The accident occurred on Saturday night in the community of Saillon, near Martigny, where the woman lost control of the minivan she was driving alone at an intersection of two roads, cantonal police said.
For unknown reasons the motorist did not properly negotiate a curve in the road and the vehicle rolled into the Salentse River, a tributary of the Rhône.
The van flipped over the four-metre bank before landing upright in the river bed.
The woman was able to extricate herself from the vehicle unaided, police said.
She then walked along the bank but it appears that she fell over and dunked into the river.
A fisherman discovered her in the river bed at 10am on Sunday, police said.
Police and an ambulance with a doctor were called to the scene and the injured woman was transported to the Sion hospital.
Her life is not believed to be in danger.