PORTLAND — A mother accused in the death of her 4-year-old son, who fell 75 feet into the Willamette River early Saturday along with his older sister, has a history of domestic violence and recently filed for separation from the children’s father.

Amanda Jo Stott-Smith, 31, was taken into custody on the ninth floor of 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 daughter, who has yet to be identified, was taken to an area hospital after surviving more than a half-hour in the 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, who has witnessed people take their lives by jumping off the bridge south of downtown.

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.

There was no answer at a telephone number associated with Jason Smith on Sunday.

Police said the 911 recordings from the incident will be released midweek.

At about 1:20 a.m. Saturday, residents reported hearing screams.

When officers and rescue workers arrived, they tried to find the source of the screams but they bounced off the water, making the search in the early morning darkness even more difficult, Marshman said.

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

Pati Gallagher called 911, she said, while her husband and a woman out on a river trail yelled out, asking the girl to tell them her location.

“At first it was, ‘Help me,’” Pati Gallagher said of the girl’s screams. “Then it went to anguishing horrible screams.”

The operator on the other line could hear the girl, said Gallagher, who made three emergency calls by the time the incident ended.

From the couple’s perspective, it seemed as though the children splashed down closer to the river’s west bank, which is about a third of a mile from the Gallaghers’ home.

Then, the brother and sister began to float downstream.

By 2:10 a.m., police said, the children were pulled from the water and taken to authorities near the Oregon Yacht Club by an unidentified couple who joined the search in their boat.

“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.