The first guess is the 8-foot-long whale, which often feed on fish in estuaries and the mouths of rivers, swam away from the ocean in search of food.
"What are the alternatives?" asked Link Olson, a curator at the University of Alaska Museum of the North.
It was highly unlikely that someone was perpetrating a hoax along a remote section of river with a whale carcass, he said.
"If you were ever close to a dead marine mammal, even for a few hours, you would know why no one in their right mind would do that."
Canoeists found the whale June 9 on the Tanana River about 40 miles southwest of Fairbanks.
Sylvia Brunner, a marine mammals researcher at the museum in Fairbanks, identified the decomposing carcass and oversaw its recovery on Wednesday.
The "bloated, black thing on the beach" was about 12 feet from the river's edge, she said.
It could have died in the river last fall and frozen during the winter, Brunner said. On the other hand, the whale could have entered the river this spring seeking fish heading for the ocean.
"When you get a carcass like that, there are a lot of unanswered questions," she said.
The carcass was taken to the museum, where it will be "cleaned and prepared as a full skull and skeleton and we will preserve tissue samples," Olson said.
Belugas are toothed whales and belong to the same group as sperm whales, killer whales, dolphins, and porpoises, according to the Alaska Department of Fish and Game.