One of two main support beams underneath the 120-foot bridge has broken, creating a sag that in turn has triggered a similar break in a roof beam above.
State Sen. Joe Benning, R-Caledonia, is one of those raising alarm about the condition of the now-closed bridge. If it were to collapse, he said, it "would probably dam up the river" and "send the water right down Main Street" into the village of Lyndonville.
Eric Paris, owner of the nearby Freighthouse restaurant and president of the local historical society, agreed that a bridge collapse could create a "huge problem. It would be a huge cleanup."
Jeanne Elliott, who with her husband, Arthur, owns the bridge, said surmising what might happen if the bridge collapses is "pure speculation."
"It certainly would be removed before it caused any flooding," she said.
The nearly 150-year-old Sanborn Covered Bridge has drawn interest from history buffs who say it's rare in at least two respects: It's privately owned, while most of the estimated 750 covered bridges around the country are in municipal or county control. It's also of a design developed by famed 19th-century New Hampshire bridge builder Peter Paddleford.
A team from the National Society for the Preservation of Covered Bridges is due in Lyndonville on Sunday to evaluate the condition of the bridge and develop recommendations on whether and how it can be preserved. The Elliotts have said they don't have the money - estimated in the hundreds of thousands of dollars - to fix it themselves.
Officials from several state agencies said the bridge and its problems were not in their jurisdiction.
The Agency of Transportation likely wouldn't step in unless the covered bridge fell into the Passumpsic and the river backed up and began threatening a state highway bridge just upstream, said Shauna Clifford, a project manager based in St. Johnsbury.
The Agency of Natural Resources might have to issue a permit for work in the stream to repair the bridge but wouldn't initiate a project to fix it, said Mike Kline, river program manager.
And Mark Bosma, spokesman for the state Division of Emergency Management and Homeland Security, said his agency would respond in the event of flooding.
"While we're concerned about any scenario that could potentially pose a threat to personal property or safety, we haven't been contacted by anyone with a concern," he said.