This is a big can of worms. The term Navigatable comes into play. Most land owners will tell you they own to the middle of the channel and the water over it. If the river is "navigatable" for commerce its supposed to be public. But, all of iowa's rivers are public water. If you are on the water, its public. Step on the connected land, its private.
This is a fight that has been going on in the south for a long time now as to what "navigatable" means specifically when it comes to fishing and fishing access on rivers and connected oxbows and connected water...