The border between the United States and Mexico is defined by a river-- the Rio Grande (the Big River), also known as the Rio Bravo del Norte-- the Wild River of the North-- and runs for almost 2,000 miles.
One of my pet peeves is when English speakers call this body of water the Rio Grande River-- they nearly always pronounce it the Rio Grand River, which is to call it the Big River River
Here are some photos of the Rio Grande.
These sites are plainly unsuitable for the installation of a border wall.
Even worse, the river moves, which could easily have the effect of the wall not matching the border.
In the past, I submit, a presidential candidate who was considering advocating a wall between the U.S. and Mexico )would have had a staffer do a little research (like what I've done here), looked at the photos and would have not advocated that project. Such statements don't bother the President; the truth is not in him.