"I came across these guys on my recent trip to Munich. I'd walked through the Englischer Garten (English Garden - Munich's equivalent of New York's Central Park) when I saw a crowd gathered on a bridge. Where the small river spilled out of a tunnel it created this wave just downstream. There were seven or eight guys taking turns surfing the standing wave. They'd wait for someone to fall or get swept downstream, then expertly jump from the edge onto their boards on the face of the wave - almost like a skater hoping into a half-pipe. I saw a couple of guys ride for three minutes or longer, doing big roundabouts and cutbacks and occassionaly slapping the noses of their boards against the river wall.
"I couldn't tell if it was always ride-able or just after the recent rains - the river looked offcolor and swollen with runoff. I did hear it was illegal and that an Australian had died a few weeks earlier, but no one hassled the surfers while I was there."