Sharks aren't exactly freshwater fish. A specific kind is adaptable to both saltwater and freshwater of course. However, none live in lakes. At least I don't think so.
Yes, I know that they don't live in lakes necessarily, but if one swam up the Mississippi RIVER, then how did they not make it up to huge lakes, which more people would swim in?
It isn't a shark's goal to reach the end of it...
Not many sharks have swam up the rivers anyway.
I say it's unlikely that one would even reach the Iowa border.