The Austrian flag (of usual flag proportions) is red-white-red in three equally broad horizontal strips.
Anywhere on rivers in Europe, you may find it (or a sign whose proportions are not very different) on the vaults or beside the arms or confluences where passing through is forbidden. Either because it is not deep enough (in which case an exception, like for canoes, may be marked as such) or because it is reserved for boats going the other way.
In Austrian history too, the flag means in a very different and much more warlike way: "thou shalt not pass". St Leopold died in a battle but won it, against the then still pagan Magyars who tried to invade the Duchy Austria.* The flag is a diagram of his shirt: bloody above and below the belt, still white where the belt had prevented the blood from soaking the shirt.
Georges Pompidou, Paris
St Anne, 26 July 2009
*This is what I heard as a child. Wikipedia gives another legend, about another Leopold. Also, according to Wikipedia, St Leopold is not the Babenberg who defended Austria against Hungarians, indeed he lived well about a Century after St Istvan/Stephen of Hungary. On this link you will find more than one early Babenberg killed in battle.