By law, the US & Mexico cannot build any structure that disrupts the Rio Grandeโs flow. https://t.co/9ZzdPBm42Q pic.twitter.com/YkvcHGD65T
โ AS/COA Online (@ASCOA) September 19, 2016
At the link, there is a fabulous zoomy-map-thing to look at some of the difficulties of creating a barrier at the border.