Glossary of Hydropower Terms

The U.S. Department of Energy’s hydropower glossary contains definitions for technical terms related to hydropower. Visit Hydropower Basics to learn more about the renewable energy source and Types of Hydropower Plants to view hydropower plant illustrations.

Hydropower Terms

Note: Many of these terms are broadly used across energy sectors, and the definitions listed below refer specifically to their usage within the hydropower industry.

Adjustable-speed technology: This refers to machines that can enable variation in the power consumed (like pumps) or generated (like turbines). This provides greater flexibility as it allows facilities to dictate their power production based on grid needs.

Aerating turbines: Turbines that use low pressures created by flows that exit the turbine to induce additional airflows. These turbines enhance downstream water quality by increasing dissolved oxygen levels.

Ancillary services: Also know as capacity, grid,  and energy services. This refers to facilities that can respond to drastic grid demands on short notice. Hydropower facilities typically provide ancillary services through stable electricity delivery and optimizing grid reliability.

Balancing authority: An entity responsible for integrating future resource plans, maintaining load-interchange-generation balance within a balancing area, and supporting interconnection frequency in real-time.

Baseload: The minimum energy demand on an electrical power system over a specific period of time.

Basin-scale: A way to describe  the activities that take place in an area of land drained by a river and its tributaries.

Biodiversity: The variety of life in the world or in a particular habitat or ecosystem.

Biogenic: Produced or brought about by living organisms

Biologically-based design: A method of creating hydropower equipment (i.e. turbines) that considers its direct or indirect biological effects on fish and other aquatic species.

Black start: The process of restoring a power station to operation after a complete or partial shutdown without relying on the external electric power transmission network. This can be seen in practice when hydropower facilities are used to jump-start the grid after a blackout.

Bulk power: The large-scale generation and transmission of power from generation facilities to the grid. This is necessary to maintain reliability of the grid.

Bypass reach: The portion of a natural waterway where water is diverted for power generation or other uses. This also refers to the area at which the water returns to the main waterway. 

Capacity factor (net): A ratio of a power plant’s actual output over a period of time compared to its potential output if it were possible for it to continuously operate at full capacity over the same period of time.

Cavitation: A phenomenon that creates cavities on the metal surface of hydropower turbines. They are typically formed when vapor bubbles form and implode, due to rapid pressure changes, generating shock waves that create the cavities.

Civil works: This refers to the infrastructure of a hydropower project, such as dams, conduits, powerhouses, tunnels, and penstocks.

Closed-loop pumped storage hydropower: A pumped storage hydropower facility that consists of two reservoirs that are not connected to naturally flowing sources of water.

Condition-based maintenance: A program that recommends maintenance actions based on information collected from monitoring equipment.

Conduit: A manmade structure for moving water. Examples include canals, tunnels, and pipelines.

Control gate: A barrier that regulates the water released from a reservoir to the power generation unit in a hydropower facility.

Critical infrastructure: Assets that are considered vital to the energy, economy, health, safety, and/or security of the United States. These include storage reservoirs for water supply and flood management, dams for power production, and the electrical transmission grid.

Curtailment: Reduction of output (ramp down or shut down) of a generation unit as a response to a grid operator's request, or to market signals.

Cyber surrogate capabilities: Systems designed to help identify intrusions into the hydropower network by assessing suspicious network traffic or inconsistencies in system signals/operation. This is used to help detect cyber-attacks against hydropower facilities.

Denitrification: A decrease of dissolved atmospheric nitrogen at a reservoir.

Digitization: The translation of analog systems to digital control systems. This solves traditional challenges for hydropower facilities and enables access to a new range of opportunities for the industry.

Digitalization: The process of how translated digital control systems (see: digitization) are used to modify business practices. Typically, this process enhances how hydropower plants operate fundamentally.

Digital Transformation: The application of digital capabilities to solve traditional challenges for hydropower and enable access to a new range of opportunities for the industry.

Dispatch: The process of operating of a generating unit (i.e. turbine) within a power system (hydropower facility) at a designated output level to meet the demand for electricity.

Distributed generation: Small, grid-connected energy generation systems located close to the load (communities) they serve.

Diversion: A facility or piece of infrastructure that channels a portion of a waterway through a canal or penstock.

Draft tube: A water conduit (either curved or straight), that allows water to flow from the point where water exits a turbine  (the turbine outlet) to where it rejoins the original waterway downstream.

Economic dispatch:  The process of a hydropower facility producing power for the grid at a designated output level to meet demand for electricity and generate energy at the lowest possible cost.

Electrical demand: The rate at which electricity is being consumed at a given instant. This can also refer to power consumption averaged over a specified period of time.

Electricity generation: The amount of electricity a generator produces during a specific period of time.

Energy arbitrage: Purchasing (or storing) energy when electricity prices are low, and selling (or discharging) energy when electricity prices are high.

Energy imbalance services (reserves): A market service provided for the management of unscheduled deviations (situations where the actual amount of power generated and consumed deviates from the planned or scheduled amount without prior notification or approval) in an individual facility.

Entrainment: The unplanned passage of a fish or aquatic organism through a turbine or cooling water intake pipe at the hydropower facility.

Environmental flows: The quantity, timing, and quality of water that must move through the hydropower facility to downstream locations to protect ecosystems and human well-being.

Fish ladder: A transport structure for safe upstream fish passage around hydropower projects.

Fish passage structure: Structure on or around a dam to facilitate the movement of migrating fish.

Fixed-speed technology: Pumps and turbines that are operated at a constant speed.

Flexibility: The ability of the power system or individual unit (like a hydropower facility) to quickly respond to changes in supply and/or demand.

Flow: The volume of water (expressed as cubic feet or cubic meters per second) passing a point in a given amount of time.

Flow regime: The magnitude, duration, timing, seasonality, and rate of change of flows in a natural waterway.

Forebay: An impoundment or reservoir directly above a dam or intake structure at a hydropower plant.

Frequency regulation: Efforts by a grid balancing authority to maintain scheduled power flow in the grid.

Frequency response: The ability of power generating facilities to increase and/or decrease output to maintain the required amount of power on the grid.

Generator: Device that converts the rotational energy from a turbine to electrical energy.

Grid: An electricity transmission and distribution system.

Head loss: The energy lost due to friction as water moves through a dam. This can be due to factors such as turbines, valves and turbulence.

Headwater: The water located upstream from the dam or powerhouse.

Hydraulic head: A measure of liquid pressure expressed in terms of the height of a column of water. This is used to represent the potential energy of the water.

Hydroacoustics: This can refer to both underwater sound as well as technology to monitor fish passage, abundance, and distribution.

Hydrologic cycle: Earth’s natural water cycle. The cycle consists of evaporation, condensation, precipitation, interception, infiltration, percolation, transpiration, runoff, and storage.

Hydropeaking: The process of releasing water through turbines at irregular intervals to meet peak energy demands. This can sometimes cause downstream water flow fluctuations.

Hydropower: The harnessing of flowing water—using a dam or other type of diversion structure—to create energy that can be captured via a turbine to generate electricity. Also called hydroelectric power.

Impoundment: A body of water created by a structure that obstructs the flow of a waterway, such as a dam.

Independent Power Producer: Any entity that owns or operates an electricity generating facility that is not owned by a utility organization.

Independent System Operator: Organization that coordinates, controls, and monitors operation of the electrical power system within a specified geographic region.

Intake: A structure that diverts water from a natural waterway into the turbine.

Interconnection: Major points within a grid where smaller, regional grids interconnect with each other.

Load: The amount of electrical power delivered or required at a specific point or points on a system.

Load following/Load shifting: The ability of a hydropower plant to adjust its power output as electricity demand changes throughout the day.

Load-following reserves: Additional capacity available to accommodate load variability and uncertainty. This is critical to maintaining a reliable grid as power demands will fluctuate over time. 

Marine and hydrokinetic technologies: Devices that capture energy from waves, tides, ocean currents, the natural flow of water in rivers, and marine thermal gradients--these devices are also broadly referred to as marine energy or marine renewable energy technologies. Typically, these technologies do not leverage hydraulic heads as part of their power capture approach.

Modernization: This refers to upgrading or adding new hydropower system capabilities to a facility.

Nameplate capacity (installed): This refers to the maximum output of a generator, or other electric power production equipment, under specific conditions designated by the manufacturer.

New stream-reach: This refers to waterways that have not been developed for hydropower. They are somtimes referred to as greenfield sites.

Non-powered dams: Dams that do not have any electricity generation equipment installed.

Non-spinning operating reserves: Additional capacity that is not connected to the system but can be made available to meet demand. They are also known as supplemental reserves.

Open-loop pumped storage hydropower:  Two reservoirs that are continuously connected to naturally flowing sources of water.

Peaking: A method of operating a hydropower facility in which power is produced only during periods of peak demand.

Peaking power plant: The process of operating a power plants to help balance the fluctuating power requirements of the electric grid.

Penetration: A way to define the portion of energy produced for a grid by select generating sources (such as hydropower and geothermal) as compared to the total generation.

Penstock: A closed conduit or pipe for moving water from one area of the hydropower facility to another. Usually, it moves the forebay to turbines in the powerhouse.

Power: The rate of production or consumption of energy. Electric power is the rate at which electrical energy is transferred by an electric circuit (i.e. the grid).

Powerhouse: The structure that contains generators and turbines at a hydropower facility.

Practical resource: A portion of a technical resource that is available when other constraints—including economic, environmental, and regulatory—are factored in.

Pumped storage hydropower (PSH): Type of hydropower project where energy can be stored and generated by moving water between two reservoirs of differing elevations.

Qualified Hydroelectric Facility: A facility owned or solely operated by a non-Federal entity that generates hydroelectric energy for sale. These facilities are added to an existing dam or conduit.

Ramp rate: The rate at which flows from the powerhouse into the tailwater and downstream into the natural waterway are increased or decreased.

Ramping capability: The ability of a power station to change its output over time.

Reactive supply: Portion of electricity from a power facility used to sustain the electric and magnetic fields of alternating current (AC) equipment, such as transformers.

Regional Transmission Operator: These are organizations responsible for moving and monitoring electricity over specific interstate areas. They are similar to Independent System Operators, which coordinate, control, and monitor operation of the electrical power system within a specified geographic region.

Regulating reserves: This refers to the capacity available for providing fast, real-time balancing services.

Rehabilitation: The process of expanding, upgrading, and improving efficiency of existing hydropower facilities.

Relicensing period: The period during which a hydropower licensee must notify regulators if it intends to seek a new license for its project. The process begins when the licensee files a 'notice of intent'. This must happen at least five years before a license expires. If the licensee declares its intent to seek a new license, it must file the application for a new license at least two years before its current license expires. 

Reregulating reservoir: A reservoir located downstream from a hydropower peaking plant with the capacity to store fluctuating discharges and release them according to environmental flow needs.

Reservoir: A body of water that builds up behind a dam. This can also be referred to as an impoundment.

Resource potential: The amount of power that could be generated from a particular resource. This can also be referred to as theoretical, technical, and practical potential.

Rotor: The rotating inner portion of a generator consisting of a series of windings that surround the field poles (electromagnets).

Rough zone: This refers to the part of the range between the minimum and maximum output of a facility that should be avoided. It is typically avoided due to deteriorating impacts on plant equipment that can be caused from vibration.

Runoff: This refers to water created from precipitation, snowmelt, glacial melt, or irrigation. This water is in uncontrolled surface streams, rivers, drains, or sewers.

Runner: The rotating part of the turbine that converts the energy of falling water into mechanical energy.

Run-of-river: A type of hydropower project in which a portion of a waterway is channeled through a canal and/or penstock. This allows the facility to utilize the natural decline of the waterway's elevation to produce energy. 

Spillway: A structure used to provide the release of flows from a dam into a downstream area.

Spinning reserves: This refers to power generating systems (in the case of hydropower, turbines) that are connected to the power grid but are intentionally unused. This allows hydropower facilities to rapidly create power in a short period of time to meet sudden changes in electricity demand. 

Storage: The storing of water in a reservoir during periods of high inflow that can be used later to generate electricity.

Sustainable hydropower/sustainability: To construct of modify a hydropower facility or facilities so that it is sited, designed, constructed, and operated in a manner that provides long term net benefits to the public owners of a resource. This may involve balancing social, environmental, and economic objectives at multiple geographic scales (e.g., national, regional, basin, site).

Tailrace: The channel that carries water away from a dam.

Tailwater: The water immediately downstream of the powerhouse or dam.

Technical resource: Portion of a theoretical resource that can be captured by using a specific technology.

Theoretical resource: Portion of a resource that could potentially be captured using a specific technology under ideal circumstances.

Transformer: A device for changing alternating current (AC) to higher or lower voltages.

Transmission: Conveyance of electrical energy from generation facilities to local distribution systems.

Turbidity: Measure of the relative clarity of a fluid. This is commonly used as a measure of water quality.

Turbine: A machine that produces power by water flowing through a wheel or rotor causing it to spin and generate energy.

Variable renewable generation resource: A renewable energy source that fluctuates due to natural circumstances not controlled by the operator, such as wind and solar.

Watershed: Land that water flows across or under on its way to a stream, river, lake, or ocean.

Weir: A barrier built across a stream or river to alter its flow characteristics (i.e. speed, turbulence, and the way it moves).

Wholesale power market: A market where any entity that can generate power and connect to the grid can compete to sell their power output.

Wicket gates: Adjustable elements that control the flow of water to the turbine.