With little current, 5lb mushroom anchor might hold you - I always used heavy clothes-line for such a small anchor when I used it on my float tube. In heavier current, you'll probably want something a lot beefier - both for anchor line and anchor weight. You'll also want to have a plan to get back to your vehicle, as if you wind up in the big C, you might have a hell of a time trying to row against the current. Not sure a trolling motor is going to be a lot better either. Is your raft one with a wood transom for mounting a motor, or is it rubber all around (thus leaving with the question of how, exactly, would you mount the electric safely?)
Summertime flows are a lot lower than they are right now - if you're thinking of floating that section in a raft right now - I would reconsider. Water temps being low, with low air temps would spell disaster in a hurry if you wind up in the water. PFD use is a must, as is some form of signaling device should you wind up in distress - an air horn would probably be better than a whistle in that regard. Both would be better than just one.