MONTPELIER, Vt. — For more than 50 years the Army Corps of Engineers has monitored the flow of the Black River through the North Springfield dam.

The dam was built in the late 1950s to help reduce downstream flooding, and over the years it has prevented flood damage, but probably never more so than last summer.

After Tropical Storm Irene dumped inches and inches of rain on Vermont, some of the communities up the Black River from North Springfield, including Cavendish and Ludlow, had heavy damage. Springfield, about three miles down the Black River, escaped almost unscathed.

The reason for the difference was that the water stored behind the dam was released slowly, helping to prevent flooding downstream. Without the dam, the flooding would have been 10 times more severe, said Greg Hanlon, an engineer with the Army Corps’ reservoir regulation section in Hanover, N.H., who monitored the North Springfield dam during Irene.

“If the North Springfield dam wasn’t there, Springfield would have been destroyed,” Hanlon said.

The worth of the dam wasn’t lost on Springfield officials.

Police Chief Doug Johnston said the floodwaters held back by the dam probably would have inundated the shopping plaza near the intersection of Vermont Routes 11 and 106, and could have threatened the police and fire stations farther downstream, not to mention dozens, if not scores, of homes and businesses.

“There’s no doubt in my mind there would have been a lot of damage in the town if we wouldn’t have had it,” said Johnston

The Army Corps estimates that during Irene its flood control projects in Vermont prevented $31 million in damage across the state. Throughout New England, the estimate is $1.4 billion.

Johnston said he felt the number was probably low.

The need for flood control dams in Vermont, with its mountains and narrow valleys that can funnel heavy rain into flash floods in a short period of time, was first recognized after the flood of 1927, the state’s worst natural disaster. The 1927 flood killed 84 and destroyed 1,200 bridges, as well as hundreds of miles of roads and railroad tracks.

A study at the time suggested that a system of 85 flood control dams across the state could prevent another catastrophe on the scale of the 1927 flood. But the dams would have inundated thousands of acres of prime farmland, changing the face of Vermont.

During the 1930s, a number of flood control dams were built, including dams on the Little River in Waterbury, the North Branch of the Winooski River in Middlesex and the East Barre dam on the Winooski’s Jail Branch.

The North Springfield Dam was one of a series of flood control projects that grew out of flood damage caused by a series of hurricanes and other storms that hit New England in the late 1930s. Construction in North Springfield began in 1957.

The dam is 2,940 feet long, 120 feet high and 610 feet wide at the base. It can hold 16.6 billion gallons of water from a drainage area of 158 square miles.

Hanlon and his co-workers spend their professional lives monitoring the dam, preparing for a big storm, like the one that happened in August.

“We have smaller events that don’t get the publicity all the time,” Hanlon said.

“It is a little gratifying to see it work this good. We take a lot of heat very often for the environmental concerns of the projects. The arguments are always, you know, these are here for a reason and someday we may know why they are here,” he said.

“The main purpose of our job is to perform this emergency operations function, to regulate these dams day-to-day, but really the most important part is during emergencies like this,” Hanlon said. “It’s pretty obvious this doesn’t happen every day, thankfully. But what we do do every day is, we prepare for it.”