The county coroner said Taylor Vejar of Rancho Cucamonga was hiking near a waterfall in Forest Falls on Monday when a boulder fell on her head.
She was pronounced dead at the scene.
Two others were hurt, including a teenage boy who broke a foot.
County fire officials said heavy rain the night before may have led to the rockslide.
Vejar's family has set up a memorial fund. Donations can be made to Bank of America Account Number 0056227203.