Both Nicole Waring and the child died, and a search continued Sunday for Waring's 2½-year-old daughter.
"For whatever reason, it was a deliberate action," said Vermont State Police Capt. David Covell.
Waring, 40, was reported missing early Saturday after she disappeared from her parents' Wardsboro house with the two girls, the state police said.
As troopers began preparing a search party, State Police Sgt. Robert McCarthy spotted Waring and a child standing on the edge of Wardsboro Brook, about 100 yards from the parents' home, Covell said.
Normally placid, the brook was swollen with rain and snowmelt and had a swift current.
McCarthy tried to talk to Waring but she ignored him and walked into the water holding the child, police said.
She was standing near a rock in turbulent, waist-deep water when McCarthy reached for her, but she pushed away from the rock and eluded his grap, plunging into the brook with daughter Dakota in her arms before being swept away, Covell said.
"The sergeant tried to communicate with her. She didn't respond at all. He was in close enough proximity that she should've recognized his presence," Covell said.
The bodies of Waring and her 6-year-old were recovered downstream Saturday and pronounced dead.
A search for 2½-year-old, Grace, was suspended Saturday night and resumed Sunday, with police, firefighters and game wardens searching the brook's banks and nearby woods.
Waring had been despondent, but police wouldn't say why. "There appears to be, based on some interviews, a history of some issues present," Covell said.
Police have spoken with Waring's husband, who was in Massachusetts at the time, Covell said.