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

full_context: Document 1: 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.
Document 2: 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.
Document 3: Blauen (Badenweiler): The Blauen or Hochblauen is a 1,165 m mountain in the southern Black Forest. The peak lies within the municipalities of Schliengen and Malsburg-Marzell in Landkreis Lörrach and the community of Badenweiler in LandkreisBreisgau-Hochschwarzwald. It is an ideal viewpoint with views of the Black Forest, Vosges, Jura Mountains, and the Alps.
Document 4: Blauhöhle: The Blauhöhle is the largest cave system in the Swabian Alps in southern Germany. The Blauhöhle presumably originated in a time when the Danube still flowed through the Blau valley. Since the shifting of the Danube, several small rivers, the Schmiech, the Ach, and the Blau, have flowed through this valley. The cave system begins about 21 meters under water at the base of the Blautopf. It continues west and northwest, rising and falling several times until after a horizontal distance of about 1200 m it comes above the level of ground water and opens into the second big air-filled chamber. The maximum depth of the cave under water is 42 m .
Document 5: 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.
Document 6: Sink (geography): A geographic sink is a depression within an endorheic basin where water collects with no visible outlet. Instead of discharging, the collected water is lost due to evaporation and/or penetration (water sinking underground, e.g., to become groundwater in an aquifer). If the sink has karstic terrain, water will sink at a higher rate than the surface evaporation, and conversely if the lakebed or sink bed has a layer of soil that is largely impervious to water (hardpan), evaporation will predominate. Since dry lakes in sinks with hardpan have little penetration, they require more severe aridity/heat to eliminate collected water at a comparable rate as for a similar sink with appreciable penetration.
Document 7: Blue Duck River: The Blue Duck River is a river of New Zealand. A tributary of the Dart River, it rises to the south of the Blue Duck Glacier, flowing into that river east of Cattle Flat.
Document 8: Blue Lagoon (geothermal spa): The Blue Lagoon (Icelandic: "Bláa lónið") geothermal spa is one of the most visited attractions in Iceland. The spa is located in a lava field in Grindavík on the Reykjanes Peninsula, southwestern Iceland. Bláa lónið is situated approximately 20 km from the Keflavík International Airport and 39 km from the capital city of Reykjavík, roughly a 21-minute drive from the airport and a 50-minute drive from Reykjavík.
Document 9: Hohle Fels: The Hohle Fels (] ) (also Hohlefels, Hohler Fels, German for "hollow rock") is a cave in the Swabian Jura of Germany that has yielded a number of important archaeological finds dating to the Upper Paleolithic. Artifacts found in the cave represent some of the earliest examples of prehistoric art and musical instruments ever discovered. The cave is just outside the town of Schelklingen in the state of Baden-Württemberg, near Ulm. In 2017 the site became part of the UNESCO World Heritage Site "Caves and Ice Age Art in the Swabian Jura".
Document 10: Sources of the Rhine: Lake Toma in the Swiss Canton of Graubünden is generally regarded as the source of the Rhine. Its outflow is called "Rein da Tuma" and after a few kilometers, it forms the "Vorderrhein" (English: "Anterior Rhine"; Romansh: "Rein Anteriur"). The course of this river is not particularly representative, after about two kilometers, its water is diverted into Curnera reservoir. The water is released at the Tavanase plant and flows into the Rhine at Ilanz. The river begins to be called "Rhine" in the vicinity of Chur, more specifically, at the confluence of the Vorderrhein and Hinterrhein (river) in Reichenau.
Document 11: Geisenklösterle: Geisenklösterle (German: 'Geißenklösterle' ) is an archaeological site of significance for the central European Upper Paleolithic, located near the town of Blaubeuren in the Swabian Jura in Baden-Württemberg, southern Germany. First explored in 1963, the cave contains traces of early prehistoric art from between 43,000 and 30,000 years ago. In 2017 the site became part of the UNESCO World Heritage Site Caves and Ice Age Art in the Swabian Jura.
Document 12: Speleonaut: Speleonaut (named from the Greek words for "cave" and "sailor") is the submersible used by the cave diver Jochen Hasenmayer. After the 1989 decompression accident that left his legs paralyzed, Hasenmayer designed the "Speleonaut" with his friend Konrad Gehringer in order to continue exploring the Blauhöhle cave system, which begins at the base of the Blautopf spring in the Swabian Jura mountain range. The "Speleonaut" is 72 cm wide and has nine engines, making it easy to maneuver in all directions. It is the first submarine designed specifically for the exploration of caves. According to Hasenmayer, the "Speleonaut" has been tested in Lake Constance to a depth of 105 m and has a design limit of 180 m .
Document 13: Blue Waters F.C.: Blue Waters, also known as Langer Heinrich Mine (LHU) Blue Waters due to sponsorship reasons, are a Namibian football club from Walvis Bay. The team is nicknamed Blue Birds, The Birds or Omeva (which means 'water' in the local OshiWambo language by its dying supporters. The team trains at its current field, Blue Waters Sport Field in Kuisebmund, a suburb of Walvis Bay. The team has a local rivalry with Eleven Arrows F.C., which was formed by former players of Blue Waters in early 1960s.
Document 14: Sirgenstein Cave: The small Sirgenstein Cave, German: "Sirgensteinhöhle" is situated 565 m above sea level inside the 20 m high "Sirgenstein", a limestone rock. The cave sits 35 m above the Ach River valley bottom in the central Swabian Jura, southern Germany. Archaeologist R. R. Schmidt excavated the site in 1906 during which he identified indices of prehistoric human presence. He recorded the complete stratigraphic sequence of Palaeolithic and Neolithic origin. In his 1910 analysis Schmidt inspired future archaeologists with his pioneering concept of including the excavation site within its geographic region, contextualizing it within a wide scientific spectrum and demonstrated valuable results as he correlated the Sirgenstein layer structure to those of prehistoric sites in France.
Document 15: Wutach Gorge: The Wutach Gorge (German: "Wutachschlucht" ) is a narrow, steep-sided valley in southern Germany through in the upper reaches of the River Wutach with three gorge-like sections, the lowest of which is also called the "Wutachflühen". The gorge cuts through the southern part of the Baar region from the eastern side of the High Black Forest heading eastwards to the "Trauf" the steep, northwestern flank of the Swabian Jura, which transitions to the Randen mountains here.
ground_truth: Blautopf