An off-duty sheriff's deputy heard the couple calling for help. He was able to save the young woman, but the man was swept away.
The body of the victim, a Rancho Cucamonga man in his 20s, was found shortly after 7 p.m., Prater said.
A powerful storm cell packed with heavy rain and hail moved through, creating a quick-moving river runoff that trapped the young couple.
Parts of the Inland Empire saw flash-flood warnings Tuesday as Riverside and San Bernardino counties were pelted with hail.
A flash-flood warning were in effect for Riverside and San Bernardino counties for most of the day.
A thunderstorm producing heavy rain between Hesperia and Apple Valley caused flash flooding and mudslides.
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Residents were warned that excessive rainfall could cause flash flooding or mudslides, particularly within the Lake Fire burn area.
The Lake Fire, which erupted on June 17 in a remote area of the San Bernardino Mountains, burned over 30,000 acres.
The NWS said most flood-related deaths occur in automobiles and warned people not to attempt to cross water-covered bridges, dips or low-water crossings.