Practice description
The partial or complete removal of dams to restore natural connections between adjacent aquatic habitats.
Goals and Ecological Processes
Dam removal is one of the best restoration actions to increase the climate resiliency of Massachusetts rivers and streams. Dams degrade river habitat by impounding water, altering river function, and blocking fish and wildlife. Impoundments trap sediment and create stagnant conditions with warmer water and lower dissolved oxygen than the rest of the river system. This change impacts the ecology of the river. For example, stream sections once dominated by species like Eastern Brook Trout, Fallfish, or White Sucker shift to favor Largemouth Bass and Bluegill in heavily dammed rivers. Dams also block the movement of fish that need different habitats in different areas of the watershed to complete their life cycle. Dams also drown riparian wetlands and floodplains and so can increase the risk of flooding to nearby towns. With more than 3000 dams in Massachusetts, the ecological impacts of dams are widespread.
Target Habitats and Species
Massachusetts has more than 3,000 dams throughout all its watersheds. However, even streams without dams are affected by dams located in tributaries or downstream mainstem rivers. Because dams restrict animal movements and disrupt biological and hydrological cycles (e.g., streamflow and water quality), dam removal should be considered as a restoration practice in all aquatic habitats. Dam removal will benefit all native fish species using upstream and downstream habitats but especially Brook Trout, Sea Lamprey, Blueback Herring, American Shad, and American Eel. However, benefits to species will depend on site conditions and locations. Several prioritization tools exist to help focus dam removal efforts (see Additional Resources below).
Associated Practices
- Culvert Replacement
- Channel and Shoreline Restoration
Methods
The goal of most dam removals is to restore the natural stream or lake conditions so that organisms, water, nutrients, and sediments can readily move between previously dammed and undammed areas. Each dam removal project is unique.
Practice Details
The extent of work necessary to complete a dam removal will vary from site to site. In general, dam removals will remove all infrastructure within aquatic habitats as well as the full removal of supporting infrastructure on stream banks. The goal is to reroute all water back to the natural river channel or lakebed and provide unobstructed movement of organisms, water, nutrients, and sediments.
Although full dam removal is the preferred restoration option, partial dam removal can be considered in cases where full removal is not feasible. In some cases, abutments or structures associated with the dam can remain in order to protect existing infrastructure. Partial dam removals are unlikely to result in fully restored river functions. From a restoration perspective, partial dam removals should only be considered when full dam removals are not feasible as benefits from the two are not equivalent.
Site conditions will also dictate associated actions necessary to fully restore habitats. In steep habitats, removal of dams within aquatic habitats is often sufficient to meet project goals. However, exposure of sediments previously inundated by impoundments are likely to need invasive species management, including native species revegetation, depending on the density and dispersal ability of invasive species in nearby habitats. Projects located in gently sloping habitats will usually require more extensive work. Beyond invasive species management, these sites may require testing and potential removal of sediments stored behind dams, stream channel recontouring, substate grading to establish natural stream and lake bottom elevations, and restoration of floodplain connectivity.
Invasive Species Management: Prevention is the best management strategy for reducing the introduction of invasive species in restored sites. Revegetation of exposed sediments with native plants is a cost-effective way to reduce invasive plants. In areas, like headwater streams, where invasive plants are not generally a problem, vegetation is likely to establish itself within one or two growing seasons.
Sediment Management: Many dams have accumulated sediment behind them. Some sediments may be contaminated, and testing of impounded sediments is part of the dam removal feasibility, design, and permitting process for dam removals in the state. Information is available at: https://www.mass.gov/guides/deciding-to-remove-your-dam.
Dam Removal Planning
Although widely seen as an effective restoration action, dam removal proponents must plan projects thoughtfully. Feasibility and other assessment work will help the proponent determine the technical complexity and potential cost of a dam removal project. Common issues that should be considered include evaluation and management of sediment trapped by the dam; timing, magnitude, and seasonality of flushing fine sediments downstream; timing and footprint of “wet” work; current uses of the impoundment; infrastructure that could be impacted by removal of the dam; potential impacts on rare species; and potential community and regulatory concerns such as recreation and impacts on historic structures. Construction planning must also include a plan for addressing invasive species that may colonize unvegetated areas of the site. Additionally, restoration expectations need to be set in context of other barriers (e.g., additional dams, incorrectly-sized culverts, dysfunctional fish ladders) within the stream or watershed and have specific goals. For instance, the removal of dams in steep coldwater streams will most benefit local ecology by linking coldwater climate change refugia and decreasing the likelihood of infrastructure failure, while removal of dams in large coolwater rivers can be more beneficial to anadromous fishes and water quality. In some cases (e.g., steep rivers and streams), removal of the dam structure itself combined with the natural flow regime of the system is sufficient to reestablish natural habitat conditions within a couple of years. In other cases (e.g., gently sloping rivers and streams), additional restoration actions may include excavation of sediments retained by the dam, reconstruction of stream channels, and revegetation of streambanks. Associated practices needed for successful restoration should address individual site conditions.
Financial Assistance: There are several Federal and state funding sources available to dam removal projects. State Programs include the Dam and Seawall Repair or Removal Program, Massachusetts Environmental Trust Program, Municipal Vulnerability Preparedness Program, and the Division of Ecological Restoration’s Priority Projects. Federal funding opportunities include the Partners for Fish and Wildlife Program, the NOAA Restoration Center, the US Fish and Wildlife Service, the National Fish Passage Program. The National Fish and Wildlife Foundation also has grants that fund dam removals. Information for these programs is available at: https://www.mass.gov/guides/deciding-to-remove-your-dam#-funding-opportunities-for-dam-removal-.
Return Intervals
Dam removals usually result in self-sustaining conditions with little subsequent work necessary beyond invasive species management. However, invasive species management may need to continue for multiple years to be effective.
Examples
- Mill River Restoration
- River Run - A Story of Dam Removal in Massachusetts
- Millie Turner Dam Removal & Nissitissit River Restoration
- Bartlett Pond Dam Removal & Wekepeke Brook Restoration
Additional Resources
Removing Small Dams: A Basic Guide for Project Managers
DER's Restoration Potential Model Tool
Dam Removal and the Wetlands Regulations
Dam Removal Creates Jobs, Stimulates the Economy