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In page Weser:

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The river, when combined with the Werra (a dialectal form of Weser),[citation needed] is 744 km (462 mi) long and thus, the longest river entirely situated within Germany (the Main, however, is the longest if the Weser-Werra are considered separate). The Weser itself is 452 km (281 mi) long. The Werra rises in Thuringia, the German state south of the main projection (tongue) of Lower Saxony.