I'm not convinced a wall or fence will do anything except cut off access to the river by Americans. At best it will divert the problem to other areas.
We already know that the Mexican coyotes and drug smugglers are expert at digging tunnels under the border.
And even if we somehow prevent that, all any Mexican needs to do is catch a plane to Canada and enter from the north. For what they pay coyotes to smuggle them, I don't know why they are not doing that already. Maybe they are.
And then there are thousands of miles of unguarded coastline where illegals could simply unload and walk freely onshore.
I think the people who believe a fence will end illegal immigration haven't really thought this through.
Very few people that I know of have actually said a fence will end the problem The goal is to slow it down and make it more manageable.