that is a big hazard!!
At least post sign's warning of the fences and mark with orange marker poles by the rivers edge about 15 feet high or better yet don't string wires across river's at all, out of consideration, legal or not. I understand that they use the fences to keep life stock in by the rivers edge, especially at low water times, so to ensure the fencing works at very low river levels they just string it across the whole river, very dangerous situation for boaters that are cought off guard. My uncle had a farm and would set up an electrical wire up to keep his cattle in, during the winter months the snowmobile riders would use this part of his land to ride on despite the no trespassing signs, so he would mark the wire with orange flagging tape and tall orange poles so the riders would be aware of the whereabouts of the fence, commen courtesy. On a side note, in my repair box I would carry a small set of bolt cutters, they would work great as ring cutters to save a finger and one time a gentleman lost his key to his family's cable lock for his bikes on the back of his mobile home and the bolt cutters worked very well to cut the cable, they have many uses. They could save injuries to you or any boater behind you. They have smaller 8 inch one' that are very compact.