This will not stem any amount of illegal immigration. Most illegal immigration happens when the person legally visits the US bit does not leave at the end of their specified stay duration. Boats by ocean and planes by air will still be making it past that wall no problem. The environmental impact of a border wall is too great and the cost is too high for something so ineffective and unnecessary. It would be, at best, a security blanket for those in states that are unlikely to ever be personally involved with illegal immigration.