Giacomini's daughter, 57-year-old Ann Phillips of Petaluma, was driving from Olema to Petaluma for lunch with her father in a 2005 Nissan Altima when the collision occurred around 11:50 a.m., CHP Officer Curt Lubiszewski said.
Phillips fell asleep while driving on D Street near Windsor Avenue just south of Petaluma city limits, CHP Officer Jon Sloat said.
When she awoke, she swerved and ran off the road and down an embankment into a seasonal drainage ditch, Lubiszewski said.
The car overturned onto its roof and was wedged between both sides of the narrow ditch, trapping Phillips and her father inside, Lubiszewski said.
The ditch was dry except for the 20-by-12-foot pool of water where the Nissan landed, Lubiszewski said.
Phillips managed to free her arm from her seatbelt and keep her head above the water in the car, but Giacomini was under three feet of water for at least 15 minutes and drowned, Lubiszewski said.
Phillips suffered minor injuries and was taken to Santa Rosa Memorial Hospital. An autopsy on Giacomini is scheduled for Friday.
Sloat said drugs or alcohol are not believed to have been factors in the crash.
The Giacomini family is well known in west Marin County for its ranching and dairy businesses, Sloat said.
Toby Giacomini, Ralph's brother, left behind a trucking business worth millions of dollars when he died at age 88 in July 2007, according to his obituary in the Point Reyes Light.