The so-called Peralta was reported shortly after 9:30 p.m. in the Santa Ana River bed near Riverview Drive and Kern Drive, according to the Riverside county and city fire departments, which were in unified command.
No injuries or structural damage were reported.
The fire was 10% contained as of 3 a.m. and its forward-rate of progress was stopped, officials said. By 8 a.m. the containment level had risen to 30%.
The cause of the blaze is under investigation.
Meanwhile, a red flag warning of critical wildfire conditions will be in effect this afternoon into Saturday morning in the Los Angeles County mountains, with forecasters calling for dry and windy weather.
According to the National Weather Service, the warning will be in place from 1 p.m. Friday to 6 a.m. Saturday for a swath of the Los Angeles County mountains.