn, which rises near Augsburg; the Würm, which is the outflow of Lake Starnberg; and the Maisach.

Ammersee is a Zungenbecken lake in Upper Bavaria, Germany, southwest of Munich between the towns of Herrsching and Dießen am Ammersee. With a surface area of approximately 47 square kilometres (18 sq mi), it is the sixth largest lake in Germany. The lake is at an elevation of 533 metres (1,749 ft), and has a maximum depth of 81 metres (266 ft). Like other Bavarian lakes, Ammersee developed as a result of the ice age glaciers melting. Ammersee is fed by the River Ammer, which flows as the Amper out of the lake. Like neighbouring Lake Starnberg - deeper, bigger in surface area, similar in shape - it is a popular location for watersports.