Inn (ĭn) [key], river, c.320 mi (515 km) long, rising near the Lake of Sils, SE Switzerland. It flows NE through the Engadine valley, then through W Austria, past Innsbruck and Solbad Hall (the head of navigation), and into S central Germany. The Inn forms part of the German–Austrian border before entering the Danube River at Passau. There are more than 20 hydroelectric power plants on the river's swift-flowing stream.
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