Two male bears tussle over a fish that the bear at left caught and was eating (you can see it in the bottom left corner of the photo). This skirmish lasted only a few seconds, but showed how quickly the big, lumbering bears could turn into formidable half-ton fighters.
Many (in fact most) of the big males positioned themselves in the foamy water at the base of the falls, picking off salmon that either hadn't jumped yet, or else had jumped too "horizontally," stunning themselves on the rocks behind the falls, and then floated back into these foamy waters. This fellow seems to be sizing up a seagull, perhaps wondering if he could reach it in time to add a little variety to his feast of fish...