Using a float in rivers is a little more complicated because of the different current speeds between the surface water and the water on the bottom. The deeper the river, the bigger the difference in the current speeds.
This is important because your float is at the surface and will be travelling at the same speed as the surface water. It will also be dragging your bait at the surface speed. The problem is that water near the bottom (and all the little bits of food there) will be travelling at a slower speed, so your bait will not look natural, and the fish will not take your bait. To remedy this problem, you need to hold back or "check" the float and slow it down. Only through trial and error can you get the correct speed.
Good floats help, too. Get some floats specifically designed for river fishing. Avon floats would be good to start with. You can get them at Wacker Baits or fishusa.com.