ST21435 is a section of the Danube River that runs through the southeastern corner of Germany, beginning at the town of Regensburg and ending at Passau. This part of the river is known as the Upper Danube, and is especially scenic and picturesque, with its centuries-old cities, villages, castles, monasteries and fortifications lining the banks. The Upper Danube is the most heavily visited section of the river, and there are numerous boat cruises that are available, which show off the beauty of the river and its surroundings. The Upper Danube is also an important agricultural area, and it is home to several hydroelectric dams, providing clean energy to the region. Finally, it is a key part of both the Eurovelo 6 and 9 cycling routes, which pass through Austria, Slovakia, Hungary and Germany.