by Compass Vermont

State dam engineers will spend three weeks this fall drilling into Wrightsville Dam, the 115-foot earthen dam on the North Branch of the Winooski River that holds back floodwater above Montpelier. From about Oct. 5 to Oct. 23, crews will take samples from inside the dam, the soil beneath it and the bedrock around its auxiliary spillway, according to the Vermont Department of Environmental Conservation.

Wrightsville Dam ranks second of 18 on the state’s own dam safety priority list, behind only the East Barre Dam. The drilling is part of the state’s effort to learn more about the vulnerabilities and potential failure scenarios identified in its assessment.

What “second” means, and what it doesn’t

The ranking comes from a risk assessment the state commissioned on the 18 high- and significant-hazard dams owned by the Agency of Natural Resources. RTI International completed it for about $445,000. The state says the list is meant to help decide where limited money for dam studies and repairs goes first.

The ranking weighs both what could go wrong with a dam and what would happen downstream if it did. Because Montpelier lies below Wrightsville Dam, what a failure would mean for the city is part of the dam’s score. The ranking is not a prediction that Wrightsville Dam is the second-most likely dam in Vermont to fail.

The same goes for the dam’s “High Hazard” label. That label describes what’s downstream if a dam fails, meaning a likely loss of life. It says nothing about the dam’s condition. After the 2023 flood, The Bridge reported that state and federal inspectors had rated Wrightsville Dam “Satisfactory” in their previous inspection, and that post-flood inspections turned up no new deficiencies.

The night the reservoir nearly reached the auxiliary spillway

Wrightsville Dam was built by the Civilian Conservation Corps under the Army Corps of Engineers in 1935, after the Great Flood of 1927. It works without gates. During a flood, tunnels through the dam let out a fixed amount of water, and the reservoir rises to hold back the rest. The auxiliary spillway is a separate overflow channel cut into the rock beside the dam, built to pass water safely if the reservoir gets too high.

In July 2023, the reservoir nearly got there. Before dawn on July 11, Montpelier officials warned that the reservoir had about six feet of storage left. They added that water had never gone over the auxiliary spillway since the dam was built. By late morning, the water stood about a foot below the auxiliary spillway. City police warned that each additional foot going over the spillway would double the water flowing from the dam into Montpelier. That evening, the city said the water was receding.

Water going down the auxiliary spillway would not have meant Wrightsville Dam had failed. The spillway exists for exactly that situation. But it would have sent more water into a city already underwater.

Two months later, the state asked the Army Corps for help. Neil Kamman, then interim commissioner of the Department of Environmental Conservation, wrote that the flood showed Wrightsville and East Barre dams were “susceptible to a back-to-back storm event”. In that scenario, a second big storm arrives before the reservoir has drained from the first.

Six failure scenarios engineers are checking

The state’s assessment identified six potential failure modes. “Failure mode” is engineering language for a specific way a dam could fail, which engineers then check for. In plain terms, the six for Wrightsville Dam are:

1. Internal erosion of the embankment: water seeping through the earthen dam slowly carries soil out from the inside.
2. Erosion of the auxiliary spillway: fast water wears away the spillway channel or its wall.
3. Failure of the outlet tunnel: the tunnel that carries water through the dam cracks or collapses.
4. Erosion along the low-level outlet: water seeps along the outside of the lowest passage through the dam, washing soil away.
5. Not enough release capacity: in an extreme storm, the dam can’t let water out fast enough, and water rises over the top of the dam.
6. Slope instability: the dam’s sloped sides shift or slump.

Two studies, two questions

Engineers are running two studies at once, and they ask different questions.

What’s inside the dam? The $500,000 study behind this fall’s drilling examines the earthen dam, the soils under it and the rock around the auxiliary spillway. It looks at how water moves through the dam, how stable its slopes are and how easily the spillway rock could erode. The state says the work will reduce uncertainty around four of the six failure modes, the ones tied to seepage, internal erosion and slope stability. Target completion is December 2026.

Can the dam handle floods differently? The second study is being run with the Army Corps. It costs $700,000, split evenly between state and federal money. It began in May and is expected to take about two years. It takes on the problem Kamman raised in 2023: getting stored floodwater out faster before and after a storm. The options on the table include:

• a gate in the auxiliary spillway, or a gate system at the base of the dam
• changes downstream on the North Branch, including removing old dams
• a revised operating manual for the reservoir

The state says that study will partly address the fifth failure mode, not enough capacity to release water during an extreme flood.

Neither study is a repair. Together they are how the state decides what Wrightsville Dam needs, if anything, and what it would cost.

East Barre Dam, which protects Barre and ranks first on the same list, was also completed in 1935. East Barre, Wrightsville and Waterbury are Vermont’s three state-operated Winooski River flood-control dams. Compass reported this month on the $76.2 million Waterbury Dam spillway project.

During the work

The state expects no change to reservoir levels or river flows, and says the dam will stay fully able to handle a flood while crews are on site. For about three weeks, work vehicles will occupy one lane of Horn of the Moon Road across the top of the dam, with traffic control in place. Reservoir users should expect noise but no closures.

What you can check

• Follow the Wrightsville Dam project. DEC posts updates and the project contact at dec.vermont.gov/wrightsville-dam. Dam Safety Program engineer Ben Green: 802-622-4093, Benjamin.green@vermont.gov.
• Get dam emergency alerts. Montpelier sends its emergency alerts through VT-Alert, the state system. Sign up at vem.vermont.gov/vtalert with your home address. The state says life-safety messages, including a dam breach, override your other settings. If you signed up before Dec. 1, 2025, you need to create a new account to change your settings. Your existing alerts still arrive.
• Look up any dam near you. The Vermont Dam Inventory lists dams statewide, with hazard class and location.
• Know what living below a dam means. DEC’s Living Near Dams page covers what downstream residents should know.