A Contra Costa County man was arrested for suspected drunken driving after a late Saturday afternoon motorcycle crash near Dillon Beach that resulted in the death of his girlfriend, a passenger on the bike, authorities said.

The 4:27 p.m. crash occurred on Dillon Beach Road near a landmark known as Elephant Rocks in northern Marin County, the CHP said.

Antioch resident Rick Dow, 53, was driving a new 2015 KTM 500 toward the coast when he lost control of the bike just north of Valley Ford Franklin School Road, CHP Sgt. Erik Egide said.

The bike overturned, and both Dow and his girlfriend were ejected, Egide said.

Napa County resident Rachel Moir, who was traveling inland from the beach and came upon the crash scene, said it appeared the motorcycle slid across the road and into a ditch, where the badly injured woman lay afterward, her helmet still on her head.

Moir, of American Canyon, said the driver was clearly in shock and complained of abdominal pain as bystanders tried to comfort him.

“He was pretty upset,” she said. “Clearly, he was not doing well.”

She said someone attempted to resuscitate the woman before paramedics took over, but they were unsuccessful and she died at the scene. The woman’s name was withheld pending notification of her family, authorities said.

Dow was taken to Santa Rosa Memorial Hospital and was arrested but released to medical personnel for treatment, Egide said.

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.