The discovery came a day after the body of the woman’s 3-year-old grandson was recovered after he also drowned in the same area of Henry Hagg Lake, authorities said.
The family lived in nearby Hillsboro and had gone to the lake for a picnic on Monday.
It was not clear exactly what happened to the family, but law enforcement officials said they found no sign of foul play.
“We’re just considering this a tragic accident,” said Sgt. Bob Ray of the Washington County sheriff’s office.
Deputies identified the three bodies found Tuesday as those of Jova Ixtacua-Castano, 42, her daughter Gabriela Garcia-Ixtacua, 25, and son Michael Garcia-Ixtacua, 13.
The 3-year-old boy found Monday night was identified as Jeremy Scholl, the son of Gabriela Garcia-Ixtacua.
Because of its size of nearly 2 square miles, the lake has no lifeguards, but the sheriff’s office uses a vessel on weekends for patrols.
The three bodies were discovered about 40 feet offshore in 8 feet of water, and about 50 yards from the spot where the 3-year-old was found.
Authorities worried about the rest of the family after they recovered the boy’s body at the site Monday evening and found personal items nearby, including a beach towel, cooler, shoes, identification, as well as a small dog on a leash.
They also discovered that a vehicle left at the parking lot near the lake was owned by a family member. The family’s neighbors and relatives told officials the four had gone to the lake and had not been heard from since.
The lake is a busy recreation site, drawing 800,000 visitors a year — 6,000 a day on busy summer weekends, officials said. It also provides drinking water for Portland suburbs.
There have been previous drownings and near-drownings at Henry Hagg Lake, near the same area where the four family members were found.
That side of the lake can be dangerous because it has a steep drop-off formed by the channel of a former river bed running beneath the reservoir.
In 2012, eight children nearly drowned after wading into the lake and stepping from that underwater drop-off. They were saved by six people at a family reunion who heard the children yelling and formed a human chain to pull them to shore.