Firefighters Thursday evening were battling a brush fire in the bed of the Ventura River that's grown to 17 acres, and crews were working overnight to prevent the blaze from spreading into nearby oilfields.
As of 9 p.m., the fire was about 35% contained.
The fire began just before 4 p.m. in the 5300 block of Ventura Avenue in Ventura, said county Fire Department spokesman Capt. Mike Lindbery.
The river bottom is typically occupied by a sizable homeless population, many of whom have been displaced by the fire.
No injuries have been reported, and no structures have been damaged.
Fueling the blaze are the mix of plants, including bamboo, sumac and grasses, Lindberry said. The bamboo in particular, he said, has burned "like a torch," sending flames 20 to 30 feet in the air and producing significant smoke.
The nearby Brooks Institute evacuated its 150 students and staff members as the fire spread in the late afternoon.
About 100 firefighters were working overnight, prompted by worries that northerly winds could send flames dangerously close to key power lines and a nearby oil field, Lindbery said.
"We're planning on being here most of Friday to make sure we have this stopped," Lindbery said.
Investigators believe the fire was caused by humans but have not determined exactly what sparked it, Lindbery said.