Firefighters on Saturday extinguished a brush fire that burned 35 to 40 acres, officials said.
The blaze began about 2:30 p.m. in the Santa Ana River bed before spreading to Fairmount Park, The Press Enterprise reported.
The fire was 100 percent contained by 11:20 p.m., the Riverside Fire Department said on Facebook.
There were no reports of injuries or structural damage after some nearby structures were threatened by flames, the Fire Department said. No evacuations were announced.
Units from Cal Fire and the Corona Fire Department assisted Riverside firefighters at the scene, which is near residential neighborhoods.
Fire crews planned to remain in the area overnight due to the possibility of "significant flare-ups or re-kindles," the Riverside Fire Department said.
The cause of the blaze was under investigation.