Durham Scientists Endorsing Dam Removal

To The Editor,

We write in regard to the upcoming referendum in Durham on whether or not to support the Town Council’s decision to remove the Mill Pond dam. We are all scientists with expertise in ecosystems, limnology, marine biology, hydrology, civil engineering, toxicology, water policy and planning, ecological economics and other relevant disciplines. Some of us have specifically studied the effect of dams and reservoirs on the environment and human communities. All of us are also Durham residents.

We acknowledge the many different aspects of the dam removal decision, such as costs, recreation, history, and environmental considerations, which may lead to different opinions regarding whether to keep the dam and the pond behind it, or remove the dam and begin to restore the river. This is a difficult decision for our community. Regardless of how any Durham resident chooses to vote, we feel that it is important to have a clear understanding of the science underlying this decision. We hope to provide clarity on these issues in support of informed decisions by the community by explaining why we believe that the weight of evidence suggests that removing the dam is more beneficial for the Oyster River and Great Bay ecosystems.

Removing the Mill Pond dam will restore the natural transfer of sediment from the watershed to the estuary. Currently, the Mill Pond is filling in with sediments that are being blocked from flowing downstream by the dam. Water depth in much of the pond is now very shallow and will only get shallower if the dam stays in place. It will be very difficult, if not impossible, to get permits to dredge the pond if the dam stays in place. If the sediments that are settling in the pond were instead flowing down the river and into the estuary, they would have the positive effect of replenishing and building up sediment in saltmarsh habitats of the Great Bay and making them more resilient to sea level rise.

The sediments within the Mill Pond are also contributing to the pond’s poor water quality. Sediments that settle to the bottom of the pond contain phosphorus, a nutrient that is typically scarce in freshwater ecosystems. Under the low oxygen conditions of the Mill Pond, phosphorus is released from the sediments, stimulating plant and algal growth, especially during the warm summer months. Removing the Mill Pond dam will restore the downstream movement of phosphorus into the estuary, where it is less likely to cause water quality problems. We expect dam removal and river restoration will convert the degraded freshwater habitat of the Mill Pond to a thriving tidal ecosystem, with revegetation much like the living shoreline restoration Durham is undertaking along the Oyster River at Wagon Hill farm. The restored river will be a high quality habitat, different from the current freshwater pond habitat given the open connectivity between fresh and estuarine water. Such habitats are rare in New England given the preponderance of head of tide dams.

Sediments stored behind the Mill Pond dam contain mercury at high enough levels to create harmful ecological impacts. Engineering best practices call for active, intentional management of contaminated sediments rather than passively allowing unmanaged accumulation. Keeping the dam in place would not guarantee that these sediments will remain where they are or that heavy metals stored in them (including mercury) will not pose a risk. If the dam is removed, at least some of these sediments will be removed from the system as part of the accompanying river restoration actions, while others will be stabilized as part of restoring the stream channel. Removing the Mill Pond dam will likely reduce the risk of mercury to ecosystem health.

The dam is also a barrier to anadromous fish, such as blueback herring, alewife and others, which have declined along the eastern seaboard, especially in the Oyster River over the last ten years. These species provide a vital ecological linkage as they move between marine, estuarine, and freshwater ecosystems, while serving as important prey for a wide variety of predators along the way. As a “head-of-tide dam”, the Mill Pond dam is the first barrier adult fish encounter when they try to swim up the river in the spring, inhibiting their access to spawning areas. The Mill Pond dam also blocks young fish from migrating down the river to reach the estuary. We expect that removing the Mill Pond dam will improve populations of fish by allowing more adult fish to swim up the river to spawn, and young fish to migrate freely back to the estuary. A second dam, owned by UNH, is located 1.5 miles upstream, limiting full use of the watershed by anadromous fish. There is no incentive to improve its management with the current physical and water quality barriers created by the Mill Pond dam. Removing the Mill Pond dam is likely to create stronger impetus to improve fish passage and flow at the next river barrier.

Removing the Mill Pond dam, while reducing some forms of recreation, will create other opportunities for increasing access to recreation. Communities across New England have created river walks, boardwalks, and riverside parks to expand access for paddlers, birders, anglers, and those just looking to take a walk along the water at lunchtime. Durham recently purchased land and built a bridge across the Oyster River to provide easy access for walkers, joggers, and bikers from central Durham to 2000 acres of public land. Removing the Mill Pond dam would expand on Durham’s investments to connect our downtown to a trail network that could extend all along the Oyster River, from tidal to riverine sections, and could generate activity for local businesses as visitors look for food, lodging, services, and a unique ecological experience of a reconnected watershed-estuarine system.

As residents of the Town of Durham, which owns the Mill Pond dam, we have the privilege of voting on its future but also the responsibility to consider how our decision will affect others who have no say. The Oyster River flows across town boundaries and its water, fish, and wildlife are state resources that belong to the current and future citizens of New Hampshire, not only those of us living in Durham. Research on public opinions shows that most NH residents think aging dams like the Mill Pond dam should be removed for the benefit of free flowing rivers and fish. A decision to remove the Mill Pond dam better advances the broader public interest and avoids imposing financial costs and risks on future residents.

As a result of the above considerations, we believe removing the Mill Pond dam is an important first step toward improving the ecology of the Oyster River watershed and Great Bay estuary. There is much that our town will still need to do after this decision, including limiting road salt, reducing non-point nitrogen and phosphorus sources, characterizing and eliminating other types of contaminants, and reconnecting the entire Oyster River watershed with the Great Bay estuary. Removing the Mill Pond dam at the head of tide is an essential starting point, which is why we will be voting No on 2 to remove the dam and restore the river.

Sincerely, Wilfred Wollheim, Associate Professor of Aquatic Ecosystem Ecology (UNH*), Durham resident since 2011 Catherine Ashcraft, Assistant Professor of Environmental Policy and Planning (UNH*), Durham resident since 2014 Shannon Rogers, Associate Extension Professor of Nature-Based Economic Development (UNH*), Durham resident since 2017 David Buck, Associate Director of Shoals Marine Laboratory*, Durham resident since 2008 James Houle, Director of the UNH Stormwater Center*, Durham resident since 2004 Anne Lightbody, Associate Professor of Hydrology (UNH*), Durham resident since 2011 Matthew Brown, State Conservation Engineer, USDA-NRCS*, Durham resident since 2015 Jake Kritzer, Marine Ecologist and Chair of the Durham Conservation Commission*, Durham resident since 2016 Rachel Rouillard, Director of Conservation Strategy - The Nature Conservancy* in NH, and former Executive Director of PREP, Durham resident since 2000 Steve Jones, Research Associate Professor of Marine Science and Natural Resources, (UNH*), Durham resident since 1997 Mary Adamo Friedman, Program Coordinator, Principal Lecturer, Community and Environmental Planning, Department of Natural Resources and the Environment, UNH*; Durham resident since 1999. Derek Sowers, Oceanographer, National Oceanic and Atmospheric Administration*, Durham resident since 2008 and UNH Alumni *Affiliations are provided for identification purposes only and do not necessarily imply institutional endorsment.