The California Highway Patrol first received reports of the crash shortly after 6:30 a.m. Saturday on Highway 20 about four miles west of Walker Ridge Road, east of Clearlake Oaks.
The semi tank truck had gone over the size and initially was not visible from the roadway, the CHP reported.
When authorities accessed the truck, they confirmed a fatality, shortly before 7:30 a.m., according to reports from the scene.
CHP separately issued a notification just before 8 a.m. confirming a fatality, and reporting that Highway 20 west of Walker Ridge Road was open but impacted by emergency traffic as work at the crash site continued. The agency asked drivers at that time to use caution or take an alternate route of travel.
The early scene reports stated that the tanker appeared to be carrying a load of milk and that the crash resulted in milk going into Grizzly Creek.
Reports also estimated that up to 100 gallons of diesel fuel could have been spilled had the truck’s fuel tanks been full.
Fish and Game and the Office of Emergency Services were requested due to the hazmat concerns, according to CHP reports.
The recovery efforts required heavy equipment, with two wreckers responding but other assistance needed to remove the fuel.
The CHP also reported challenges with recovery due to the crash being on a blind curve on the highway.
The work to remove the semi continued into the afternoon, the CHP said.
Additional information will be published as it becomes available.