Authorities were working to extinguish flames that erupted near the San Fe Dam Recreation Area in Irwindale on Wednesday.
The blaze grew to cover 35 acres by 8:40 p.m. after breaking out about two hours before, the Los Angeles County Fire Department said. It was being dubbed the Arrow Fire.
It was about 50 percent contained, and officials said forward progress had halted around 7 p.m.
Motorists were asked to avoid the area as the fire was burning south of the 210 Freeway and east of the 605 Freeway, sending heavy smoke was billowing north across the 210 toward the foothills.
Firefighters were requesting the Metro Gold Line be shut down because of the thick smoke. In a tweet, Metro officials said service was not interrupted and the Irwindale station not impacted.
No structures were immediately threatened, and no evacuations had been ordered.
However, it was burning near a MillerCoors brewing facility that sits at the recreation area's northeast corner. The location employs around 510 people, according to the company's website.
Sixteen salvage flat-bed trailers did burn, the Fire Department said. It was unclear whether they were connected to the brewing operation.
Aerial video showed fire crews attacking small patches of flames throughout the undeveloped area from the air and on the ground.
Firefighters would remain on scene into the night to continue mopping up flames and monitoring hot spots, officials said.