Witnesses reported that the child, later identified as Denson's 4-year-old daughter, Je'Hyrah Daniels, was screaming.
Shakayla Denson, 26, also faces aggravated child abuse and grand theft auto charges.
She was pronounced dead at St. Joseph's Hospital.
At around 4 p.m. calls started coming into police from witnesses who saw the mother park near the river then drag the girl by her arms from the auto into the water.
"Witnesses say she waded into the Hillsborough River with her child".
According to Dugan, the female suspect was walking a short distance from the scene when police located and arrested her. Her mother, Shakayla Denson, is facing a number of charges, including first-degree felony murder.
Police Chief Brian Dugan called the incident "completely tragic". At one time, both the child and the mother were screaming, according to officials.
Tampa Police officers responded to the scene at N. Rome Avenue and W. Aileen Street around 4:06 p.m. Thursday.
Denson, authorities say, first stole a vehicle from a local auto shop and then drove to the water with her daughter. Just before 4 p.m., witnesses saw her drag Daniels into the Hillsborough River by the arms and then pull her close to her chest. Witnesses were shaken up, the police chief said.
On Jan. 8, 2015, 5-year-old Pheobe was thrown off the Dick Misener Bridge into Tampa Bay by her father, John Jonchuck. He said it appears she drove to the bridge, but police couldn't release further details Thursday afternoon as they continue to investigate.
"Our diver, when he found her and he brings her lifeless body to shore.I walked up to the diver and said, 'How old do you think she was?' He looked at me and he said, 'She's the same age as my child, '" Dugan recalled.
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