BANGKOK — Every year when the moon is full and the rainy season draws to an end, Thailand’s waterways fill with millions of floating lotus-shaped lanterns — a symbolic, centuries-old gesture once meant to placate to the country’s goddess of water.

Today, many Thais still believe the candlelit boats launched during Loy Krathong can carry misfortune away with them, allowing past sins to be cleansed and life to begin anew.

This year, flood-ravaged Thailand has plenty of reason to pray for rebirth — and little reason to celebrate.

The festival, due Thursday, comes on the heels of a cataclysmic waterborne disaster that’s drowned one-third of the country in three months, killing 529 people and wiping out rice fields and factories and livelihoods along the way. The flooding is the worst in Thailand since World War II, and it’s not over yet. Damage so far is likely to exceed $6 billion. Recovery will take months.

“Most people don’t feel like celebrating this year — there’s been too much sadness and suffering,” said Saithong Sateankamsoragai, a Bangkok flower vendor who sells the tiny boats, called krathongs, that are an integral part of the
annual festival.

Saithong fled her own home late last month after chest-level water engulfed it. Now she lives with her sister in a drier part of the capital, a refugee forced to flee by the water this Southeast Asian kingdom is ironically paying tribute to.

Tragedy in mind, the Tourism Authority of Thailand has canceled all official celebrations in Bangkok, including those along the Chao Phraya river — the chocolate-colored waterway that snakes through the city of glittering condominiums and decrepit apartment blocks.

In recent weeks, the river’s banks have brimmed to record levels, forcing a halt to dinner cruises and fueling fears the mighty waterway could swamp downtown.

Outside the capital, in cities floodwaters have spared, festivities are going ahead. They include the northern town of Sukhothai, where the tradition is believed to have been born. Revelers there have already begun setting off fireworks this week, filling the skies with the spellbinding spectacle of balloon-like lanterns.

The mood in Bangkok, where many neighborhoods remain submerged, is far more subdued. The Culture Ministry is calling for revelers to float just one boat per family, or float them online through websites on which you can light digital candles and incense and watch yours float on a full-screen rendering of lake.

The Bangkok Metropolitan Authority, meanwhile, is urging people in flooded zones not to launch any at all.

Close to a million krathongs are typically set adrift annually in the capital alone, and there is concern they could trigger fires in abandoned homes or clog drains and canals critical to helping ease the massive pools of runoff bearing down on the metropolis of 9 million people.

Most krathongs are made from hardened, painted bread or ornately curled banana leafs filled with yellow marigold flowers and metallic-purple globe thistles. Some are built from environmentally unfriendly non-biodegradable plastic foam.

Thais joke they won’t have to go far from home to find water this year. “We probably can float the krathongs right in the house,” tweeted one.