Hey Bert - you would have to be beyond stupid to try to build a fence in a river.
On or back a bit from the riverbank - that’s another story. (Shrug) I’m not trying to be an Engineer, but it is plenty doable.
In one of my fiction books I refer to The Trench. In that story, the US just digs a huge trench on the border.
Then, anything trying to cross gets shot.
Ya, a pretty lousy fiction world, eh?
My guess is that landmines are out of the question, right?
What around electricity - high voltage within the wall?
We need a prison type wall.
so, you are for ceding Mexico American territory. The border is the center of the river
No trench is necessary. just a bright line on the existing fences and
a tower with riflemen every quarter mile or so