I heard a radio report that the barrier is blocking navigation along the river. No idea if that’s actually the case or not, but if there is any sort of regular boat traffic (even just police boats etc) along there then that has the potential to be a big deal.
There are both international and national regulations pertaining to navigation, intended to ensure uniformity of those regulations no matter what body of water you’re on. The international regulations apply along coastal waters, while the US regulations apply to inland lakes, rivers, etc. I’m not sure which would apply here given the Rio Grande river is also an international border. But which one applies may have to do with whether the state/governor can legally overrule them in the name of “security”.