Police on Monday identified the girl as Tina Fontaine, who was in the care of Child and Family Services and was reported missing Aug 9.
Winnipeg police Sgt. John O'Donovan said Tina had only been in Winnipeg for a month, but was rebelling and had run away. Described as five-foot-three-inches tall and weighing only about 100 pounds, she was last seen in the city's downtown Aug. 8. She was wearing a white skirt, blue jacket and pink-and-white runners.
Her body was pulled from the river Sunday.
"At 15, I'm sure she didn't realize the danger that she was putting herself in," O'Donovan told a news conference.
"She's a child. This is a child that's been murdered. Society would be horrified if we found a litter of kittens or pups in the river in this condition. This is a child.
"Society should be horrified."
Thelma Favel and her husband had been caring for Tina for about 10 years. Favel is Tina's great aunt, but added Tina called her mama.
Favel had also cared for Tina's father, Eugene Fontaine, for many years, and when he was diagnosed with cancer, he asked her to look after his children.
"He asked me and my husband if we would look after his kids seeing that he lived with us for so long when he was younger, and he said he knew the kids would have a good home and food to eat all the time."
Favel said the last time she saw Tina was July 1, when she took her to Winnipeg to visit her biological mother. Tina had run away a few times starting last summer.
"She ran away twice and then she sort of settled down and she went back to school and she was doing so good," Favel said, sobbing.
Favel said Tina called her sister Sarah about a week and a half ago.
"The last time her sister heard from her, she told her, 'Tell mama and papa I love them, I miss them, but I'm not ready to go home yet."
No arrests have been made, O'Donovan said. Investigators are calling on anyone who saw Fontaine in the last 10 days, or who knows what happened to her, to contact police.
Fontaine, who had just turned 15, had a history of running away, but O'Donovan said police knew roughly where she went each time she was previously reported missing.
Police aren't saying how Fontaine died or whether she was sexually assaulted, but O'Donovan said she was already dead by the time her body was dumped in the river.
"The autopsy has been conducted and it's obvious that this child didn't put herself in the river in that condition," he said. "It's something that we're going to hold back on and we're not going to get into exactly how this girl died."
There are only a handful of people who know how Fontaine died, including those responsible for her death, he said.
"She's definitely been exploited and taken advantage of, murdered and put into the river in this condition," O'Donovan said. "Somebody may have seen her. We would like to talk to anybody who has seen her."
Last week, Manitoba unveiled a monument to almost 1,200 missing and murdered aboriginal women. The two-metre-high granite statue stands just by the junction of the Red and Assiniboine rivers in downtown Winnipeg.
In May, the RCMP issued a detailed statistical breakdown of 1,181 cases since 1980. The report said aboriginal women make up 4.3 per cent of the Canadian population, yet account for 16 per cent of female homicides and 11.3 per cent of missing women.
Also on Sunday, the body of a man was pulled from the river near a north-end park. That man has not been publicly identified, but police say the death is not suspicious.
Police have not identified the man, but several Winnipeg media outlets reported Monday night that a family member said the man was Faron Hall, also known as the "Homeless Hero."
Hall received national media attention for saving two people from the Red River on separate occasions in 2009.