The incident took place on the Foresthill Bridge in California - the highest bridge in the state.
A friend of the unnamed woman told KOVR that the woman and her friends 'were taking a picture on the bridge, and then the big bolts that are holding the beams together, she stepped on them kind of weirdly and lost balance and fell backwards.'
She fell around 20 metres onto a trail below, and suffered broken bones, gashes, and was knocked unconscious. She was airlifted to hospital.
After the incident police have begun to patrol the dangerous area under the bridge and say they will arrest any trespassers.