True that some of the Rio wouldn't require a fence as the canyon walls to a large extent
provide the barrier. Then you have the problem in towns, cities with the structures being built
up to the river.
All said I guess it could be done but it isn't going to be an easy thing. However we built and
continue to build major freeways across the land and take property along them for the ROW.
If I were going to build a wall to keep out illegals I would build it away from the river maybe 5 or 10 miles inside Texas. Which entry points at existing roads. No way would I build at the physical border. Except in densely populated areas, there I would build it at the border.