Question: What 15 km river, located in southern Germany, whose name means "Blue Pot" flows east through Blaustein to the city of Ulm? 

Context paragraphs:
San Juan River (Nicaragua): The San Juan (Spanish: "Río San Juan"), also known as El Desaguadero ("the drain"), is a 192 km river that flows east out of Lake Nicaragua into the Caribbean Sea.  A large section of the border between Nicaragua and Costa Rica runs on the right (southern) bank of the river.  It was part, with the lake, of a proposed route for a Nicaragua Canal in the 19th century.  The idea of the project has been revived in the last decade, including the possibility of other routes within the country.  The Ecocanal project has obtained a Concession from the National Assembly of Nicaragua to re-open the San Juan River to commercial barge traffic.

Kosasthalaiyar River: Kosasthalaiyar River, also known as "'Kortalaiyar" in Manali a distributary, is one of the three rivers that flow in the Chennai metropolitan area.  The 136 km river originates near pallipet in thiruvallur District and drains into the Bay of Bengal.  Its northern tributary Nagari river originates in Chitoor district of Andhra Pradesh and joins the main river in the back waters of Poondy dam.  Its catchment area spreads in Vellore, Chitoor, North Arcot, Thiruvallur and Chennai districts.  It has catchment area in North Arcot District with a branch near Kesavaram Anicut and flows to the city as Cooum River and the main Kosasthalaiyar River flows to Poondi reservoir.  From Poondi reservoir, the river flows through Thiruvallur District, enters the Chennai metropolitan area, and joins the sea at Ennore creek.  There are two check dams across the river at Tamaraipakkam and Vallur.  The excess discharge in the river is controlled by the Tamarapakkam Anicut located across the river in the downstream of Poondi reservoir.  Vallur Anicut is a small check dam constructed near Minjur across the river to control water levels and feed irrigation channels in the area.  It flows to a distance of 16 km in the Chennai metropolitan area.  The total catchment area of the river is 3757 km , and the bed width ranges from 150 to .  The discharge capacity of the river is 110000 m3/s , and the anticipated flood discharge capacity is about 125000 m3/s .

Kakahu River: The Kakahu River is a river of south Canterbury, New Zealand.  It flows east and then southeast from its source 15 km east of Fairlie, joining with the Hae Hae Te Moana River before flowing into the Waihi River close to the town of Temuka.

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

Ultapani Reserve Forest: Ultapani, the name means "The Reverse Water", the river which flows through the forest in the direction west to east unlikely other rivers flows east to west.  It is located in Kokrajhar Tehsil of Kokrajhar district in Assam, India.  It is situated 55 km away from Kokrajhar, which is both district & sub-district headquarter of Ultapani Forest.

Paine River: Paine River ("Río Paine") is a river located in the Magallanes Region of Chile.  The river rises from its source in Dickson Lake and flows east for nine kilometres (km) to Paine Lake.  After flowing through the lake, the river runs for 15 km, first southward and then westward to Nordenskjöld Lake.  This part of the river's course includes the Paine Cascade.

Blautopf: The Blautopf (German for "Blue Pot"; "blau" means "blue", "Topf" means "pot") is a spring that serves as the source of the river Blau in the karst landscape on the Swabian Jura's southern edge, in Southern Germany.

Waterhen River (Saskatchewan): The Waterhen River is an east-flowing river in northwest Saskatchewan in the basin of the Beaver River (Canada).  It is north of and parallel to the east-flowing part of the Beaver River and joins the north-flowing part of that river.  It is at the southern edge of the boreal forest belt.  The large Primrose Lake discharges south through the Martineau River into Cold Lake on the Alberta-Saskatchewan border.  Cold Lake discharges east into the Cold River through Pierce Lake then into Lac des Iles.  The Waterhen River begins on the east side of Lac des Iles and flows east to Waterhen Lake.  Most of the river from Lac des Iles to Waterhen Lake is in Meadow Lake Provincial Park.  East of Waterhen Lake the river leaves the park and flows east and somewhat north into the north-flowing part of the Beaver River.

La Souche: The commune is located in the "Parc naturel régional des Monts d'Ardèche" 15 km east of Aubenas; its main hamlets are located on the right bank of the river Lignon, which rises in the commune's western part, then flows east through the commune.

Andelva: Andelva is a 15 km river that runs through Eidsvoll, Norway.  It flows from the lake Hurdalssjøen at Eidsvoll Verk to the river Vorma in the town center of Eidsvoll.

Supporting paragraphs: Blau (Danube) Blautopf
Answer: Blautopf