A tall thick concrete fence.
The kind the #Nevertrumpers and Free Traitors were terrified might happen.
Trump said they are using 90ft concrete slabs as in parking decks.
What bothers me is that the existing fences along the river areas have been set back from the river into the US side, effectively giving up US territory. There is a treaty that says no barriers can divert the flow of the river, so there has never been a barrier right down the middle of the river where it belongs. Ranchers actually have to use locked gates to get their livestock to the river and back in those areas. In other places, no barrier exists at all and the BP has to constantly wave off smugglers in rafts.
It seems like a fence could be driven into the riverbed that does not interfere with the flow of the river. That should be studied and a design decided upon so those ranchers can get full access to their land back and we can avoid fights over taking any private land to build the fence on the riverbank. If a solid concrete barrier is out, then one with openings too small for people but large enough to not impede water should be built.