Cass Jack stumbled across the sack near the bank of the river while walking her dog in Alexandra on Tuesday.
On closer inspection she also found part of an animal stuck in a tree, animal skins on the bankside, dismembered limbs possibly from deer and a large carcass floating in the water.
It was her dog that brought the gruesome scene to her attention.
"He wouldn't come back.
"I went over to get him and thought, `Oh my God, that's disgusting'."
She initially thought the first carcass she saw was a piece of plastic. But, it was a dead animal that was wound around a tree that [ducks] were pecking at."
It appeared someone had pulled up to the bank and "chucked" the animals over the edge, she said.
Mrs Jack immediately told the Central Otago District Council (CODC), which agreed to clean the area up, she said.
The river's water quality was investigated last month following the Otago 4WD Club Superwinch Mainland Challenge, an event Mrs Jack took part in.
Mrs Jack, who is a member of the Central Otago 4WD Club, wants to keep it that way.
"This is where people come to swim," she said.
"What do people think will happen when they chuck things in the river?"
A district council spokeswoman confirmed it had been notified of the situation and a contractor was being arranged to clean up the carcasses on the bankside.
She said anything floating was the Otago Regional Council's responsibility.
