“President Trump ceded the Rio Grande river to Mexico”
No legal concessions have been made. Some border defensive measures (like roadblock inspections) are up to a hundred miles from the border.
Border Patrol Maritime Units continue to Patrol the River, and are being well resourced.
I ran the Rio Grande on Google earth. I thought the border was the middle of the river. When you look, in lots of places it’s hard to tell where the river is or was.
Somehow I learned of the controversy about a similar situation with Oklahoma and the Red River. There is considerable acreage made and lost by the meandering of the river. As I recall, there are (or were) some big legal battles of the new/lost acreage