The Donauinsel is a 20 kilometer long and about a hundred meters wide island that divides the Danube river as it passes through Vienna. On one side of the island is the Danube river where big ships go, and on the other side is the New Danube river, where small boats, swimmers, etc. go. The "Old Danube" isn't a river at all anymore - it is a lake that was formed when the riverbed of the Danube river was straightened out as it passes through Vienna. The leftover U of the river became the Old Danube.
So to summarize this very short lesson in unapplicable geography - there are three Danubes in Vienna - Danube (big river), New Danube (small river), and Old Danube (lake).
The island has these insanely powerful sprinklers placed along it that drenches anything within a semicircular area in front of it.
View from the - for lack of a better word - battery.
The island has great bicycle and rollerblade paths. While some paths use cobblestones (as can be seen on this picture and the next), most have a very smooth asphalt surface.
View of Vienna International Center.