Exeter NH Great Dam Removal Offers Lessons for Other Communities

EXETER – It has been five years since the Great Exeter Dam was removed and all qualitative indications show that this is a successful restoration of the Exeter River ecosystem.

What was more difficult, however, was getting a quantitative analysis of the success of restoring the Exeter River alewife population, says Michael Dionne, marine biologist at New Hampshire Fish & Game.

The alewife has played such an important role in Exeter’s history that it has featured prominently on the town’s seal, and by all accounts its population has rebounded since the dam was removed.

Around the coastline and throughout New England, anti-tide dams supplied textile factories and manufacturing plants with hydroelectric power for their looms and machines when they were first built in the 18th and 19th centuries.

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“Alewives are baitfish,” Dionne said. “They’re at the bottom of the food chain (and everything) feed on them from when they’re juveniles until they’re adults. They’re important to seabirds, striped bass.”

Dionne said other key environmental benefits of dam removal are better water quality.

“With the dams, usually upstream has become too rich in nutrients and below them it becomes starved of nutrients,” Dionne said. “Slow moving water in the reservoir is more stagnant and will heat up and not hold as much oxygen.”

Feedback on the removal of the dam

When the Great Dam was removed in 2016, so was its fish ladder, which anadromous species of herring used to migrate from the ocean, up the Squamscott River at tide, to the Great Dam and into the River Exeter to lay eggs.

While in operation, Dionne said the Grand Dam’s fish ladder was one of the least efficient in the state. He said that since 1970, the Grand Dam’s fish ladder has averaged only 1,628 alews per year swimming there and In in the River Exeter to lay eggs, the highest annual total recorded in 1981 at 15,626.

Dionne said that before the removal of the Grand Dam, during the annual alewife harvest for commercial and recreational fishermen, they regularly returned with tens of thousands of fish migrating to the ocean. The alewife migration into Exeter begins in late April and lasts until early June. He said the harvest usually takes place in the second or third week of May.

“There is an annual alewife harvest on the String Bridge for lobster boats and personal striped bass fishermen to catch bait and Fish & Game issues them a free license to fish the saltwater portion of the river,” said Dionne. “They can only harvest two days a year and only fill one bag per person and they caught over 20,000 fish a year. So even with these restrictions, the fish harvest far exceeded the use of the fish ladder, so we knew we had a problem at the Grand Dam.

Exeter Natural Resources Planner Kristen Murphy talks about the removal of the Exeter Great Dam in 2016 and whether the removal has met its planned environmental goals on November 22, 2021.

“The only downside to dam removal, if you can call it that, is that ladder removal removes the place where it’s easiest to count fish,” Dionne continued.

Dionne said that since the removal of the Grand Dam, between 2016 and 2020, Fish & Game would qualitatively observe the river several times a week and that the removal was deemed successful given that fish could migrate much more easily through the river.

This year, Dionne said, Fish & Game has focused on establishing reliable data to quantify alewife migration in the spring and has increased its monitoring to three times a day by staff during the alewife race. He said the data had not yet been finalized as he wanted input from other marine biologists for his counting methodology to ensure that the estimates truly reflect migration.

“It is safe to say that the upward migration of the Exeter River exceeds 100,000 fish (per year),” Dionne said. “To put this into perspective, we consider the Lamprey River fish ladder (in Newmarket) to be one of the healthiest fish ladders; this averages around 34,000 fish per year.

An etched glass rendering of the original dam is on display at the Exeter Public Library on November 22, 2021.

Kristen Murphy, natural resources planner for Exeter, said the recovery of alewife habitat has been so strong that the city is looking to bring back its alewife festival next May, which was halted in 2008.

“There is a lot of energy behind the return of the festival,” said Murphy.

Murphy said the process of removing the Grand Dam began when the city received a failure letter from the state Department of Environmental Services dams office in 2000.

After years of debate, the city decided on three alternatives, which were to remove, repair and maintain, or modify the dam. The city ultimately decided to remove the dam, at a cost of $ 1.2 million in 2016. However, the final feasibility study published in 2013 noted that the removal of the dam opened up multiple grant possibilities for the city. restoration of herring habitat that provided 50-100% of design, licensing and construction costs.

Exeter Natural Resources Planner Kristen Murphy explains whether the removal of the Exeter Great Dam in 2016 met its planned environmental goals on November 22, 2021.

“We all got a glimpse of what the Exeter River would look like naturally,” Murphy said. “We were able to restore 10 miles of free flowing river, and it’s great to see the wildlife returning; there are many more hawks and eagles that hunt fish.

Yet further up the River Exeter, the fish are blocked by the Pickpocket Dam on Pickpocket Road. Dionne said the Pickpocket fish ladder is much more efficient than the old Grand Dam ladder, but alewives fail to climb the ladder as much as they would have liked.

Dionne said that in order to achieve an almost complete restoration of the alewife population in the Exeter River ecosystem, he would like to see more alewife migrating up and down the river, through the Pickpocket Dam, to Crowley Falls in Brentwood.

Dionne said the falls were the natural barrier where herring would stop to lay their eggs long before the dams were built. He said Fish & Game started collecting fish under the Pickpocket Dam and transporting them to the other side of the ladder to familiarize them with this section of the river in the hope that when their offspring return to lay eggs. eggs, they will go all the way. Crowley Falls.

“These are generations of fish that don’t know how to migrate further (past the Pickpocket fish ladder),” Dionne said. “It’s a pretty common practice to restore river herring to this section of the river to truck it and haul it around to start this habitat restoration process. “

Elsewhere on the coast, Durham currently faces many of the same questions Exeter has faced before as it debates the removal of the Mill Pond Dam.

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Durham City Administrator Todd Selig said if a citizens’ referendum was passed in the March municipal election to overturn the city council vote to contract with engineering firm VHB to begin the design and engineering to remove the Mill Pond Dam, the city will have to deal with getting the dam re-classified by the state from “low risk” to “non-threatening” dam if the dam is to remain as it is. is today. He said the city had to make repairs to the dam if it remained a “low risk” designation by the state, Durham would be on the verge of rebuilding it wider, taller and more stable than its current state.

To be eligible for reclassification without doing this full restoration, Selig said, Durham must obtain a waiver of liability from an abuter, who would not be required to sign it, and would effectively become a permanent easement on this land.

“This is the precursor process for any kind of repair to the Mill Pond Dam,” said Selig. “It would be a significant challenge to meet modern engineering standards at this location on the Oyster River.”

Dionne said the blueback herring that swim up the Oyster River in the spring are at risk. In 1992, Dionne said more than 150,000 herring migrated and climbed the Mill Pond Dam fish ladder to lay their eggs upstream.

Since then, Dionne said, the blueback herring population, which makes up the vast majority of the overall herring population in Oyster River, has fallen to around 8,000 in recent years.

Dionne said Fish & Game attributes the decline in the herring population at Oyster River to the lack of constant flow of the Mill Pond River in summer and early fall, when blueback herring spawn, hatch and attempt to migrate out to sea. The Oyster River slows down to a trickle and the resulting stagnant water loses oxygen to the fish, he said. As a result, many newly hatched blueback herring die before they can swim to the Gulf of Maine in late summer and early fall, he said.

“What we’re hoping for here is improved water quality by removing the dam,” Dionne said. “When adult herring go up the fish ladder in the spring, there’s a lot of water flowing; they lay their eggs and return to the ocean. In the summer, when they hatch behind the impoundment, there is often not enough water to pass the weir and they cannot escape the habitat.

Ted Diers, administrator of the Watershed Management Office of the State Department of Environmental Services, said that when communities assess the removal of dams that have played a vital role in servicing factories in the region there are centuries, several considerations must be taken into account beyond the environmental benefits.

“These are tough conversations to have, even though dams don’t provide the service they were originally built for, they often serve recreational or aesthetic purposes,” Diers said. “But we have to recognize that they have an ecological cost. This is why the public process is so important, as it was in Exeter.


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