There are many areas where a wall isn’t feasible, and may not even be the most effective means of protecting the border. And you are right, Trump has mentioned this, and others for a LONG time have been talking about it.
What bothers me is the media hyping this, and knowing that many people that voted for Trump based mostly on his talk of a “wall” will get pissed at Trump.
“There are many areas where a wall isn’t feasible”
I have seen some areas of the Rio Grande around the Big bend area towards Presidio that a wall is not needed.