23-year old Nicholas Lovato of Menifee was hiking with four other people into the Santa Margarita Ecological Reserve.
His companions say he was sitting on a rock outcropping when it suddenly gave way.
Lovato fell about 30 feet into a river bed and was struck by a falling boulder.
Officials say he died instantly.
The 4,300-acre wilderness reserve is managed by San Diego State University and is off limits to the public.