Dozens of police, firefighters and rescue workers converged on the bridge a half-mile north of downtown Vallejo, the Chronicle said.
Vallejo resident Albert Wong said he saw sleeping bags, backpacks, clothing and other items being removed from the cave.
"It was a lot of stuff," he said.
"It's really common to have homeless encampments under bridges but this is the first time I've seen something like this," Walker said.
Construction workers last week began rebuilding a decades-old two-lane bridge that serves Mare Island and hope to have the new span finished by year's end, Walker said.
Mare Island is about 37km northeast of San Francisco.
- AP