A 38-year-old man helping firefighters battling flames in the hills above the Napa Valley died Monday morning when his large water truck left the Oakville Grade and plunged down a ravine, according to the CHP.

The man, whose name wasn’t released as of Monday evening, had delivered a load of water up on the hill for the Nuns Fire fight and was headed down winding Oakville Grade to get more, said Napa CHP Officer Marc Renspurger.

It was about 6:45 a.m., not long before sunrise. The truck was near Walnut Drive, not far from the bottom of the hill, when it left the roadway, broke through a guardrail and dropped about 20 feet, Renspurger said.

“He was going to refill. For some reason he lost control and went off the road,” Renspurger said. The man died in the crash.

The driver worked for a private company contracting with Cal Fire to help on the regional fires. Such contracts are common as the state seeks help in fighting big blazes. The CHP didn’t release the name of the company.

As the vehicle, a 1993 Kenworth water tanker, rolled off the roadway, the tank detached from the rig. Efforts to pull the two pieces up the hill and onto the roadway still were underway late Monday afternoon.

“I’m sorry for the loss of the gentleman driving the water tender. He was helping fight the fires.” CHP Capt. Mike Palacio.

CHP investigators are examining the cause. The man’s speed wasn’t known.

There were no other vehicles on the road as the notoriously steep and curving route between Oakville and Glen Ellen has been closed at Highway 29 in Oakville since the fire began more than a week ago.

The Nuns fire started on the Sonoma Valley side of the Mayacamas mountains and flames have crested the top and been burning down into Napa County.

Numerous people Monday morning were in the area to watch the firefighting activity.

Monday’s death marks the 41st fatality from the Northern California fires, which have been burning for eight days.

You can reach Staff Writer Randi Rossmann at 707‑521-5412 or randi.rossmann@pressdemocrat.com. On Twitter@rossmannreport.