While preservationists swung into action as word spread the historic Bridge Street bridge might be endangered after the state declared it “severely deficient,” few appear to know how the span over the Saugatuck River actually functions.

But a few boaters know — even if many town officials who were asked do not — that simply calling a phone number posted on the side of the William F. Cribari Memorial Bridge, as it’s formally known, can trigger the labor-intensive opening of the metal swing span within about two hours.

“If someone wants it opened, they need to give at least two hours advance notice and call the number posted on the bridge, which is 203-866-7691,” according to Judd Everhart, spokesman for the state Department of Transportation, which owns and operates the bridge carrying Route 136 over the river.

Opening the bridge is no easy feat, starting with the fact that the phone number is difficult to view, except from the water below. The bridge opening itself requires at least five professionals — two police officers and at least three state Department of Transportation employees — to travel to the scene to physically activate the operation and maintain traffic safety.

“That phone rings in Norwalk and a DOT crew of three is dispatched to Westport to open the bridge … Westport police provide traffic protection during the opening and closing,” Everhart said. No fee is charged to the boater who passes through the opened span.

Motorists, meanwhile, may have to pay in terms of time lost, because when the bridge is open, traffic is diverted around the span back to the Post Road by way of Riverside and Imperial avenues, respectively, along the west and east banks of the river.

“The opening is a manual process,” Everhart said, though the bridge swings open by an electronic motor and takes roughly 10 to 15 minutes. Once upon a time, however, before the bridge was reconditioned in the 1980s, the original structure had to be opened with a hand-turned crank by four DOT workers and could take 45 minutes or more.

Statistics provided by the state indicate the complicated process of opening the Bridge Street bridge is not an everyday event. Over the last few years, according to the state, the bridge was opened 25 times in 2011, 18 times in 2012, nine times in 2013, 22 in 2014 and 19 so far this year.

“This is the oldest swing bridge in the country,” Everhart said, “built in 1870,” though local historians put the year at 1884. “Some of the original parts are still in use,” he said.

“It is 287 feet long,” he said, noting it’s one of only nine moveable bridges in Connecticut.

“We are now preparing a ‘Rehab Study Report’ to determine the scope of the rehab work necessary,” he said, which isn’t news to many Westporters. In fact, controversy over the future of the bridge began brewing as word spread that the state — after identifying a series of serious problems — is studying whether it should be overhauled or replaced.

“This bridge was last rehabbed in the 1980s,” Everhart said, when it was electronically mechanized. While the mechanics underneath were updated at the time, the distinctive metal superstructure atop, which at this point is decorative, was left in place.

According to state statute, hours for having the bridge opened are restricted, including during key rush hour times on weekdays. Also, more notice must be given at certain times, including during winter and nighttime, which requires 24-hour notice.