Shortly before 2 p.m., a brush fire was reported near Forest Falls and Mountain Home in the area of Highway 38 and Valley of the Falls Drive.
Authorities issued mandatory evacuations for the Forest Falls area as well as the surrounding trails as the fire grew to 1,000 acres by the evening hours. It was 0 percent contained.
Fire authorities said the fire, dubbed the Valley Fire, was spreading rapidly up steep hills and rough terrain amid temperatures in the triple digits.
As many as 500 firefighters were battling the blaze and all recreation sites, as well as trails were closed until further notice.
An evacuation center was created at the Inland Leaders Charter School, 12375 California St. in Yucaipa. Animals are not accepted at this shelter, but authorities are working to create one for pets.
Highway 38 was closed between Angelus Oaks and Lake Williams.
The cause of the fire was under investigation.