Leeanne Ericson was rescued Saturday by a handful of Camp Pendleton beachgoers, including one who used a tourniquet fashioned from a surfboard leash to stanch the bleeding.
She was flown to Scripps Memorial Hospital in La Jolla, where hospital spokeswoman Janice Collins said she was critical condition but declined to release further details.
"All of the back of her leg was kind of missing," Thomas Williams, one of several witnesses who pulled the woman ashore, told the Orange County Register.
The shark attacked at a beach known as Church on the northern tip of the base.
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Ericson is a single mother of three young children, according to a GoFundMe page to raise money for her recovery.
The page, which had raised a little more than USD 23,000 by yesterday afternoon, was created by Christine McKnerney Leidle, who did not immediately respond to email messages. Members of Ericson's family could not be immediately reached. Authorities immediately shut down a three-mile stretch of shoreline extending from a mile north of Church to nearby San Onofre and San Clemente state beaches.
"If we do have additional sightings, aggressive shark behavior or something that warrants extending the closing beyond Wednesday we would certainly reassess, but right now we haven't had anything," said Todd Lewis, superintendent for the central sector of California State Parks, Orange Coast District.
It didn't appear anyone recorded video of the attack, and it wasn't known if the shark was a great white, although several have been seen in the area over the past year.
While shark sightings are not uncommon along the California coast, attacks are rare and fatalities even more so.
A shark chewed into her sides, ribs and liver but she recovered and swam in a triathlete competition in San Diego last October.