European Sea and River Cruises

Were there really chains across the Danube in the Middle Ages?

I am asking about the area near Aggstein. I have heard that medieval Lords put chains across the Danube to stop boat traffic and make them pay tribute. I know this did happen on the Hudson River upstream from New York during the Revolutionary War (not for tribute, but for defense). But did it really happen during the Middle Ages? Is there any evidence?
Thanks for your answer, Ines. I am fluent in German, so no problem with German-only info.

Have a look here:

http://www.sagen.at/texte/sagen/oesterreich/niederoesterreich/waldviertel/kuenring.html

Stefan Fadinger (Bauernführer) tried to put a chain across the Danube near Aschach (Upper Austria) to block the Bavarian transports.

Also have look here:

http://de.wikipedia.org/wiki/Turmbefestigung_Linz

scroll down to “Donauanschlüsse”

So it was more or less common practice – either to collect tolls or for defense.

Herbert von Karajan conducts The Blue Danube Waltz