In Santa Rosa, a woman on Porter Creek Road drifted into another lane. She tried to correct herself but lost control, flipping her car.
It hit another vehicle before slamming into the guardrail. Investigators said she would have likely survived but her seatbelt was only across her waist, not her shoulder.
A father and his 9-year-old son were in the other vehicle. They had minor injuries.
"Given the wet roads, she didn't have that good of traction. And that's what caused her to go sideways in front of the other vehicle when she tried to correct it real quick. So slow down in the wet weather, it definitely makes a difference," CHP Sgt. Brent Townsend said.
In Petaluma, investigators said a driver tried to drift going around a curve on D Street but lost control.
He slammed into a truck carrying a family. Both drivers had serious injuries.
Two passengers in the truck were also taken to the hospital.