I have been down that road to a bird preserve on the Rio Grande. It is scary wandering in the woods along the river with expensive camera gear. I got out of there and haven't been back.
I did see a Border Patrol SUV on the levee that the fence is on parked about 50 yards from the road watching the road crossing.
Here is an interactive map of walls and fences all along our Mexico border. If you look at the area along the Texas border you can see that the barriers cut off significant chunks of land as the river weaves back and forth as rivers do. I don’t know if the estimated length of walling it all means up against the river or not. If it does mean right along the river, then that may be 3 times the current wall length in those areas.