A 37-year-old man from Red Granite, Wisc., was driving the big rig southbound on Highway 99 in the north part of Bakersfield when he started coughing, the CHP said Tuesday.
The driver allowed the truck to veer across lanes and plow over guardrails, down an embankment and into an irrigation canal.
Highway traffic was backed up for hours as some lanes in both directions were temporarily closed.
The truck's driver sustained only minor injuries, the CHP said. His 32-year-old passenger, who was in the sleeper compartment, was killed.
The Kern County coroner's office identified the passenger Tuesday as Keith Michael Zeamer, of De Pere, Wisc. The CHP withheld the name of the driver.
The Kern County Fire Department said Zeamer's body was located under parts of the submerged cab.
The truck was hauling canned food, the fire department said.