It happened in the Riverstone area, where deputies say witnesses called 911.
According to dispatch audio, the bystanders also reported the woman, identified as 36-year-old Aisee Mwembo, was screaming.
When deputies and the Missouri City Fire Department arrived, they found Mwembo's car underwater.
She was originally taken to Memorial Hermann Southwest Hospital before being transferred to Memorial Hermann in the Texas Medical Center.
Authorities believe the fog may have caused the woman to crash.
"It's quite possible with the fog," Sgt. Danny Beckworth with the Fort Bend County Sheriff's Office said. "We did learn she works nearby and just started. It might have been the fact that she turned somewhere and she didn't realize where she was."
A dive team was called out to the scene to recover the woman's car and to check if any other passengers were inside. The woman was the only person in the car and an investigation is ongoing.