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a river in the Federal Republic of Germany; a right tributary of the Rhine. The Lahn is 245 km long and drains a basin area of 5,900 sq km. The river flows through a winding valley, mainly within the bounds of the Rhenish Slate Mountains. The average discharge of water at the river’s mouth is 57 cu m per sec. There are winter and spring floods. The Lahn is navigable and sewered from its mouth to the city of Giessen (a distance of 148 km). The cities of Marburg, Giessen, Wetzlar, and Limburg are located on the Lahn. The resort of Bad Ems is located on the lower course of the river.