The mother was seen trying to save her 18-month-old from a flooded parking lot when they were swept away into a ditch, a Beaumont fire-rescue official said.
Surrounded by high water and unable to reach an ambulance, a good Samaritan with a truck helped bring the mother and child to the medical transport waiting for them at a nearby intersection.
A rescue crew was able to save the child, but her mother never regained consciousness, police said.
The mother was unresponsive and efforts to revive her failed. The woman then took her daughter, exited the auto and was swept about a half-mile away.
"Water was up to the trestle and first responders would not have been able to save the child if they had floated under it", police said in the statement.
First responders performed CPR until they could get her to an ambulance.
'The baby also had a backpack that was helping her float on her back and she was holding on to her mom, ' she added.
"But it's heart-wrenching to think of her growing up without her mother". Authorities said that the mother pulled into a parking lot, got out of the vehicle and grabbed her child.
Harvey has drawn comparisons with Hurricane Katrina, which devastated New Orleans 12 years ago, killing 1,800 people and causing an estimated $108 billion in damage.
Staff at the hospital said the child was in a stable condition.
On Tuesday, only hours after Sulcer was found dead, Riley told PEOPLE the devoted mother "absolutely saved the child's life".
At least 18 people have now been killed by Harvey since Friday, when it made landfall in Texas as a Category 4 hurricane. Harvey has since weakened to a tropical storm.
The New York Times reported Wednesday that the death toll from the storm had risen to 30 confirmed and suspected flood-related deaths. Hundreds of thousands of residents of Texas and Louisiana have been ordered to evacuate their homes.