NAPA, Calif.—Standing on a bridge over the Napa River, 77-year-old Ging Chan watches the sun-flecked water slide lazily around a muddy curve and thinks of a forgotten past.

"This was Chinatown," he says, waving a hand toward a scrubby patch of land that once hummed to the rhythm of hundreds of laborers, empty now save for a sprinkling of bright orange poppies.

The workers who lived here left their signature in pickax marks chiseled into the rocky bones of wine country. But few people know about the Chinese laborers who helped lay the foundations of California viticulture, planting vineyards, bridging creeks and digging caves.

"No one knows that we were so much a part of the fabric that established the industry in California," says San Francisco wine merchant Raymond Fong.

For those who discover Napa's secret past, the results can be fascinating.

"I was surprised to see how many worked on tunnels and planted vineyards," says Cherise Chen Moueix, who with her husband, winemaker Christian Moueix, is hosting a dinner at their Dominus Estate winery honoring the contribution of the Chinese—one of the lead-up events to next month's Napa wine auction.

The history is of particular interest to Moueix, who is of Chinese descent, and one she'd like to share. "Maybe it will launch a lot of dialogue," she says.

Old newspaper articles and other 19th-century accounts show hundreds of Chinese workers in both Napa and Sonoma counties.

Many were farmers who brought their agricultural skills to the industry, helping establish vines and working in cellars. "There's more to this story. There's this whole human side of how the valley was developed," says Fong, who has researched the region's history.

A 1967 paper by a Napa school official on file at the Napa County Historical Society records that when rains turned the 1887 grape harvest into a muddy mess, keeping wagons out, Chinese workers waded in barefoot and hauled out the grapes.

But 19th-century Chinese in California faced fierce discrimination, including laws banning them from owning property and campaigns urging farmers not to hire them. In 1882, Congress passed an immigration ban on Chinese. Populations dwindled and rural Chinatowns disappeared as workers headed to cities.

Chan's family history dates to the 1860s, when his grandfather first came to Napa. Later, the family ran a general store that was a social center of Chinatown. The settlement was hit by two fires, but Chan's father, Shuck Chan, lived there until residents were moved out in 1930 to make way for a planned, but never built, yacht harbor.

The wine industry had its own problems by the '30s, as depression, vine disease and Prohibition took a toll that lasted until the revival of the 1960s.

"The thing that's remarkable is that by the time the wine industry here in Napa was born again, the Chinese were nowhere to be found," notes Hugh Davies, president of the Schramsberg winery in the Napa Valley that has some celebrated wine caves dug by Chinese workers.

Still, there are glimpses of what once was.

A plaque on the First Street bridge commemorates old Chinatown. And local historians are keeping memories alive with tours and exhibits.

Fire has destroyed the wooden markers identifying Chinese workers buried in Napa's Tulocay cemetery. But a few stone markers remain, etched in English and Mandarin.

Chan is glad to see renewed interest in the Chinese legacy and has been inspired to learn a bit more about his own family's role, something that didn't get talked about much when he was growing up.

"We weren't that curious about the past," he says. "You think back and you kind of wish you had been. There's a lot of history that's gone."