Warner’s Pond Dam

The pond is in an advanced state of eutrophication—excessive aquatic plant growth fueled by excess nutrients—with deep nutrient-rich sediments. This results in degraded habitat for native fish, loss of biodiversity, the visual appearance of eutrophication, and loss of recreational opportunities as the pond gets shallower.

More: Dams exacerbate eutrophication by slowing the natural flow of streams and rivers and trapping nutrients and sediment. Dams can create conditions for invasive plants that thrive in slow-moving systems such as water chestnut, fanwort, and variable water milfoil, which are all abundant in Warner’s Pond. Dams can also starve the river downstream of the silt, sand, and gravel needed to maintain the riverbanks, the natural riverbed, and aquatic wildlife habitat. The sun warms the water impounded behind the dam, further degrading fisheries’ habitat. Observations of eutrophication in Warner’s Pond date back to at least the early 1980s.

The pond’s water comes from a large watershed, mostly (97%) located in other towns, so it would be very challenging to control the sources.

More: The pond’s water comes from an approx. 47-square-mile watershed located primarily in Acton, Boxborough, Carlisle, Littleton, Stow, and Westford. These communities plus state agencies would have to cooperate and make significant investments to reduce nutrient input into Warner’s Pond, and the feasibility of other communities make costly investments to improve a pond outside their communities is highly unlikely. Further, the amount of nutrients already within the pond is far higher than the critical load, so even if nutrient inputs stopped today, the effects of the nutrients in the sediment would continue to contribute to poor water quality. The two tributaries that flow into the pond, Nashoba Brook and Fort Pond Brook, merge just upstream of the pond inlet on the western shore. Water discharges into the pond through a broad delta of emergent wetlands on the western shore. The pond discharges at the dam on Commonwealth Avenue, and Nashoba Brook flows downstream to the Assabet River.

There have been two pond management plans (1999, 2012). Mechanical harvesting and hand-pulling (water chestnut) have been undertaken but with only short-term benefits.

More: Multiple invasive aquatic plant species with high aquatic plant cover and volume were documented in a 1999 survey. Mechanical harvesting was employed across 21 acres in July 2004 but wasn’t continued because of the potential to spread fanwort and milfoil from fragments. The Town also initiated hand-pulling water chestnut by volunteers and staff in the 1980s, which has been repeated annually with temporary success. Herbicide treatment was conducted in 2011, with some success, but not continued because the pond’s high flushing rate doesn’t allow a long contact time of herbicides to the invasive plants. A 2012 Watershed Management Plan recommended short-term options (herbicides and biological controls) and long-term options (controlling upstream nutrient and sediment loading through resident education and stormwater management; sediment removal from the pond). The Town identified dredging to remove sediment from about 6 acres of the pond as a costly yet feasible means of addressing eutrophication. The Town also developed and distributed a brochure on efforts residents could take to help.

Dam removal was proposed as an alternative that would provide an enduring solution to the eutrophication, reduce flood and other risks to public safety, remove the economic and public safety liability associated with maintaining the dam, and restore the ecological health of Nashoba Brook.

More: In 2021, the Town began the design and permiting process for the proposed dredging project; permits were received in 2022 and the project was put out to bid. The results of the public bidding process indicated that the dredging project as designed would not be economically feasible. The town then explored an alternative approach to limited dredging of the pond and relocating dredged sediment within the pond, rather than disposing of it offsite, as well as removing the dam as a technically and economically feasible alternative. The alternatives analysis showed that removal of the dam was the only option that would meet all restoration goals. See: Warner’s Pond Alternatives Analysis Report, Concord Massachusets, EA Engineering, May 2023

Table 1-1 Project Goals

Dredging is extremely expensive and has been pursued for only 6 acres of the 59-acre pond. Dredging would have to be repeated over time, and the Town would have the ongoing maintenance and liability of the dam and its flood risks. Dredging would not provide nearly as many ecological and resiliency benefits of dam removal.

The low-level outlet is not designed to be used for flood control and does not drain the water low enough to be effective for invasive aquatic plant control.

Warner’s Pond dam is currently rated a “Significant Hazard Potential” dam in Satisfactory Condition. The latest dam inspection report (2023) showed several deficiencies and recommended additional studies and analyses, minor repairs, and remedial modifications, as well as yearly monitoring, inspection, and ongoing maintenance. A Significant Hazard Potential dam is one “where failure may cause loss of life and damage to home(s), industrial, or commercial facilities, secondary highway(s) or railroad(s) or interrupt use or service of relatively important facilities.” Dam failures can also have significant negative environmental impacts downstream.

When diadromous fish can reach a dam, state law enables the director of the Mass. Division of Marine Fisheries to “determine whether existing fishways, if any, are suitable and sufficient for the passage of such fish in such brooks, rivers, and streams or whether a new fishway is needed for the passage of fish over such dam or obstruction; and he shall prescribe by written order what changes or repairs, if any, shall be made therein…” (MGL Ch. 130/Sec. 19). In practice, the Division of Marine Fisheries uses this statute to educate dam owners (in this case, the Town of Concord) about their responsibility to provide passage and then find and implement the solution that is best for the dam owner and the fish in question. There are no federal laws that specifically require that dam owners provide fish passage unless the dam is licensed by the Federal Energy Regulatory Commission.

Other dam removal projects in New England show a rapid return of migratory fish once a dam is removed. When the Talbot Mills dam in Billerica is removed (2025/2026), there will be only two dams between the mouth of Nashoba Brook and the ocean, one in Lawrence and one in Lowell. Both have fish passage. In recent years, the fish lift at the Lawrence Dam in the Merrimack River has passed as many as 450,000 river herring and 90,000 shad in a single season. There is currently a fishway at the Centennial Island dam on the Concord River in Lowell and work is underway to design a “rock ramp” natural fishway that will improve upstream fish passage effectiveness.

More: The Concord River has been identified by both federal and state fisheries agencies as one of the best possible places in the Merrimack watershed to remove a dam for the benefit of restoring migratory fish. To help quicken the pace of restoration, the U.S. Fish and Wildlife Service and Mass. Division of Marine Fisheries has been transporting spawning herring and shad from the Essex Dam on the Merrimack River in Lawrence to suitable areas in the SuAsCo watershed. Offspring of these fish should imprint on the rivers and navigate their return in three to five years as adults.

Fishways are expensive, are not nearly as effective at passing fish as dam removal, and often need significant maintenance to operate correctly. Fishways would not meet the needs of all the fish species that would benefit from a free-flowing brook. Fishways also don’t provide the same benefits for other species that live in and near the water, such as amphibians and reptiles. During the fish migration season the dam owner would be required to provide daily operations and cleaning to assure continuous function of the fish ladder. A fishway would not result in water quality improvements either. The dam owner would still have to repair the dam and maintain liability insurance. Where there is space and other requirements are met, more nature-like fishways can be built that act less like dams and allow fish to pass.

A 1983 survey showed a warm water fish community typical of shallow lakes and ponds in our region rather than supporting riverine fish which would naturally occur in Nashoba Brook. Pond conditions have deteriorated since then. Dam removal would allow migrating alewife to access the natural remaining pond for spawning and contribute the fry eaten by larger fish as well as the adults being a recreational fish. OARS monitoring shows that the dam consistently raises the water temperature which is a stressor on fish. The pond is currently too warm for native book trout. However, a tributary to Nashoba Brook upstream of the pond is a mapped coldwater stream, providing suitable habitat for native trout; there is no structural impediment between Warner’s Pond and this tributary.

Different state agencies have different priorities and focus areas that often overlap when it comes to dam removal. The Division of Ecological Restoration (DER) works to restore ecological function and climate resilience and supports many dam removal projects around the state. As does the Municipal Vulnerability Preparedness (MVP) Program with its focus on climate resilience and nature-based solutions. The Office of Dam Safety (ODS) does not have issue with a dam that is in safe condition but could still support removal for other reasons.

Without the dam, Nashoba Brook would flow much more swiftly. A free-flowing stream allows sediment to be naturally deposited along its length. Some sediment and nutrients would be deposited in the floodplain and wetlands when there is high water, where they would not contribute to aquatic biomass growth. This would also prevent the recycling of nutrients by the aquatic weeds that currently grow, die, and sink to the bottom of the pond.

Aquatic and riparian invasive plants will be largely eliminated since few grow in fast-moving water with little sediment typical of free-flowing streams. Consider that water chestnut are a significant problem in the slow-moving Sudbury River, but virtually non-existent in the faster-moving water of the undammed sections of the Assabet River. In general, dam removal projects have not seen problems with wetland or floodplain invasives, but any project should monitor and control them if needed over several years. The exposed sediment becomes green quickly, drawing on its bank of native seeds.

Recreational boating in the pond has declined in the summer months due to dense aquatic plant growth and shallow water. Fishing for pond fish is still popular where the water is accessible. Informal ice skating still occurs for a few days each winter when the pond is sufficiently frozen. All these uses are likely to decrease in light of warming winters and summers, and deeper droughts. Dam removal with careful planning and design would increase opportunities for recreational boating in Nashoba Brook to and from the Wild & Scenic Assabet River and maintain boating access to Scout Island. Fishing would continue with increases in species diversity possible. It may be possible to maintain ice skating in a more limited area.

Warner’s Pond is not a public or private drinking water supply.

Dam removal would remove the possibility of loss of life and damage to homes, businesses, other facilities, roads, and infrastructure.

Warner’s Pond is not located within or immediately adjacent to a Local Historic District, Massachuset Historic Landmark, or an area listed on the National Register of Historic Places (NRHP). The dam’s role in the early industrial history of West Concord could be presented and memorialized for the public through displays, etc. The long natural history of Nashoba Brook as part of indigenous people’s lives and economies could also be presented.

There is inadequate flow to justify the investment needed, and there is a lengthy and complex process of licensing through FERC.

The town has made it very clear that any newly exposed land will still be within the deeded boundaries of Warner’s Pond and is protected as conservation land. It will still be a wetland resource area under the state Wetlands Protection Act and Concord’s Wetlands Bylaw, among other protections.

Numerous dam removal projects in New England show rapid plant growth in the newly exposed banks and wetlands, greening up within a couple of months. Exposed sediments behind dams have been found to contain significant quantity of dormant seeds that can now germinate and revegetate any exposed land. Seeding and planting can also be part of the plan if needed. This will stabilize as a biodiverse and healthy wetland and floodplain ecosystem with Nashoba Brook running through it.

The Natural Resources Commission has decided to pause on a 2024 Annual Town Meeting article asking the community for a decision on dam removal and has established a Task Force that will make recommendations. See: Warner’s Pond Task Force.

OARS supports the Dam Removal Alternative with a project design that incorporates enhanced recreational access to Nashoba Brook and Scout Island (see: OARS Position). Removing the dam at Warner’s Pond and restoring Nashoba Brook to its free-flowing condition is the best option to regain the ecological function and environmental aesthetics that so many Concordians cherish. Restoring migratory fish passage will also improve conditions for fish, which are key components of a healthy watershed and forage species for wildlife and the commercial and sport fisheries of the Gulf of Maine and North Atlantic. Removing the dam will help to build climate resilience in our river system that will serve both Concord and the region well into the future and restore the beauty and vitality of a healthy stream and wetland habitat.

There are currently many sources of state, federal, and private funding for the engineering studies, permitting, and removal of dams to reduce the cost of the dam removal option to the town. There are far fewer sources of funding for repair or dredging.

More: See chart below

• Natural Resources Commission Warner’s Pond website, OARS website, Warner’s Pond Dredge Feasibility Study (ESS 2018)
• Warner’s Pond Dam Phase I Inspection/Evaluation Report (Pare 2018) Concord River Diadromous Fish Restoration Feasibility Study (Gomez and Sullivan Engineers 2016)
• Concord Open Space and Recreation Plan (Town of Concord 2015)
• Warner’s Pond Watershed Management Plan (ESS 2012)
• Project Completion Report for Nuisance Aquatic Plant Management Program at Warner’s Pond (ACT 2004)
• Classification of the Natural Communities of Massachusets (Swain 2020)
• Warner’s Pond Management Plan (ACT/NEE 1999, revised 2012)
• Fisheries Survey of Warner’s Pond (MassWildlife 1983)
• Talbot Mills Dam Removal project website
• Geospatial data layers available from MassGIS2
• Historical maps of West Concord are available from the Concord Free Public Library Special Collections.

Questions are good! We revise this FAQ as new questions come in.

Contact: Heather Conkerton, OARS’ Ecological Restoration Coordinator: HConkerton@oars3rivers.org