I don’t think this was ever explained in the movie. If it was, I didn’t hear it because I muted the sound whenever Kevin Costner spoke.
Why was the pond near the base filled with animal carcasses?
I wanna say maybe he said they’d killed all the cattle as part of the defense of the fort?
Or maybe the savages killed the cattle as part of the onslaught?
-watch the movie and pay attention, his voice isn’t so bad, at least he’s not trying to do the brittish accent like in robin hood!
I don’t think it was filled with animal carcasses; it was one deer or elk carcass, and the rest was just a bunch of crap that had accumulated.
It was pollution. It showed how the white men did not live in harmony with nature.
I think the animals died from the gases emitted by the alien spacecraft that later appeared over Devil’s Tower.
the book actually has backstory concerning this… but the short answer is that the troops at the fort were basically in a state of mutiny, and were fending for themselves… there had been no resupply in a long, long time (as I recall the skeletons that Dunbar and the freight guy found were the resupply). They ate their horses, they lived in holes dug into the hillside (hence the state of that hill), and morale and discipline were low enough that they were not particularly tidy. In the final days before they basically deserted and started walking back toward civilization they used that area as a dump.
This is all from my memory and it has been years since I read that book… but the basic point is correct.
The DVD has a deleted scene that makes it clear that the animals had been shot for sport.
Isn’t the name of the book/movie Dances with Wolves?
huh! I didn’t even notive! but yeah!
Actually in the new DVD release there is additional scenes of John Dunbar cleaning out this area and there are several animals (deer/elk) in the pond. The original “Dances with Wolves” really only shows him pulling out one carcass (the antlers were sticking out of the water) but with the extended scenes it shows him pulling out several more and the sled is full by the time he’s finished.
For the record: that’s the new extended DVD that was just released this past July.
nitpick alert!
Or how about Dances with Wolves? 
(that is to say, the OP got the “with” correct, but messed up by calling it "Dancing with Wolves)
Thanks for the replies, folks. And sorry for messing up the title.