Elk Valley RCMP say the crash occurred at about 2:30 p.m. Sunday, eight kilometres west of Fernie, as the family of five was travelling back to Alberta on Highway 3 in slushy conditions.
The woman killed in a van crash near Fernie has been identified as Kunthea Altvater of Coaldale, Alta., CTV News reported.
The white Chevrolet minivan veered out of control on a straight stretch of highway and plunged upside down into the frigid water.
A man and woman in a passing car managed to pull all five people from the van and other witnesses performed CPR on the woman, but she could not be revived and was pronounced dead at the Fernie hospital.
Two of the children are also severely injured and have been flown to Alberta Children's Hospital for treatment, while the father, remaining child and the male rescuer were treated for hypothermia and other minor injuries.
An investigation of the crash is underway but police say the minivan driver may have been attempting to pass another vehicle when he lost control in the slushy accumulation in the centre of the highway.