Frequently Asked Questions About River Restoration

What Is A Dam?

There are a number of technical and legal definitions of a dam, but generally, it is any structure that impounds or diverts water.

How Many Dams Are There In The United States?

The exact number of dams is not known. There are more than 91,000 dams in the U.S. Army Corps of Engineers’ (Corps) National Inventory of Dams (NID), which is the most comprehensive inventory of dams nationwide. However, this inventory only covers dams that meet minimum height and impoundment requirements, so an unknown number of small dams are not included in the inventory. The actual total number of dams in the U.S. is likely several times the number of dams included in the NID. No one has a record of where all dams are located.

Who Regulates Dam Operations?

A number of state and federal agencies are responsible for regulating dams. Dams owned by federal agencies are self-regulated. Non-federal dams that produce hydropower are regulated by the Federal Energy Regulatory Commission (FERC). Non-federal dams that do not produce hydropower are sometimes (although not necessarily) regulated by the state in which they reside. Often state oversight is focused on ensuring the safety of dams of a certain size and hazard classification.

How Many Dams Actually Produce Power?

According to the National Inventory of Dams, seven percent of dams in the U.S. produce power. As of September 2019, FERC was managing 1,045 active hydropower licenses. FERC also allows license exemptions to many small hydropower operations. According to the National Hydropower Association, when federal, other public, and privately-owned facilities are included, there are a total of 2,198 active hydropower plants in the U.S.  Approximately half of the hydropower produced in the U.S. comes from facilities in Washington, California, and Oregon.

What Is The Largest Dam In The Country?

The size of a dam can be measured in a number of different ways. According to the NID, Oroville Dam, on the Feather River in California, is the tallest dam in the United States, measuring at 770 ft. The dam with the largest impoundment is Hoover Dam, on the Colorado River in Nevada, which stores approximately 30 million acre-feet of water. The dam that provides the most hydroelectric power in the United States is Grand Coulee Dam, on the Columbia River in Washington, which produces an average of 21 billion kWh of power annually.

Why Are Some Dams Being Removed?

There has been a growing movement to remove dams where the costs – including environmental, safety, and socio-cultural impacts – outweigh the benefits – including hydropower, flood control, irrigation, or recreation – or where the dam no longer serves any useful purpose. The goal of removal can be multi-faceted, including restoring flows for fish and wildlife, reinstating the natural sediment and nutrient flow, eliminating safety risks, restoring opportunities for recreation, and saving taxpayer money.

How Are Dams Removed?

Because dams and rivers vary greatly, physical removal strategies and techniques may also vary on a case-by-case basis. Here is more in-depth information on how dams are removed.

How Many Dams Have Been Removed To Date?

At this link, you can view an interactive dam removal map and find a link to our database of all known U.S. dam removals.

How Much Does It Cost To Remove A Dam?

Because the size and location of dams vary so greatly, the cost to remove an individual dam can range from tens of thousands of dollars to hundreds of millions of dollars.

Who Owns The Dams That Are Being Removed?

Private businesses or individuals, federal agencies, state agencies, local governments, or public utilities may own dams. Most of the dams removed to date have been owned privately, by local government, or by public utilities.

Who Pays For Dam Removal?

Who pays for the removal of a dam is often a complex issue. In past cases, removal has been financed by the dam owner, local, state, and federal governments, and in some cases agreements whereby multiple stakeholders contribute to cover the costs.

Who Decides That Dams Should Be Removed?

The decision to remove a dam is made by varying entities, depending on the regulatory oversight of the dam. In most cases, the dam owner itself is the decision-maker, often deciding that the costs of continuing to operate and maintain the dam are more than removing the dam. State dam safety offices can sometimes order a dam to be removed if there are major safety concerns. State fish and wildlife offices are also often involved in the decision-making, particularly when the goals of the project include the restoration of habitat for migratory and resident aquatic species. If the dam in question is a hydropower facility, the Federal Energy Regulatory Commission can order a hydropower dam under its jurisdiction to be removed for both environmental and safety reasons.

Can Rivers Be Restored Through Dam Removal?

Although most rivers cannot be completely restored to historic conditions – simply because of the amount of development that has occurred on and along them – dam removal can often recreate conditions that move the river towards those historic conditions. For example, fish are returning to historic stretches of river that had been previously obstructed on Butte Creek in California, the Souadabscook River in Maine, and the Clearwater River in Idaho, as a result of dam removals. Natural flows and transport of sediment and nutrients can help restore downstream reaches and reconnect oceans to headwaters. Communities can gain resilience to climate change by removing river barriers and allowing rivers to function more naturally.

What Benefits Do Dams Provide?

Dams may provide a variety of benefits, including water supply, power generation, flood control, recreation, and irrigation.

How Can The Benefits Of A Dam Be Replaced When It Is Removed?

While dams serve a number of human needs, society has developed ways to address many of these needs without dams. For instance, flood control can often be accomplished more effectively and at a lower cost by restoring wetlands, maintaining riparian buffers, setting back levees or moving people out of the floodplain. Updating antiquated irrigation systems and replacing inappropriate crops can dramatically reduce the need for dams and reservoirs in the arid West. Rather than plugging rivers with multiple hydropower dams, a cheaper and less environmentally harmful solution is to use existing energy efficiency technologies. For example, the 3MW of power lost in the removal of the Edwards Dam, on the Kennebec River in Maine, could have been replaced simply by replacing 75,000 light bulbs with energy-efficient bulbs. In addition, many dams that have been removed no longer had any beneficial use or that use was very limited, as is illustrated by many mill dam removals. [Learn about alternatives that exist by reading Beyond Dams: Options and Alternatives.]

Is It Cost-Effective To Remove A Dam?

Dam removal can be expensive in the short term, but in most cases where dams have been removed or are being considered for removal, money is actually saved over the long term. Removal eliminates the expenses associated with insurance coverage, maintenance and safety repairs, as well as direct and indirect expenses associated with fish and wildlife protection (e.g., fish ladders and mitigation for fish mortality). In addition, removal often generates income from newly available recreation opportunities – including fishing, kayaking, and rafting – which may actually result in a net economic benefit. In some areas, dam removal may allow the resumption of commercial fishing activities.

Will The Removal Of A Dam Matter If Other Dams In The System Are Not Removed?

Some rivers are so heavily developed and dammed that the removal of one dam on that river will only return flows to a small portion of the river. Generally, dams that have been targeted for removal are strategically located – removal will open up a section of the river critical to fish and wildlife and/or recreation. In some cases, this additional section of the river is enough to sustain crucial populations of endangered or threatened species of fish, mollusks, and other wildlife.

How Does Dam Removal Affect Fish?

Dam removal benefits fish in many ways, including: (1) removing obstructions to upstream and downstream migration; (2) restoring natural riverine habitat; (3) restoring natural seasonal flow variations; (4) eliminating siltation of spawning and feeding habitat above the dam; (5) allowing debris, small rocks and nutrients to pass below the dam, creating healthy habitat; (6) eliminating unnatural temperature variations below the dam; and (7) removing turbines that kill fish.

What Are The Potential Downsides To Dam Removal?

Dam removal does result in fundamental changes to the local environment. The reservoir will be eliminated and with it the flat-water habitat that had been created. Wetlands surrounding the reservoir may also be drained, although new wetlands are often created both in the newly restored river reach above the former dam site and in the river below. Sediment that collects behind a dam, sometimes over hundreds of years, may contain toxins such as PCBs, dioxide, and heavy metals. Removal of these toxic materials is often extremely expensive, and the threat of re-suspending these toxin-laden sediments in the process of dam removal has the potential to damage downstream water quality and threaten the health of fish and wildlife, and water users. These impacts, however, can be prevented through proper removal techniques. Short-term impacts of the dam removal itself can include increased water turbidity and sediment buildup downstream from releasing large amounts of sediment from the reservoir, and water quality impact from sudden releases of water and changes in temperature. It has been demonstrated that these short-term impacts and greatly outweighed by the quick recovery of the system and the long-term benefits that result.

How Quickly Do Rivers Recover After Dam Removal?

Rivers are very dynamic and resilient systems. Experience has shown that natural river systems can be restored relatively rapidly after dam removal. For example, American eel were seen crawling through the breached section of the Harvell Dam in Virginia amidst its removal, spawning fish returned to the Souadabscook River in Maine only months after a dam was removed, and the flushing of the sediment from the Milwaukee River in Wisconsin following the Woolen Mills Dam removal took only six months.