The Alte Donau is a piece of the Danube that the engineers left behind. When the river was straightened between 1870 and 1875 this bend was left out of the new channel. Put your hand in the water now and nothing moves; it stands, fed from below by groundwater rather than by the river, and reeds grow thick along the banks. Wooden boathouses line the shore, one after another, and boats have been hired out here since 1880. This is where Vienna keeps its sailing dinghies and rowing boats, because there is nowhere else in the city to put them.
On the eastern side lies Gänsehäufel, a sand island in the old river bed. A naturist named Florian Berndl opened it as a bathing beach, and the city then took the idea over and built a proper lido there in 1907. It was flattened in the war and rebuilt by 1950, and on a hot weekend it still fills with tens of thousands of people. The water itself nearly died in the early 1990s, when blue-green algae took over, and it is now kept alive by a constant feed of fresh water.