Blau (Danube)
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The Blau is a 15 km long river in Baden-Württemberg, southern Germany, left tributary of the Danube. The source of the Blau is the karst spring Blautopf, in the town Blaubeuren, in the Swabian Alb. It flows east through Blaustein to the city of Ulm, where it flows into the Danube.