Quick thinking by onlookers at San Onofre State Beach near San Diego, including the use of a surf leash by the people who came to the woman's rescue, arrested the bleeding caused by the shark's bite.
Thomas Williams, a 29-year-old who only recently received his EMT certification, saw several people carrying a bloody woman out of the water on a surfboard, and along with others nearby, leapt into action, applying a tourniquet to stop the bleeding.
"It was definitely to the point, her hamstring was gone," said Williams. "If she didn't receive immediate care, it was life-threatening."
"All of the back of her leg was kind of missing," he said.
The shark attack, which happened at 6:24 p.m., resulted in the woman being bitten along her "glute and down her thigh," according to state park employee Travis Lara, as reported by the Orange County Register.
The woman had been swimming, along with many others, in the warm waters of a popular family beach near the halfway point between Los Angeles and San Diego. Her current condition is unknown.
The second such attack in a year, San Onofre beach officials quickly shut down access to the ocean for a 72-hour period.
A woman heading to the beach that evening to watch the sunset was turned away by park rangers. "There's so many kids," she commented to the Orange County Register, "you don't really want to think about it when you're out in the water."
The injured woman was wearing swim fins and wading in shallow water when the attack occurred. The size and exact species of the shark that attacked her is not known, although there have been several other shark sightings in the area recently.
Two videos posted recently appear to depict sharks breaching near surfers at locations just north of Saturday's attack site, and a video showing a 16-foot great white shark taking chunks out of the corpse of a dead whale at Dana Point, some 23 miles north of Saturday's attack site, was posted last week.
About one year ago a swimmer was attacked by a shark, estimated to have been about ten feet long, about 32 miles north of San Onofre, suffering bite marks across her chest, down her hip and over a shoulder.