The third lane is expected to be reopened at 2:30 p.m.
The tanker crash was reported at about 1 a.m. on southbound Highway 101 near Tennant Avenue.
Cal Fire Battalion Chief Brandon Leitzke said that approximately 1,200 gallons of gasoline leaked out of the tanker.
Crews from Caltrans are in the process of removing the fuel remaining inside two tanks on the overturned truck before they can lift the vehicle back onto its wheels, Leitzke said.
The truck was lying on its side partially on the shoulder and partially in one of the southbound lanes of the highway and it will take Caltrans hours to extract the spilled gas, Leitzke said.
"They will be out there all day, just because of the cleanup work that Caltrans has to do," Leitzke said.
Rainfall complicated the cleanup because the water on the ground spread the spill and the flammable gas can still be lit and can burn on top of water, according to Leitzke.
Crews installed a dike to block the spill and are using absorbing pads to skim off the gas, which is floating on the surface of the water, Leitzke said.
Fumes from the spill also prompted the evacuation of some residents living near the highway, he said.
The evacuation was later lifted around 6 a.m. and the residents have been allowed to return home, Leitzke said.
He said Cal Fire has accompanied several local fire departments in responding to the spill.
There was no immediate estimate for when other two southbound lanes of the highway will be reopened, according to the CHP.