Houston RCMP was called to a car accident on near the 23 km mark on Morice River FSR around 5:30 AM on Tuesday.
Police say a truck left the road and rolled down the embankment into the river.
First responders rescued the female passenger, but the male driver remains missing.
Search and Rescue crews are still in the area looking for the driver.
The cause of the incident is under investigation and RCMP say that alcohol is considered a factor.
Police have not released any names at this time.