That route on top of the dam seems to be forbidden to use. Every few meters you will face like up to three signs telling you that it is forbidden to use that way ( "(...) Betreten verboten"). If i remember correctly, the "publisher" of those signs were something like an offical instituation.
The first sign was to the left, near the "Am Kanal" tunnel. The last one somewhere after "Am Sperrtor" to the right of that path.