The Blau (ˈblaʊ) is a 15 km long river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in Baden-Württemberg

Baden-Württemberg

Baden-Württemberg is one of the 16 states of Germany. Baden-Württemberg is in the southwestern part of the country to the east of the Upper Rhine, and is the third largest in both area and population of Germany's sixteen states, with an area of and 10.7 million inhabitants...

, southern Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, and a left tributary of the Danube

Danube

The Danube is a river in the Central Europe and the Europe's second longest river after the Volga. It is classified as an international waterway....

. The source of the Blau is the karst spring

Karst spring

A karst spring is a spring that is part of a karst system. That includes the underground drainage of a much larger area, which means that karst springs often have a very large discharge...

 of Blautopf

Blautopf

The Blautopf is a spring that serves as the source of the river Blau in the karst landscape on the Swabian Alb's southern edge, in Southern Germany. It is located in the city of Blaubeuren, approximately west of Ulm...

, in the town Blaubeuren

Blaubeuren

Blaubeuren is a town in the district of Alb-Donau near Ulm in Baden-Württemberg, Germany.It has 11.963 inhabitants as of December 2007.-Coat of arms:...

, in the Swabian Jura. It flows east through Blaustein

Blaustein

Blaustein is a municipality in the district of Alb-Donau Baden-Württemberg in Germany. It is situated on the Blau River, 6 km west of Ulm and has about 15,000 inhabitants.It was created in 1968 after the union of Ehrenstein and Klingenstein...

 to the city of Ulm

Ulm

Ulm is a city in the federal German state of Baden-Württemberg, situated on the River Danube. The city, whose population is estimated at 120,000 , forms an urban district of its own and is the administrative seat of the Alb-Donau district. Ulm, founded around 850, is rich in history and...

, where it empties into the Danube

Danube

The Danube is a river in the Central Europe and the Europe's second longest river after the Volga. It is classified as an international waterway....

.

Blau valley

The valley of the Blau was formed by the Danube. When the Swabian Jura rose, the Danube initially incised a deeper and deeper valley but finally chose a course to the southwest. In the valley it left behind today, the Schmiech river flows south meeting the Danube in Ehingen (Donau), whereas the Blau flows east towards Ulm

Ulm

Ulm is a city in the federal German state of Baden-Württemberg, situated on the River Danube. The city, whose population is estimated at 120,000 , forms an urban district of its own and is the administrative seat of the Alb-Donau district. Ulm, founded around 850, is rich in history and...

.