The so-called Bain Fire was reported at 11:22 a.m. Tuesday on the north side of the dry channel, in the area of Bain Street and Limonite Avenue, just north of the Hidden Valley Nature Center, according to the Riverside County Fire Department.
The agency said that multiple engine and hand crews from the county and Riverside Fire Department were sent to the location and encountered flames moving at a moderate rate through thick brush.
Evacuation orders were issued in Riverside for the area of Arlington Avenue, between Crestview and Drive and Western Avenue, as well as Sandy Lane, Sunnyvale Drive, Valley Drive, and Fairhaven Drive, a fire department spokesperson said.
Two Cal Fire air tankers and two water-dropping helicopters were summoned to the location. At least one plane was seen dropping Phos-Chek on the flames.
There was no word on what might have triggered the brusher. The river bottom is teeming with homeless encampments, and debris, cooking and warming fires occur year-round.
City News Service contributed to this report.