The crash happened at about 2 a.m. Wednesday outside of Naselle, KING-TV, Seattle, reported.
The 26-year-old woman, from Astoria, Ore., and her two daughters, ages 2 and 4, were on their way to pick up the woman's husband in Westport, Wash., the Washington State Patrol said.
At about midnight the woman spoke to her husband, who informed her he was going to be coming into Portland. He also told just head home to Astoria, however, she apparently decided to drive to Portland.
At about 2 a.m., the woman's car ran off the road and into a tree in a very isolated area, police said. The woman likely died immediately.
The 4-year-old managed to get herself and the 2-year-old out of the car and covered both of them with a blanket. They both suffered injuries, police said.
The girls weren't discovered until about 8:30 a.m. when a passerby noticed signs of a crash.
"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 said via Twitter. "Most people would not have been so observant."