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Overview of hydroelectric technologies, description of the benefits and barriers to hydroelectric, description of the steps necessary to install hydroelectric, relevant legislation in the Commonwealth of Massachusetts, financing options, and examples of hydroelectric installations.

Icon - Hydroelectric Hydroelectric Overview

Hydropower, or hydroelectric power, refers to the use of water to power machinery or to generate electricity. Historically, humans have been using water as a source of power for thousands of years for things such as running mills and sawing wood. In fact, by 1889, over 200 electricity plants used water for all or some of their electricity generation. 

Hydropower is the most used form of renewable energy in the United States, comprising 75% of renewable energy generation and approximately 10% of our country’s total electricity generation, or over 90,000 megawatts of electricity.  The United States is the second largest producer of hydropower, surpassed only by Canada, and produces enough power to supply twenty-eight million households with electricity, the equivalent of saving 500 million barrels of oil. 

However, it should be noted that most hydropower potential in the Commonwealth has been utilized to date, meaning there very little new hydropower potential exists in Massachusetts.


How Does Hydropower Work?

Water is constantly moving through a large global cycle by evaporating from rivers and lakes, transpiring from vegetation, condensing into clouds, precipitating as rain and snow, and falling back down to the earth to start the process again; this cycle is called the hydrologic cycle (Figure 1) and is powered by the sun. 

The movement of this water, particularly of flowing water, produces kinetic energy which can be captured and turned into mechanical energy through the use of turbines. This mechanical energy can then be converted into electricity by a generator and used to power buildings, facilities, and other systems. 

CleanEnergyToolkit - Hydroelectric 1

Figure 1: Hydrologic Cycle.  Source: USGS

There are several ways to transform the kinetic energy of water into electricity:

  • Impoundment
  • Pumped Storage
  • Dam-free Hydrokinetic Systems.  These unconventional hydropower technologies do not rely on the conventional methods listed above, but instead utilize ocean wave energy and/or tidal currents. For more information, please visit the Ocean/Tidal Energy page

CleanEnergyToolkit - Hydroelectric 2Impoundment

Impoundment is the most common type of hydropower facility, where a dam is used to back up water in a pond or reservoir. When the water is released from the dam it enters a turbine, spins it and activates the generator in order to produce electricity (Figure 2).  (Sometimes the turbine is not located right at the dam, but at some distance down a canal into which the water is diverted.)  The reservoir can act like a battery, storing water until it is needed.  Alternatively, the water can be released at the same rate as it flows into the reservoir, in which case the facility is said to operate in “run-of-river” mode. 


Figure 2: Impoundment Facility. Source: US DOE Energy Efficiency and Renewable Energy


Pumped Storage
Pumped storage facilities store energy by pumping water between reservoirs. During periods of low electricity demand, a pumped storage facility pumps water from a lower reservoir to an upper reservoir.  If there is high electrical demand, the water from the upper reservoir is released back to the lower reservoir to generate electricity. However, please note that under the Massachusetts Green Communities Act, pumped storage projects are not eligible as a renewable source of energy as they are viewed as a non-sustainable source of electricity.

The capacity of impoundment, diversion, and pumped storage systems varies from large systems which are over 30 megawatts, to small capacity systems, which are between 100 kilowatts and 30 megawatts, to micro capacity facilities which generate up to 100 kilowatts of electricity.  


Benefits and Barriers of Hydropower


  • No Greenhouse Gas Emission: Hydropower is a clean fuel source (water) that doesn’t require the burning of fossil fuels, meaning its use emits no air pollution or greenhouse gas emissions (note: there are often emissions associated with pumped storage systems).
  • Less Expensive Than Traditional Energy Sources: Once a hydropower facility is built all future fuel costs are negated, however, there are costs associated with monitoring and maintaining the facility.
  • Domestic Energy Source: Water is a local energy resource which reduces our demand for foreign fuel.
  • Recreational Sites: Impoundments often have recreational uses, such as fishing, boating and swimming.
  • Aid in Flood Control: Dams and reservoirs are designed to store waters that might cause flooding during storms.



  • Fisheries and Wildlife Impacts: Dams can create barriers to migrating fish and other aquatic and riverine organisms, including reptiles, amphibians, and mammals, creating obstacles that they are unable to navigate around. This can lead to the loss or decline of certain species that rely on fluvial conditions for survival. In some cases, fish ladders have been used to partially mitigate the adverse impact of dams by allowing fish to migrate to their nursing grounds. Dams also present a significant impediment to aquatic and riverine organisms' successful adaptation to changes in temperature, rainfall patterns and water quality issues expected to result from the impacts of climate change. Dams can also create artificial drought and/or artificial floods conditions in downstream areas, significantly disturbing the natural habitat.
  • Water Quality: Impoundment may impact water quality by increasing turbidity and decreasing the amount of dissolved oxygen in water, leading to the degradation of aquatic habitats. Additionally, reservoirs may increase water temperatures and increase the amount of trapped sediment and nutrient in the water column due to lack of water flow. This can lead to undesirable growth of algae and aquatic weeds, which can further disrupt the aquatic environment.
  • Recreational Impacts: Reservoirs created by dams stop the free flow of the river preventing whitewater rafting and fishing for species that depend on colder or faster flowing waters.
  • Vulnerability in Times of Drought: In times of drought, water flows may be insufficient to power the turbines. In these cases, hydroelectric systems won’t be able to generate enough power to meet market demands (or provide revenue to their owners).


Installing Hydroelectric

If your municipality is interested in exploring the potential for hydropower at an existing dam, some first steps include identifying the property owner, identifying the owner of the water rights, consulting the local Conservation Commission to understand ecological issues in the area, and estimating the generation potential of the site based on annual available flows.  To sell electricity from the facility, you will likely need a license from the Federal Energy Regulatory Commission.  You can review the Federal Energy Regulatory Commission’s Guide to Developing Small/Low-Impact Hydropower Projects to learn more about licensing your own small hydro facility.

It should be noted that most of the sites appropriate for river-based hydropower in Massachusetts have already been developed. Due to stricter environmental regulations and increased knowledge about the adverse impacts on natural habitat, not all areas that were suitable for hydropower in the past are good candidates today.  However, another avenue to greater hydroelectricty generation in the Commonwealth is via increasing the efficiency of existing hydropower facilities through upgrades and modernization, the installation of generation at dams originally built for other purposes (such as water supply), or the installation of turbines in conduits that serve water distribution or wastewater discharge purposes. 

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  • Federal Energy Regulatory Commission: The Commission's major hydropower activity is re-licensing existing projects whose licenses are about to expire. Staff prepare either an Environmental Assessment (EA) or an Environmental Impact Statement (EIS) and base recommended license conditions on these reviews. The Commission also regulates over 1,700 non-federal dams in the US.
  • Green Communities Act: Among other provisions, the Green Communities Act enables the Massachusetts Renewable Energy Trust to continue funding clean energy projects, including hydropower.



  • Green Communities Division: The Green Communities Act provides up to $10M annually in grants and loans for communities that qualify as a Green Community as specified in the Act. The Green Communities Division in DOER is responsible for developing guidelines for qualifying communities and administering the program.  Funds are available to qualifying communities for renewable energy projects.
  • Massachusetts Renewable Energy Trust: The Massachusetts Renewable Energy Trust has a variety of different programs to help fund municipal clean energy projects, including hydropower. These programs include funding for conducting feasibility studies for clean energy and funding for project implementation.



There are many hydroelectric power plants throughout New England. Two examples are: 

  • City of Holyoke Gas and Electric (MA): In 2007, the City of Holyoke’s Gas and Electric Department generated 187,168 Megawatt hours from its hydroelectric plants. This renewable energy resource amounted to 50.6% of all electricity consumed within the City.
  • City of Norwich Public Utilities Department (CT): owns and operates three local sources of hydro-electric power. The Greeneville dam, one of the most popular, was originally built in 1829 and re-commissioned in 1880 and 1915 to widen the headgates and increase the water capacity of the canal. In 1961 Norwich Public Utilities purchased the Dam with the intention of building a hydroelectric station, which was completed in 1966 and has been in operation since.



  • Hydro Research Foundation: Through research and education programs, the Hydro Research Foundation advances knowledge in and builds public awareness of the benefits of hydroelectric power to natural resources and the electric power supply of the United States and the world.
  • Low Impact Hydropower Institute: This non-profit organization is dedicated to reducing the impacts of hydropower generation through the certification of hydropower projects that have avoided or reduced their environmental impacts pursuant to the Low Impact Hydropower Institute’s criteria.
  • Massachusetts Department of Environmental Protection: Provides information on the State's wetlands protection program for "land under water bodies" and access to Chapter 91 Waterways Licenses.
  • Massachusetts Riverways Program:  Promotes the restoration and protection of the ecological integrity of the Commonwealth's watersheds: rivers, streams and adjacent lands
  • National Hydropower Association (NHA): The National Hydropower Association is a nonprofit national association dedicated exclusively to advancing the interests of the hydropower industry. It seeks to secure hydropower's place as a climate-friendly, renewable and reliable energy source that serves national environmental and energy policy objectives.
  • Natural Resource, Agriculture, and Engineering Service (NREAS): Created an overview for the installation of small hydroelectric plants.
  • U.S. Department of Energy's (DOE's) Hydropower Program: The US DOE conducts research and development (R&D) that strives to improve the technical, societal, and environmental benefits of hydropower and provide cost-competitive technologies that will enable the development of new and incremental hydropower capacity, adding diversity to the nation's energy supply.



ICLEI's Municipal Clean Energy Toolkit was generously funded by the Massachusetts Renewable Energy Trust.