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By Anne O’Connor

PEPPERELL — When the Millie Turner Dam came down, scientists were not sure about the exact effects it would have on the Nissitissit River.

Armed with data gathered before the breach, they plan to study the waterway for years to come.

The dam, which was deliberately breached in September, provided a unique opportunity for study, said Denise Burchsted. The Keene State College fluvial geomorphologist studies the shape of rivers. She spoke at the Squan-a-Tissit chapter of Trout Unlimited meeting on Jan. 26.

“How things move and how they shape the rivers, that’s what I do,” she said.

Before the dam came down, Andy Marion, now a senior at the college, studied data about the sediment in the river and in the impoundment area, the pond created by the dam. They expected the sediment to flow downstream once the dam was gone, Burchsted said.

Instead, the fine, sandy sediment collected upstream of the dam in deposits like sandbanks. The deposits are moving slowly downstream. The day after the breach Burchsted could see the sand “just trucking along under the water.

Burchsted attributed the result to a difference between the Nissitissit and other rivers where dams have been removed. The Nissitissit is a slower, less powerful river than other rivers with similar dams.

It is unusual to be able to study a dam breach where water can flow freely, since so many dams have toxic sediments, she said.

The riverbed will continue to change, she said.

The changes in the riverbed means changes to the habitat of the animals that live there.

Aquatic ecologist Peter Hazelton of the National Heritage and Endangered Species Program, Massachusetts Department of Fisheries and Wildlife, approached preserving populations of mussels in a scientific manner. What is learned will set a template for the next dam that is removed, he said.

The Nissitissit is home to five species of native freshwater mussels. Four of these are on the Massachusetts list of endangered, threatened and special concern species.

Removing the dam could have both good and bad effects for the bivalves, Hazelton said. With no dam, the river will be connected from end to end, but as the water flows, the mussels could suffer from dewatering, burial and changes in their food.

Before the dam was removed, scientists identified the species and where they were living. Beginning in June, Hazelton used a snorkel and a glass-bottom bucket to collect data.

It took 150 hours of volunteer labor and 500 hours of staff labor for the next step, relocating the endangered mussels further upstream once the dam was breached.

Picking through the sediment, workers culled common mussels and threw them back to the center of the stream. Endangered animals, were collected and rehomed in locations where scientists hope they would thrive.

Adults will usually remain in place. “They don’t get up and swim,” he said.

Their offspring could set up home in a different part of the river. Mussel larvae mature while attached to fish. When the molluscs are big enough to survive, they fall off the fish and stay in place.

One sign of a healthy population is a selection animals at different stages of development, Hazelton said. Both juvenile and adult mussels were found in the river.

Mussels contribute to the health of the water by filtering out toxins and returning nutrient-rich material to the river, he said.

Plans are in place to monitor the population for at least the next seven years.

The Millie Turner Dam was breached in September. The project was a joint effort of federal, state and municipal government. Additional funding came from the Squan-a-Tissit and Boston chapters of Trout Unlimited, an organization for fly-fishermen.

The Squan-a-Tissit chapter will hold a fundraising dinner on March 19 at the Pepperell VFW Post at 55 Leighton St. The project manager for the dam removal, Alex Hackman from the Mass. Division of Ecological Restoration will be the featured speaker. Doors open at 5 p.m.

Follow Anne O’Connor on Twitter and Tout @a1oconnor.