River Country was at least partially fed with water from the lake and supposedly the water was deemed unsafe. I'm not clear on whether the water was treated at all but either way it was apparently not clean enough.
Also, River Country was an older and smaller park and charged less than the other two waterparks. Its main attraction was the "old swimming hole" and location. It was a very attractive park but low on the "thrill" factor.
There's been rumors that something might be created for the exclusive use of Fort Wilderness visitors (which would cause me to stay there!) but nothing's ever happened. We walked through a portion of it a little over a year ago and most structures were still there at that time so who knows?