Archaeologist Torben Rick watched with frustration as pounding surf clawed at one of North America's oldest homesteads, a massive heap of village foundations, cutting tools, beads and kitchen discards left behind over the last 13,000 years.

Here, seafaring tribal members cast fishing nets from canoes made of redwood planks, prepared dinners on stone griddles, and painstakingly chipped out tiny shell beads prized as currency.

But unless something is done, this rich trove of Native American history and several others on the island will almost certainly be destroyed by rising seas and strong storm surges along beaches that will soon no longer exist.

Rick, curator of anthropology at the Smithsonian's National Museum of Natural History, picked up a tiny pink bead.

"Things like this are golden because they can help us better understand the people who lived here and how they dealt with some of the same unstoppable forces we face today," he said. "The trouble is, a few more storms and all this valuable history will be washed out to sea."

So conservationists and archaeologists are fighting back. Half a dozen scientists armed with trowels, clipboards and global positioning devices fanned out across the island's headlands and rocky fingers earlier this month to take the first full accounting of archaeological sites heavily threatened by shoreline retreat and storm erosion.

The inventory will create baseline information to help guide conservation decisions at imperiled sites where human culture and island ecosystems have a shared history. It will also enable scientists to monitor the destructive forces of marine erosion, which are predicted to get worse.

The archaeological assessment is a collaborative effort by the Smithsonian Institution, the University of Oregon, Chumash tribal leaders and the Nature Conservancy, which manages 76% of the 96-square-mile island, the largest of eight Channel Islands, 36 miles off Ventura.

"The real tragedy, and the urgency, is that sea level rise is destroying wholesale the opportunity to learn about our past — information we can use to be better conservationists," said Scott Morrison, the Conservancy's director of conservation science. "We're trying to do something about that."

Dozens of sites, including former villages and workshops, were assigned one of five risk categories based on variables including elevation, distance to the nearest shoreline, coastal slope, soil erosion and precipitation rates.

Eight sites were designated "code red," meaning they contain significant archaeological resources and are in imminent danger of being destroyed by rising seas.

Among them is a remote cave where the team conducted an "emergency archaeological rescue" of artifacts entombed beneath thick layers of sand and driftwood: arrowheads, crude stone implements, rope and fabric woven out of sea grass.

Also recovered in the cave was a square piece of redwood smeared with brown tar, used to repair a hole in a canoe.

"The last time one of these planks was found was in the 1960s," said Jon M. Erlandson, an archaeologist at the University of Oregon and expert on Santa Cruz Island's cultural resources.

The plank is a reminder of a time when the coast teemed with fish, waterfowl and shellfish "and the shoreline was a lot further away than it is now," Erlandson said.

"There's not enough time to save everything," he said. "We're trying to record and salvage all we can before it's too late."

At a site near the cave, at least three Chumash house pits discovered within the last 40 years have been destroyed by the ocean.

The destruction eliminated the possibility that DNA and isotopic analyses could provide important information about changes in island culture and the landscape, Erlandson said.

Rising sea levels between 7,000 and 15,000 years ago submerged much of the evidence of the island's earliest occupants, seafarers who arrived when pygmy mammoths lumbered through inland forests at the end of the last ice age.

Their descendants flourished amid the rich marine environment. In 1542, explorer Juan Rodriguez Cabrillo counted several densely populated villages as he sailed past, and there were many more he didn't see.

The job of surveying evidence of their existence over centuries was complicated by the relatively large size of the island and the diversity of its largely roadless terrain: plunging valleys, conifer forests, steep mountains and wind-swept beaches.

Bringing a four-wheel-drive truck to an abrupt stop after wiggling along a rocky road for two hours, Rick said, "Let's go for a walk."

Moments later, he and Gil Unzueta, a Chumash Indian monitoring the survey effort, were striding toward the eroding remnants of an ancient settlement and refuse heap that was falling into the sea.

"We're standing on a living history book," Unzueta said. "And we're losing pages from it every day."

louis.sahagun@latimes.com

Twitter: @LouisSahagun