The mother of the CEO of Uber, the transportation network company, died in a boat accident Friday evening in California, the company said.

Bonnie Kalanick, 71, died after the boat she and her husband, Donald, 78, were riding hit a rock in Pine Flat Lake in the eastern part of Fresno County, authorities said.

They are the parents of Travis Kalanick, 40, who founded Uber in 2009. The ride-hailing company has grown to become an international operation and a pioneer in the sharing economy with a market value of nearly $70 billion.

In a memo to Uber staff, Liane Hornsey, the chief human resources officer, called the incident an "unthinkable tragedy." She wrote that "everyone in the Uber family knows how incredibly close Travis is to his parents."

About 5 p.m. Friday, officers were called to the scene of the accident and found a man and woman on a shore of the lake, the Fresno County Sheriff's office said in a statement.

The woman died at the scene, and the man suffered moderate injuries, the sheriff's office said. He told officers the boat had sunk.

Uber identified the couple as the Kalanicks. The couple have been longtime boaters.

An autopsy of the woman is planned, the office said.

Donald Kalanick is being treated at a hospital and is in stable condition, the company said.

Crews were trying to remove the boat from the lake Saturday, the sheriff's office said.

The Associated Press contributed to this report.