All lakes (AFAIK) are connected to other water systems; there will be rivers going in and out, they just might be underground ones, so you don't see them.
well being form the land of 10000 lakes I think fluttermoth is right and also I think alittle travels through the soil and gets to the Mississippi River somehow. there are also some man made structures that take in water but that's more of an anti flooding thing from our harsh winters.