have you ever driven the border to see that a wall is for miles and miles impossible, especially in the middle of the Rio Grande?
I live in Texas, that fallacy - that you can’t build a fence along our border, due to our terrain/river - has been disputed, time after time, long ago.
Besides, in the few areas you might not want a fence/wall/barrier, you have other means to man/protect/secure.
You’ve seen how this has been done in other countries (Israel, etc), so I won’t post redundant info on that.