Inn (river, Europe) (ancient Aenus), river in central Europe, flowing in Switzerland, Austria, and Germany. It rises in Lake Lughino in eastern Switzerland and flows generally northeast to the Danube River. From its source the Inn flows through the Swiss valley of the Engadine, enters Austria near Nauders, and crosses the Tirol (Tyrol) and Bavaria regions. The river joins the Danube at Passau, Germany. In the last part of its course of 510 km (317 mi), the Inn forms the boundary between Germany and Austria. The principal tributary is the Salzach River.