The Viking Tir is now on the Danube River, the final leg that started with the Rhine River, then the Main River, and onto the  Rhine-Main-Danube Canal  (106-mile long connection completed in 1992) with 16 locks).   The Danube is the second-longest river in Europe, 1051 miles in length, after the Volga River in Russia, from the Black Forest into the Black Sea. Flows through 10 countries, including Ukraine (border with Romania). The river water is opaque and greenish-brown.   In other words, the Danube is NOT blue.   You might see photos of what looks like a blue Danube, but that is more a reflection of the blue sky. The  most polluted is the Ganges River in India , with the Mississippi at #5. Rivers all pick up soils from land, and man-made waste, so the water color reflects the mineral content. But the Danube is beautiful, matched with gorgeous surroundings. There are  four Danubes  flowing through Vienna, called Old Danube, New Dan...