The riders, two adult women, were last seen leaving the Wild Horse campground Sunday at 10 a.m. Three dogs were with them, as well as their horses, according to the San Bernardino County Sheriff's Department.
They were due back to the campground about 4 p.m. and to return home by 9 p.m. When the riders had not returned home that night by 9:45, they were reported missing to the sheriff's department.
It is unclear whether Sunday's storms had anything to do with the women getting lost.
An immediate search began Sunday night and continued Monday. About 20 volunteers participated in a ground search and were assisted by the aviation division, department personnel on horseback and off-road vehicles.
The riders were located in the Mission Creek Preserve in Riverside County.