The frozen danube - actually a missleading title. The frozen water that is shown is a sidebranch of the danube that runs parallel to the "original" danube. It is man-made to take a way significant amount of water from the main stream during high-water times. As this is rarely the case, especially in winter, the watergates are closed most of the time and the flowrate is almost zero - hence it is covered by ice. That would never happen to the real danube.