Authorities have not identified the woman but said they are trying to contact relatives.
The child was believed to be with a woman that, police said, was her mother.
The chief said they quickly found Denson walking southbound on Rome Avenue and she was taken into custody.
Fresno mother drowned her 2 children before hanging herselfShakayla Denson, 26, was arrested and charged with first-degree felony murder and aggravated child abuse in the death of 4-year-old Je'Hyrah Daniels.
In a press conference Friday, Chief of Police Brian Dugan called the incident tragic and unbelievable while giving an update on the investigation.More news: Police in Missouri respond to possible Mollie Tibbetts sighting
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"I live in the building across and I was out on the balcony and I first looked to see if therew as a boat, like perhaps this person's boat had broken down", said Vicki Walker.
Police allege Denson, 26, had just stolen a Nissan Altima from an auto fix shop. Benson told investigators her daughter was in elementary school and she was trying to get her into therapy and enrollment in an autism center.
"The whole thing is just unbelievable", said Dugan. She was about 75 feet from shore.
WFLA reported that Denson then allegedly grabbed her daughter with both arms and pulled her tight against her chest as she walked deeper into the river and let the girl go. "I loved her with every bone in my body", the report said.
"Our hearts hurt for the loss of Je'Hyrah Daniels", Sheriff's Office spokesman Danny Alvarez stated. They were by themselves in the river so I don't know why I took a picture.
On Jan. 8, 2015, 5-year-old Pheobe was thrown off the Dick Misener Bridge into Tampa Bay by her father, John Jonchuck.