Just to add another possible thought to the mix:
In the olden days, you used to be able to swim in Bay Lake (the lake that runs from the Grand Floridian back to River Country). However, it was discovered that bacteria was growing in the lake, and instead of the massive amounts of money it would cause to constantly be treating the massive amount of water, it was decided that swimming would no longer be allowed. That same water was used in River Country (as a section of Bay Lake was what many of the slides emptied into).
Now, whether this was the reason River Country closed, I don't know. This was just a rumor I heard floating around.
As I side note, I used to love River Country. I remember my favortite thing was the raft ride. There were little pockets that you could get stuck in while you were on your tube and you had to wait for someone to come and knock you out. Plus there was a hill at the end that dumped you back into the lake that I always hoped that I would be able to go down backwards (granted, I was 10 at the time, so who knows if I would think it was exciting now?).
RyMickey