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.
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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.