The poor Discovery Riverboats of AK. My brother was an original boat driver that helped open Animal Kingdom. They changed a lot from not having animal trainers on board to having them, to only go half way around the river to going all the way around. They also stopped the fire breathing rock thing and other changes happened at the beginning. Then they repainted them and gave the CM's different costumes and called them something like Radio Disney boats. Then they vanished.
I thought the boats were gone before 9/11 as far as I can remember, they sure didn't last long that's for sure. My brother only worked through the summer of 98, so I don't know why they closed, but I did hear that they got a lot of guest complaints due to the long wait for such a boring ride (since you didn't really see many animals on it). Also I don't think Eisner liked them.
I wish they did bring them back and would clean up the river. It looks sad without them going around the river and when I was there last summer the river was getting over grown with algae and plants.