The driver told authorities the truck overturned after his load suddenly shifted.
The accident caused the tanks to rupture, spilling an estimated 4000 gallons of fuel on to the roadway and into a nearby creek.
"There is an ongoing leak that has been reduced down about one gallon a minute. That fuel is being captured, it is no longer working its way into the creek," said Eric Sherwin with San Bernardino County Fire.
The accident site has been declared an environmental hazard and crime scene. Authorities are concerned about possible contamination.
"The Santa Ana Water Shed represents one of Southern California's significant sources of drinking water, so all the agencies are taking this tanker spill very serious," said John Miller with the U.S. Forest Service.
The San Bernardino Flood Control District, which operates Seven Oaks Dam, has been notified and the dam has been shut down in order to pull the contaminated water. Two hydroelectric plants are also out of operation. Authorities say the tanker had the capacity to carry 8500 gallons of fuel, though it was unclear exactly how much fuel spilled into the creek.
By 5 p.m., authorities said the fuel inside the overturned tanker had been removed and hauled off in another tanker. Their next focus turned to getting the overturned tanker upright.
The spill has also shut down a 3-mile stretch of Highway 38 between Valley of the Falls Drive and Jenks Lake Road West, beginning in Angelus Oaks and ending in Cedar Falls. The closure could last for a week or more. Barton Flats Recreation Area is still open, but the hard closure begins in the Cedar Falls area.
Drivers can take Highway 330 or Highway 18 to get up and down the moutain.
For Jennifer and Brian Bazzel of San Diego, their weekend getaway to Big Bear turned into a longer ride than expected.
"We have twins and a 3 1/2-year-old in the car so that might be a bit of a challenge on the way," Jennifer Bazzel said.
Cars, trucks and motorcycles were turned away. Russ Sanders and his motorcycle club had hoped to checkout some cabins in the area.
"We were told at Forest Falls, they stopped us and said that we could not go any farther than Angelus Oaks Restaurant," he said.