You are aware that the Rio Grande flows along the Texas/Mexico border right? It is the line that separates the 2 countries along that section of the border. And the wall will be built on our side of the river in Texas. So yeah it will wall off the entire Rio Grande to Americans that rely on that river.
Here's the thing, any wall that will be built will in effect be built on lands belonging to the United States. That strip of land on the opposite side of the wall (non-U.S. side) will have a portion of land that still belongs to the U.S., that is until... the wall is completed. At that point this country will have effectively ceded land to Mexico. Does anybody truly favor the idea of ceding one inch of the United States to Mexico?