This sounds like more of a personal question to be answer - sorry
Answer:
It is correlative!
Explanation:
This conclusion can be drawn since correlative sentences are like "tag-team conjunctions." They working in pairs to join phrases or words that carry equal importance within a sentence.
I believe it's unknown since the Mississippi is the predicate and unknown is a describing word, a.k.a. an adjective but I'm not sure
Good luck, hope I helped!