Jedidah is right. to build a fence along the river you’d have to build it on the US side, cutting ranchers off from water. It’s not workable.
I have ridden on the San Felipe Ranch of 25,000 acres along the Rio Grande, it’s very arid and they need that water badly. also I know someone who leases acreage near Eagle Pass, and their grazing land is fed by a canal from the Rio Grande.
These people are also threatened daily almost by thuggish illegals coming across. they lose calves and lambs to illegals who kill and eat them.
A combination of enforcement of the labor laws, checking ID, and a lot more Border Patrol would be a better start.
Yes good fences make good neighbors.