A mother accused of throwing her two children into the Willamette River — killing her 4-year-old son and injuring his older sister — has a history of domestic violence and recently filed for separation from the children's father, police said.

Amanda Jo Stott-Smith, 31, was taken into custody at a downtown parking garage Saturday morning — some six hours after her children's screams were first heard on the river.

She had threatened to jump off a ledge when officers arrived, police said.

A state medical examiner determined that Stott-Smith's son, Eldon Jay Rebhan Smith, drowned. Her 7-year-old daughter, who has yet to be identified, was hospitalized after surviving a fall of 75 feet and more than a half-hour in the cold water. Police said Sunday afternoon that the girl's condition was improving and she had spoken with officers.

"How she's not dead is a miracle," said Sgt. Mike Marshman, a Portland Police Bureau spokesman.

Stott-Smith faces aggravated murder and attempted aggravated murder charges. She is to be arraigned Tuesday afternoon.

Police have not yet released a motive. It's unclear if the woman has an attorney.

Stott-Smith filed for separation from her husband, Jason Smith, in March, according to Washington County Circuit Court records. The parents had joint custody of the children, Marshman said.

Court records show Stott-Smith had been charged with domestic assault in 2000.

Early Saturday, the father filed a missing persons report in suburban Tualatin that helped authorities identify the children, Sgt. Detective Rich Austria said.

Early Saturday morning, residents reported hearing screams on the river. When officers and rescue workers arrived, they tried to find the source of the screams in the early morning darkness.

Pati and Dan Gallagher, both 50, live in a town home just east of a bridge. They were sitting on their patio, they said, when they suddenly heard a single splash, silence, then screams. They called the police.

"At first it was, 'Help me!"' Pati Gallagher said of the girl's screams. "Then it went to anguishing horrible screams."

About an hour later, 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.

Marshman said police 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.