The reason this is terra nullius is because the river has moved that land around for centuries, and will continue to do so. That border attempts to follow the river but the river doesn't follow diplomacy. These people who take advantage of this fact are very short sighted and mostly bored middle/upper class kids.
Maybe if a bunch of libertarians get together for mutual defense, elect leaders, set rules on how they'll pool their resources, they'll be able to prevent this from happening...
Are you giving a definition of libertarians or making a rhetorical point about the inadequacy of democracy in the short term for avoiding a crisis, or arguing against any form of government?