German Drau River, south-central Europe. It rises in the Carnic Alps and flows east through Austria, where it forms the Drautal, the longest longitudinal valley of the Alps. Flowing southeast, it passes through Slovenia and forms part of the Croatian-Hungarian border. The Drava is 447 mi (719 km) long and a major tributary of the Danube River. Its valley was the chief passage through which invaders penetrated the Alpine countries