A Cal Fire spokeswoman said Friday afternoon that the fire was reported at 3:25 p.m. west of Highway 101 near Bradley around a dry Lake San Antonio riverbed.
The fire was spreading at a moderate rate, though smoke was traveling west, away from Highway 101.
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The terrain around the area is rugged and hard to access, Cal Fire said. Of the 15 engines at the scene, four are from San Luis Obispo Cal Fire. Also responding are four water tenders, five foot crews, two bulldozers, two air tankers and a helicopter.
It was not immediately known how much of the fire had been contained as of 5 p.m.
Cal Fire says that one lane of southbound Highway 101 has been closed to traffic and the closure will remain in place until at least 9 p.m. Northbound traffic is not being affected.