The fence does not have to be mid river. At most it requires 300 feet of shoreline for fence/road/open security area.
I believe there are some requirements about fencing/walls along the flood plain which must comply with US/Mexico Treaties, Rio Grande/water.
Likely so. But that’s why you make this “officially and legally” a “National Security” issue. It places it in a whole new world related to environmental ans “Treaty” obligations. And considering Mexico’s “obligations” don’t concern them, I don’t see why we should be too concerned either.
Just as a side note to the above (and this isn’t aimed personally at you but to people in general who ask those type of questions)
Yes, there are any number of laws in place concerning environmental issues. Can you honestly imagine Washington stopping the RevWar over the fact that battles in meadows destroyed wetlands?
Can you imagine FDR not fighting WW2 over concerns that sunken ships would dump fuel into sensitive waters?
We are talking about a literal invasion here and protecting the very border and American citizens. I do not see how the endangered snarfblat should have a say in it. Our former War presidents seemed to agree.