The driver lost control of the big rig just west of the 2 Freeway at about 1:50 a.m.
"The driver said that, for some reason, he allowed the vehicle to veer over onto the shoulder. The right side of the trailer caught the bridge railing. He said he veered it hard to the left and lost control, and he flipped it over onto its side," Sgt. Michael Munoz said.
The eastbound lanes were blocked and initially expected to reopen by 2 p.m., but progress was made in clearing the scene and three lanes were reopened shortly before 8 a.m.
The driver of the big rig declined to be taken to the hospital.
CHP is leading the investigation into the cause of the crash.