Water flow speed

I have only fished on the Missouri river a couple times. Friday we were trying to pitch jigs at some wing dams and had no success feeling the bottom. A couple times I got snagged but I think it was a log or something wood. I was pitching 5/16 and 3/8 oz jigs with hair and non-aggressive plastic, with 6 pound line. The river on the inside bends was moving 3.25 mph land speed and on the outside it was moving 4.75 mph land speed. My question is, are these water speeds typical of what you usually see on the Mississippi? Do I need to try 7/16 or 1/2 oz jigs? Puzzled