Social constructs. As was pointed out, it kind of misses the point. It isn't that some structures are natural, and some artificial, it's that we box things off for reasons. The Nile isn't about some objective feature of the world, and universities some social entity that is fundamentally different, it's that they do work. They have stakes. You don't know anything, or really care about anything to do with some objective feature of the Nile. The work it is doing in this context is meant as a contrast for a purpose to make a point (and the concept exists at different resolutions, with different associations for us all, none of which matter as long as we understand how it is functioning in the discourse). When I mention the river, I mean fish, perhaps a threat of drawning, the other side of town, that place I met you, the border between two locations, the picnic we're having later.
The whole thing about structuralism is that whenever we talk about something, there is a whole lot of motivation, intention, history, associations, and things all behind its mention, and the very way it is structured. Smaller brooks run into the river, and the river runs into a lake or something, and this matters, and is cut up the way it is for reasons.
Power tends to be a big one, but this is why post-structuralism tends to be so tightly tied into the humanities, and pyschoanalysis and whatnot.