The future of US dams is dependent on the right dam (large or small), at the right location, at the right time – Michael Rogers, Global Dams Practice Leader for MWH Global.

Are the days of constructing large US dams finally over? Absolutely not.  There is still a considerable need for water storage, flood control and clean/reliable hydropower energy across the United States. However, the mature nature of the American industry means new projects will need to be vetted to a much deeper environmental and socio-economic standard than past historical periods of large dam development.  As an example, the Olivenhain Dam – a new 91m high dam in San Diego was completed in 2006 and Alaska is getting closer to making the 215m high Watana Dam a reality after three decades of planning.

Both large and small dams have always been a critical part of the infrastructure that serves the geographically and economically diverse population of the US.  However, both also provide risk to their owners.  These risks need to be addressed through ongoing inspection and maintenance, so the asset and the population at risk downstream are protected from potential impacts such as extreme floods, earthquakes, as well as, the day-to-day ageing process that will weaken structures and mechanical/electrical systems over time.

The environmental concerns of all dams – large and small – are similar and should be held in high regard to the design engineer and owner.  Large dams often overshadow smaller dams due to their design, construction complexity, or even their sheer size like Hoover Dam.  Large dams also overshadow smaller ones when it comes to consequences of failure, thus the dam safety profession must always work to minimise these risks to protect lives and property.  However, depending on the downstream development, a smaller dam could have similar risks to life and property and tendencies to relax dam safety surveillance for smaller structures is extremely unwise.

There are many lessons that have been developed in our industry for the design and construction of large dams that have been successfully applied to smaller dams.  For example, the innovative design, materials and construction methods of RCC at Olivenhain Dam have been applied to large and small RCC dams in the US and around the world.

While it may seem that the age of the large dam is over, it is not.  There are many areas across the United States as well as the world that need significant sources of hydropower, water supply or flood protection that can only be met by the construction of a dam.  Certainly the developers of the Olivenhain and Watana Dams would argue that the benefits of these single large dams provide a much more significant benefit than four or five smaller dams trying to provide the same infrastructure service with significantly more environmental and socio-economic impacts.  Large or small, however, a dam must have a sustainable purpose and operating mission to provide a greater benefit than the environmental impacts of its existence or the risks associated with its ageing structure.  Once a project moves to a negative benefit situation, the owner must consider refurbishment, re-tasking or removal to meet that critical balance of risk and reward.  The future of dams in the US is dependent on the right dam (large or small), at the right location, at the right time.  


There continues to be sharing of lessons learned on both large and small projects – Daniel Johnson, Principal of Dams and Hydropower at AECOM in Colorado; and Joe Kula, Practice Leader for Dams, Reservoir and Levees at AECOM in Maryland, US.

The major period of dam building in the US was between the 1930s and 1980s and since then, few new dams have been planned or constructed. However, over the last two decades, a few new projects have been planned and built where needs exist.  These dam projects are planned, realising that new storage is needed for expected population growth and because of variations in water supply timing (larger storms and earlier snow melt) due to climate change. Issues regarding dam construction have gotten more complex, particularly for new dams and there is a push among environmentalists and some regulators for the decommissioning of some dams for fish passage or to alleviate dam safety or public safety issues. 

The US has two general water supply conditions. The east is considered moderate in precipitation with annual averages around 1300mm. The west is considered semi-arid with annual precipitation averages of around 400mm. These generalisations are not valid everywhere but effective in looking at differences in the needs and approaches for dam in the eastern and western portions of the US

In the east, a few new dams in the small to large size are being constructed to address water supply needs, flood control and storm water management, flow augmentation, and for replacement of ageing structures.  Additional reservoirs have been planned in several locations, but interestingly, decreased demand due to conservation measures and from the downturn in the economy in recent years (since 2008) has curtailed the permitting of several of these new dams.  

In the west, larger reservoirs are being planned and moving to construction. Larger reservoirs are perceived to be more efficient even though permitting, design and construction are significantly more complex and costly than for smaller reservoirs. In the last decade, several of these larger reservoirs have been developed.

These newer dams are primarily for off-line reservoirs constructed on smaller streams which store water pumped from a nearby river or adjacent basin.  Off-stream reservoirs typically have less environmental impact and can therefore be more easily permitted. 

Smaller dams are also being constructed in all areas of the US, most of which are for flood or storm water management, either to address existing historical flooding conditions or as part of new developments to mitigate against flooding.  Most of these structures are small enough that they do not qualify as dams from a state regulatory perspective, but some of the larger structures do. Smaller dams typically result in less land and environmental impacts are therefore, easier to permit.

Dam enlargements are a desirable alternative, if available, for providing new storage. Enlargement projects take advantage of the existing reservoir and its impacts on social and environmental conditions, and typically require less permitting effort. However, in the west the enlargement projects still have the impact of taking water for storage, many times from different water basins, involving significant impact mitigation.

Increased water supply due to population growth and increasing demand have resulted in the raising of existing dams or the construction of new large dams and water supply reservoirs.  Recent examples of off-stream reservoirs include the Rocky Pen Run Reservoir (Lake Mooney) and Cobbs Creek Reservoir Projects in Virginia, Hickory Log Reservoir in Georgia, and the Duck River Reservoir Project in Alabama.  In the west, the enlargement of San Vincente, Calaveras, and Los Vaqueros dams in California have been successful. Large reservoirs are in planning in California, such as Sites and Temperance Flats dams, and Colorado, such as Glade, Chimney Hollow, and Gross Enlargement dams.     

The key to successfully implementing a new dam project, whether large or small, in the US is to have sound justification for the demand and need, demonstrating in an alternatives analysis that a new dam or enlarged storage is the best solution; avoiding and mitigating environmental and social impacts. Both large and small dams have been completed in the last couple of decades and more are moving toward completion. Perceptibly, there is greater risk in permitting and constructing a large dam, due to the high cost of the upfront social and environmental permitting, but the benefits of a large storage volume encourage developers. There continues to be sharing of lessons learned on both large and small projects to help developers negotiate the paths necessary to start permit a dam project and start construction.


There will certainly be opportunities for the construction of large dams in the future – Eric B Kollgaard, independent consultant based in California.

In addressing this issue, one must first define what is meant by large or small dams.  The International Commission on Large Dams sets the height definition as only 15m.  I believe S David Freeman, Chairman of TVA, had in mind larger size delineation in his recommendation that China construct small dams on tributaries rather than the large Yangtze Gorge development.  For the purpose of this discussion, let’s set the height divide at around 30m and ignore setting any volume designation.

As the need for storage of water becomes ever more important, new dams both large and small will be needed.  In the continental United States suitable in-stream sites are largely already used or are on environmentally designated “Wild Rivers”.  Off-stream sites for water supply storage and pumped storage power generation are increasingly looked at as the most desirable locations for future dams.  If site conditions allow, making use of a site to maximise storage by construction of a large dam or by providing for future increased storage seems warranted.  Already we are seeing projects to raise the height of existing dams for increased storage and/or increased head for power generation coming into play.  Certainly there will be opportunities for the construction of large dams in the future.

So are more small dams needed in the US? We have to bear in mind that the size and type of dam must suit both the site conditions and satisfy the objective(s) for its construction.  Small dams are still needed and will be constructed where sites are favorable and conditions are suitable.  The expansion of small hydroelectric developments utilising low diversion and storage dams will still be sought.  Similarly, the need for dams to serve for smaller scale irrigation projects, flood control on smaller tributaries, ground water recharge basins, and off-site storage of recycled water or desalinated water will continue to exist.  All of these purposes are probably best suited for the use of small dams.

While the overall consequences of a small dam failure are generally less, the risk for smaller dams appears no different than that for large dams.  Both must have a viable reason and purpose for being in existence and both require the same adherence to good engineering principles to be a suitable design.


The impact of a dam varies with the river, the dam’s design, and the projected use – Jack Cassidy, Independent Consultant based in California.

Do we think that small dams always win over large dams?

Because there are more than 100,000 small dams in the US, many studies have been conducted in an attempt to come up with a unique answer to this question.  The result is that there is no such unique answer.  The impact of a dam varies with the river, the dam’s design, and the projected use.  The idea that large dams have a larger environmental impact stems from the fact that large dams generally have a larger footprint for both the structure and the reservoir.  Because of their purpose, a series of small dams often result in more streamflow being diverted from the river than is the case with a large dam.  However, small dams, properly designed, can have some attributes that large dams do not: 

They can be designed to bypass sediment reducing the impact on the stream regime.

And they can be designed to be more effective in safely passing fish species both upstream and downstream. 

As a result a small dam can have less impact on the environment.  Small dams built early in the history of the US were not designed to address these concerns and thus, produced significant environmental impact.  As a result some dams are no longer serving the purpose they were intended for, and are targets for removal.  A number of old dams, both small and large have been removed.

When considering engineering and design achievements – do large dams overshadow small dams?

The budget for the design and site investigation studies for a large dam is generally relatively larger than that for small dams.  As a result studies and investigations for optimising the design and improving understanding of foundation behaviour of a large dam are more apt to be conducted than is the case for a small dam, resulting in improved safety, reduced construction costs, and shorter construction time. 

The Boulder Canyon Studies conducted by the US Bureau of Reclamation (USBR) in the early 1930s, which preceded the construction of Hoover Dam, are an excellent example of more comprehensive studies related to a large dam.  While dam designers in the United States continued to employ initially developed methods in RCC structures in the 1980s, China experimented with new innovative methods and procedures on their many large concrete dams, and significantly improved the technology of concrete dam construction.  In general, all lessons learned from successes and innovations in large dam design and construction can be applied to the design and construction of small dams to produce reductions in cost, improvements in dam safety, and shortened construction times.

Can lessons from large dam construction be shared for the benefit of smaller dams in the future? Reducing the potential for cavitation through the use of air-entrainment slots in locations of large velocities was initially studied and developed on large dams by the USBR.  Depending on the height of small dams, this design concept can be readily utilised on small dams.  In addition, developments in RCC design and construction can be economically and readily employed for the construction of small dams.  The concept of using stepped spillways was also developed primarily during the design of a number of large dams.  However, this technology is equally adaptable to the design and construction of small dams, and has been used quite successfully.


In the era of the Clean power Plan hydropower is more important than ever – United States Society on Dams (USSD) – compiled by John Wolfhope, President; Dean Durkee, Vice President; Manoshree Sundaram, Treasurer; Dan Wade, Board member; and Gene Guilford, Executive Director.

Many people recognise that dams commonly provide multiple benefits, including but not limited to water supply, flood damage reduction, hydropower, navigation and recreation.  In light of the current devastating drought and growing population in the far west, there is an unprecedented need for both new large regional reservoirs as well as small local reservoirs in California and other western states for the water supply benefit alone. 

However we'd like to approach the question of the future development of large or small dams and the issues specifically confronting hydroelectric dams in light of the Environmental Protection Agency’s (EPA) Clean Power Plan.

In 2015, the US generated about 4T kWh of electricity. About 67% of the electricity generated was from fossil fuels (coal, natural gas, and petroleum).  Americans love electricity and our demand for it continues twenty-four hours a day, virtually unabated. As a nation, since the 1970s we have made enormous progress using electricity more efficiently, though we still have far to go.

In August of 2015, President Obama and the EPA issued a policy document that calls for the greatest fundamental change in how we generate our electricity since the 1970s.

The EPA Clean Power Plan [CPP] to reduce emissions from power plants is based on a combination of four strategies:

  • Making existing coal-fired power plants more efficient.
  • Shifting electricity production from coal-fired power plants to natural gas-fired power plants (which emit less carbon pollution, though emissions of the potent greenhouse gas methane associated with natural gas production remain unacceptably high).
  • Shifting electricity production to power plants that produce no carbon pollution (like wind and solar) and keeping existing zero-emitting plants (like nuclear) online.
  • Reducing energy waste using energy efficiency programs.

EPA applied these four strategies to each state's electricity mix.

The overall direction of the CPP is to reduce the nation's reliance on coal, oil and even natural gas and stresses the increased reliance on renewables; with a heavy emphasis on wind and solar. However, the single largest contributor to the nation's renewable energy production today, 51% of our renewable portfolio, is hydro – a subject not heavily emphasised in the CPP's more than 1500 pages.

Hydroelectric power plants can generate emission rate credits, or "ERCs," but there are not many new hydropower projects planned in the US. Instead, some dams are "repowering:" replacing old turbines with more efficient new ones and changing how the facility is operated to better accommodate fish, wildlife, and recreation. Repowering or relicensing an existing facility does not mean the facility can generate ERCs, unless there is increased capacity as part of the relicensing, and then only the new, extra capacity can generate ERCs. Canadian hydropower plants can also generate ERCs, but only from capacity installed in 2013 or later. Dam owners like Manitoba Hydro and Quebec Hydro export electricity to the US, and would like to export more. Canada remains and will grow as an important resource for electricity generated from hydro for the U.S.

FERC currently regulates 2540 hydroelectric dams in the United States and, despite a growing reliance on imported hydropower from sources in Canada, the US will need every hydroelectric dam it has and then some in order to meet the ambitious goals of the CPP.  These goals include achieving, by 2030,  cutting harmful carbon pollution from the power sector by 32% below 2005 levels, giving states and utilities both time and flexibility to develop strategies that work for their specific needs and that are essential for clean energy innovation.

Given the importance of our existing dams, however, new projects large and small face headwinds  – even with the need for more clean, renewable energy. FERC and state permits are often opposed with legal challenges questioning the potential impact to species, interpretation of water rights limitations, and other barriers that can impede the financial and/or technical viability of the project.

USSD and its partners across the country look forward to engaging the public, media, and policymakers with discussions about how we balance our growing need for clean, renewable hydropower that enables compliance with the Clean Power Plan while at the same time protecting the environment and important wildlife habitat.