Portland police said 31-year-old Amanda Jo Stott-Smith of suburban Tualatin was taken into custody on the ninth floor of a downtown parking garage around 7 a.m. Saturday.
Portland police search on a dock near the Sellwood Bridge after a 3-year-old boy and 7-year-old girl were found in the Willamette River.
Stott-Smith was facing aggravated murder and attempted aggravated murder charges, Austria said. She is to be arraigned Tuesday afternoon.
A police spokesman said he did not know late Saturday whether Stott-Smith had contacted an attorney to represent her. Courts reopen Tuesday after the Memorial Day holiday.
Rescuers were unable to revive the boy, Eldon Jay Rebhan Smith, who drowned, according to an autopsy by the state medical examiner. The girl remained in an area hospital Saturday night, Austria said, and was expected to survive.
"She's doing well," the detective said. "She has the will to live."
Police did not release her name Saturday night and said they had not yet questioned her about the incident.
It was unclear whether the children were pushed or fell into the river, but Austria said it was certain they were on the Sellwood Bridge, one of the oldest bridges in the state.
Police were asking the public to call investigators if they saw Stott-Smith's car, a blue Audi sedan, on the bridge when residents of the area heard the children's screams and a massive search was launched.
A couple who live in the area and joined the search in their boat found the children in the darkness nearly an hour after screams that moved downstream with the current were heard.
"Had they not been awake at one in the morning," Austria said, "we would investigating a case here with no children."
Officers were called to the Sellwood Bridge area around 1:20 a.m. after the screams were reported by residents.
Upon arrival, officers also heard screams on the river but could not locate their source because of the early morning darkness, said Officer Greg Pashley, a Portland Police Bureau spokesman.
Pati and Dan Gallagher, another couple who live in a town home just east of a bridge, said they were sitting on their patio when they suddenly heard a single splash, silence, then screams. They called 911.
"At first it was, 'Help me!"' Pati Gallagher said of the girl's screams. "Then it went to anguishing horrible screams."
By 2:10 a.m., police said, the children were pulled from the water.
"It's tough to get the sounds out of my head - the screaming, the splash," Dan Gallagher said.
Sgt. Mike Marshman, a Portland Police Bureau spokesman, said officers remain in awe of the girl, whom he describes as "tiny for a 7-year-old," and how she was able to not only survive the long fall, but withstand more than 30 minutes in the cold water.
Portland Fire Bureau and Multnomah County River Patrol boats also responded to the scene with suburban Milwaukie police. The U.S. Coast Guard sent a helicopter to assist in the search.
"As you look down into the darkness it's hard to see anything," Pashley said. "I can't imagine what it must be like to be a child in this cold river."
Austria said information from family members and a missing persons report the father filed with Tualatin police helped authorities identify the children.
Authorities later said that the mother has a history of domestic violence and recently filed for separation from her husband, Jason Smith, according to Washington County Circuit Court records. The parents had joint custody of the children, Marshman said.