Two other people are in hospital in Invermere.
RCMP Const. Kris Clark said a mother and a friend took two young children for a float in a rubber dinghy down the Dutch Creek River.
He said the river was flowing quite quickly and the raft got hung up on some debris and flipped.
"The mother was able to get out with her three-month-old son, however she was unable to retrieve her one-year-old daughter. The girl went down the stream wearing her PFD, but was swept quickly downstream and eventually got hung up on, I believe, a sandbar."
All three were taken to hospital but the little girl was later flown to Calgary by STARS air ambulance.
Police say the baby is in stable condition.
The family is from Radium.