The crash happened around midnight Wednesday about seven miles south of Naselle on Highway 401 in Pacific County.
Troopers say 26-year-old Jessica Rath was on her way to pick up her husband, who is a fisherman. But the car ran off the road and struck a tree, killing the mother and injuring her 4- and 2-year-old daughters inside.
State police said it appears Rath fell asleep at the wheel.
It wasn't until hours later that a passer-by saw the marks on the road and then discovered the kids nearby. Both kids were taken to Columbia Memorial Hospital in Astoria, Ore. with serious injuries -- the 2-year-old with very serious injuries, troopers said. Both children were transferred to OHSU in Portland.
Investigators say the 4-year-old may have pulled the 2-year-old from the car and kept her warm with a blanket.
Troopers also praised the passer-by for the person's awareness in spotting the wreckage deep within a wooded area off the highway.
Photo courtesy: Washington State Patrol
"Full marks and Well done is all you can say to the person who took the time to find these baby girls at this crash," Trooper Russ Winger tweeted.
Both girls suffered broken bones. The 2-year-old was still in the hospital Wednesday night.
Winger said the girls could have easily died if they had been out there for any longer.
"It may be that they stayed down in there and got hypothermia very easily down there," he said. "They were out there for hours in the dark."
The crash is still under investigation.
KATU News reporter Emily Sinovic contributed from Portland, Ore.