Dam removal is the process of taking down a dam to allow water to flow freely in a river. People consider whether the negative effects of dams are greater than their benefits when deciding to remove them. Dams provide benefits such as electricity generation, flood control, irrigation for crops, and helping boats travel along rivers. However, dams can cause harm, including damage to the environment, such as lower plant growth, loss of animal life, and fewer native species. Some problems, like weak structures, safety risks, sediment buildup, and high repair costs, become worse as dams age. In the United States, the number of dams being removed has grown over time, partly because many dams are old. In 1996, about 5,000 large dams worldwide were more than 50 years old. By 2020, 85% of dams in the United States were over 50 years old. Between 1990 and 2015, about 900 dams were removed in the United States, with about 50 to 60 dams removed each year by 2015. France and Canada have also completed major dam removal projects. Japan’s first major dam removal, the Arase Dam on the Kuma River, started in 2012 and finished in 2017. Many dam removal projects aim to improve the environment, especially by restoring river habitats, native fish populations, and natural land features. For example, restoring fish populations led to the Elwha Ecosystem Restoration and the removal of a dam on the Allier River. Restoring native fish and a type of rock formation called travertine also guided the Fossil Creek restoration project.
Purposes and effects of dams
Many dams in the eastern United States were built to move water, support farming, operate watermills for factories, and serve other purposes that are no longer considered important. As these dams age, the chance that they might suddenly collapse increases over time. Also, many of these dams block the paths of fish that swim from the ocean to rivers to lay their eggs, such as Atlantic salmon and American shad. These dams also stop important sediments from reaching areas where rivers meet the ocean.
In the western United States, many dams were built to move water for farming in dry regions. Producing electricity was an important added benefit of these projects. One of the largest water-moving projects is the Columbia Basin Project, which uses water diverted by the Grand Coulee Dam. The Bureau of Reclamation oversees many of these water-moving projects.
Some dams in the Pacific Northwest and California block the paths of fish that swim from the ocean to rivers to lay their eggs, such as Pacific salmon and steelhead. Structures like fish ladders have not been successful in reducing harm to salmon populations.
The Bonneville Power Administration manages electricity generated by 11 dams on the Columbia River and 4 on the Snake River. These dams were built by the Army Corps of Engineers.
In the Desert Southwest, dams can change how rivers function. For example, the Glen Canyon Dam has changed the Grand Canyon’s river from warm and muddy to cold and clear. This change has harmed ecosystems downstream. Three native fish species in the Grand Canyon have gone extinct, and others, like the humpback chub and razorback sucker, are now endangered.
Some dam projects, like those on the Salt River Project in Arizona, stop rivers from flowing naturally downstream. Water is diverted into canals for farming and city use, leaving only a dry channel in the desert.
So much water is taken from the Colorado River for farming, city use, and water lost through evaporation behind dams that the river no longer reaches the Gulf of California.
Methods of removal
Dam removal can be done in several ways, and the chosen method depends on many factors. These include the size and type of the dam, the amount of sediment behind it, the environment below the dam, who owns the dam, and the time available for removal. Dam removal is expensive, and costs often increase when environmental concerns are prioritized. Usually, the cost is shared among the dam owner and government agencies. Four common dam removal methods are described below.
Sediment management is an important part of all methods. A common issue with dams is that sediment carried by rivers builds up in reservoirs, filling them with silt. This can reduce the ability of reservoirs to generate electricity, change the river channel below the dam, trap nutrients behind the dam, and put pressure on the dam itself. Sometimes, the sediment stored in reservoirs helps the area near the river by rebuilding fish habitat, providing nutrients, and adding to beaches or estuaries. Other times, the sediment can make the river cloudy, harm fish, erode land, or damage infrastructure.
Before releasing sediment, it can be tested to see if it is harmful to the area below the dam. If not tested, dam removal can cause problems. For example, when the Fort Edward Dam on the Hudson River was removed in the 1970s, harmful chemicals called PCBs in the sediment were released, harming people and wildlife downstream.
The notch and release method is often used because it is good for the environment. It is a slow process where notches are cut into the dam to drain the reservoir. New notches are added to keep water flowing out at a steady rate. Sediment flows downstream slowly, allowing the ecosystem to adjust. This method can take months or even over a year but has helped restore fish populations. The Elwha and Glines Canyon dam removal project used this method successfully. The Rindge Dam, which is completely filled with silt, is planned to be removed using a similar approach.
The rapid release method is the fastest and least expensive, but it has serious risks. A large tunnel is dug through the dam’s base and connected to the reservoir. Water drains quickly through the tunnel, causing sudden flooding and erosion downstream for miles. This can harm the environment and damage bridges, pipes, and other structures. However, if the reservoir is small and drains into a larger river or lake, this method may cause little harm.
The dig and dewater method is usually the most expensive but is needed in some situations. It involves emptying the reservoir, letting sediment dry, and moving it to a safe place for disposal. This process is costly and slow but may be necessary if the reservoir is near hydroelectric facilities that could be damaged by released sediment or if the sediment contains harmful substances.
The retained sediment method is the final common approach. It involves leaving sediment where it is by rerouting the river around the dam site. This can be expensive and difficult but may be used in remote areas where other methods are not cost-effective. The San Clemente Dam on the Carmel River was removed using this method.
Alternatives to removal
Fewer than 1% of dams in the United States are being considered for removal. However, in recent years, efforts have focused on improving existing dams to fix their problems without removing them. These improvements aim to make dams work better and reduce harm to the environment. Updating equipment and recognizing that dams have a limited lifespan are two ways to reach these goals. A plan to take the dam out of use and restore the river should be created before the dam becomes too old to function safely.
One step in restoring rivers that can begin before a dam is removed is using environmental flow. Changing how much water flows through the dam during different seasons helps copy natural changes in water levels caused by winter and spring storms. Adding fish ladders to dams can also help fish move through the river and reach where they lay their eggs. Some people debate how well fish ladders work, but usually, some fish can pass through, which is better than none being able to reach their usual spawning areas.
Managing how dams are used can also help reduce the buildup of sediment in reservoirs.
Dam removal in Europe
According to the European Commission, at least 150,000 barriers in European rivers are no longer needed or do not serve their original purpose. Removing these barriers is seen as a cost-effective and helpful solution that supports the European Union's Water Framework Directive. This directive aims to reduce pollution and ensure enough water for both wildlife and people.
To improve water flow, the EU Biodiversity Strategy plans to remove unnecessary dams and barriers from 25,000 miles of rivers by 2030. In 2022, Dam Removal Europe helped take down 325 dams or other structures, which is a 36% increase compared to 2021. A group of organizations, including the WWF, The Rivers Trust, The Nature Conservancy, The European Rivers Network, Rewilding Europe, Wetlands International Europe, and The World Fish Migration Foundation, have joined together to help return Europe's rivers and streams to their natural condition.