Is river bed private property?

[HookJaw_Brimhall]

Here's a question.
What if I'm fishing a river and a come to a barbed wire fence that goes into the river and clearly states that it is private property? The fence has a post in the middle of the river bed indicating that the property is infact the owners. Crossing this fence would indeed be trespassing, but if the river is public how can I justify crossing someones fence??? Nobody wants to be seen crossing a fence whether it's public or private on the other side.
What would you do?
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