Wasn't on a river, but I nearly saw a fight about 6 or 7 years ago out on the Redondo pier. Bunches of people casting buzz bombs and plug cut herring for salmon close to shore, and bunches of people in boats cutting far too close to the front of the dock. One yahoo came in from the north (the launch is on the south side of the pier) and when he cut across the front of the pier no more than twenty feet out, shearing off lines and making people lose gear, about a dozen people on the pier decided they'd just about had enough. They all ran down towards the shore and the smaller floating dock used for launching boats, and when this guy caught a look at the reception party, he turned around and decided Des Moines might be a better spot to come in.