More than two million dams are estimated to exist across the United States, and 75 to 90 percent of them do not currently have a functional purpose since many were initially built to serve mills that are no longer in operation. Additionally, the U.S. Army Corps of Engineers National Inventory of Dams tracks approximately 90,000 dams greater than 25 feet in height. Over 80 percent of these dams are expected to exceed their 50-year engineered life span by 2030; increased flooding associated with extreme rain events places additional stress on these structures.
As aging dam infrastructure continues to reach critical decision points, stakeholders must weigh safety concerns, environmental risks, maintenance needs, liability considerations, and restoration opportunities. Some structures may warrant continued investment and rehabilitation, while others may be candidates for removal and ecological restoration.
Societal and Environmental Impacts
The effects of aging dams extend beyond the dams themselves; they can have an impact on public safety, property, water quality, habitat, and ecological function. From a societal perspective, aging dams could pose a threat to life and property in the event of failure. The case of the Edenville Dam on the Tittabawassee River in Michigan shows just how severe the possible consequences can be. This dam was constructed in 1924 for the purpose of producing hydropower and controlling floods, and it failed in 2020. The failure resulted in the loss of 2,500 structures valued at more than $200 million. Failure is not the only safety concern. Aging dams may also create hazards for people using waterways for recreation. Low-head dams can generate reverse currents capable of pinning swimmers and boaters against the structure. These dams are estimated to contribute to an average of 50 fatalities annually in the United States.
Environmental and ecological impacts are also important. By impeding water flow, dams can increase water temperatures and potentially increase eutrophication and associated biological oxygen demand. They can also change habitat structure by increasing embeddedness and disrupting natural riffle and pool habitat complexes. Perhaps most notably, dams interrupt ecological connectivity. By blocking fish passage, they contribute to a loss of species diversity and affect species that depend on fish to complete their life cycles, including freshwater mussels.
Another key consideration is sediment. If contaminated sediments accumulated behind dams are not properly managed, sediment-related contaminants may be transported great distances downstream from the dam during a catastrophic failure or an extreme hydrologic event occurring during or after dam removal. Even when the sediment is not contaminated, increased sediment loading can adversely affect species that are sensitive to suspended sediment. It can also increase embeddedness or potentially bury habitat as sediment is deposited downstream.
Case Study: Restoring Connectivity in the Delaware River Basin
The ecological impacts of dams are particularly evident in the Delaware River Basin. Migratory fish species in the basin include American Shad, river herring, Hickory Shad, American Eel, Sea Lamprey, and Atlantic and Shortnose Sturgeon. These species, especially American Shad, hold significant social, economic, and ecological value; however, populations of these fish have been declining since the 1800s due to overfishing, pollution, and dams blocking fish passage. An estimated 40 percent of historical spawning habitat for American Shad is obstructed by dams and other barriers throughout the United States and Canada. Many tributaries have remained blocked for more than a century.
Although improvements have been made in fish passage, water quality, and fishing regulations, stock assessments for the Delaware River show that these improvements have not produced a corresponding rebound in the American Shad population. This demonstrates the need for a more aggressive restoration strategy, including the removal of aging dams to enable fish passage. One example of this approach is the Cedar Grove Dam removal project on New Jersey’s Pequest River, a tributary identified as a Tier 1 restoration priority within the Delaware River Basin Restoration Roadmap. The project reflects broader efforts throughout the basin to improve habitat connectivity and restore access to migratory fish habitat. In addition to the benefits for migratory fish, dam removal can improve ecological function and benefit recreational users by restoring more natural flow conditions.
Rehabilitation or Removal?
Whether an aging dam should be rehabilitated or removed will depend on the particular circumstances affecting the structure and is usually not a straightforward decision. Factors such as safety, the cost of long-term maintenance, environmental liabilities, sediment conditions, and restoration goals all inform the best course of action.
Liability Reduction
If impacted sediments are present behind an aging dam, proactive assessment and management may help address potential liabilities while those sediments remain localized and contained. Owners responsible for maintaining aging structures may also find value in evaluating the long-term costs of rehabilitation and maintenance against the costs and benefits associated with removal. For dams that pose safety hazards to recreational users, removal may eliminate those hazards while also supporting restoration objectives.
Due Diligence During Property Transactions
Dams can also become an important consideration during property acquisitions and divestitures. When ownership of a property includes a dam that may be retaining impacted sediments, assessment tools may help determine the potential for sediment impacts and guide additional characterization efforts.
Restoration Credits
The potential benefit of receiving credit for restoration may also be part of that evaluation. Dam removal may be considered as a restoration project. Removing a dam may establish restoration credits against a Natural Resource Damage claim or another environmental offset. These projects can provide significant ecological uplift in waterways, which may have considerable value relative to the project cost, depending on the availability of other restoration projects or the cost of mitigation.
Supporting Informed Decisions
Aging and obsolete dams can create hazards for communities, limit ecological function, and, in some cases, present environmental liabilities associated with sediments that have accumulated behind these structures. Understanding these risks, alongside the potential benefits of restoration, is an important part of making informed decisions about a dam’s future.
EHS Support identifies opportunities to support these evaluations through sediment assessments, liability reduction strategies, due diligence investigations, and restoration planning. By considering both risk reduction and ecological outcomes, stakeholders can better determine whether rehabilitation or removal represents the most practical path forward for a given structure.
To learn more about our dam removal & river restoration services, please contact Greg Murphy.
Greg Murphy
Principal Scientist, Sediment & Risk Assessment Service Line Director
