The incident happened near the O'Chiese First Nation, near Rocky Mountain House in central Alberta.
The 25-year-old woman was last seen on July 14, at the north end of the reserve.
She had been with a group of people driving on a dirt road when the vehicle broke down, said RCMP Const. Nick Munro. The occupants of the vehicle started walking home and the woman found herself alone on the road with a man she barely knew, he said.
That's when she was allegedly attacked by the man.
"She fled into the bush as a result of the altercation and became lost," he said. "She did get quite a severe blow to the head. She got lost and was just kind of wandering, trying to find her way out."
She survived on berries and river water, said Munro.
After she was reported missing, police started searching for her with the help of local residents, search and rescue crews and a helicopter. She didn't turn up.
An oil field worker found her walking along a road, 12 days after she had gone missing, about 10 kilometres from the place the vehicle broke down.
She is now recovering among family and friends, say police.
A 36-year-old man is charged with sexual assault, aggravated assault, and obstructing a peace officer.
He is set to appear in Rocky Mountain House provincial court on July 31.