Glossary of Terms


Abutment: The part of the valley edge against which a dam is constructed. An artificial abutment is sometimes constructed where there is no suitable natural abutment. Right and left abutments are those on respective sides of an observer when viewed looking downstream. The wall between a spillway or gate structure and the embankment can also be referred to as an abutment.

Adaptive Management Plan: A system of management practices based on clearly identified timeframes and outcomes with monitoring to determine if management actions are meeting outcomes, and, if they are not, facilitating management changes that will best ensure that outcomes are timely met or to reevaluate the outcomes. Adaptive management recognizes that knowledge about natural resource systems is sometimes uncertain and is the preferred method of management in those cases.

Aerator: Forced air systems used to re-aerate water released downstream of a powerhouse.

Ambursen Dam: A buttress dam in which the upstream part is a relatively thin flat slab usually made of reinforced concrete.

Anadromous Fish: Fish that begin their life cycle in freshwater, then migrate as juveniles to the ocean, where they grow into adults before migrating back into freshwater to spawn.

Angled Bar Racks: Angled bar racks and louvers are used to direct downstream migrating fish toward bypasses and sluiceways rather than through a hydropower plant. These structural guidance systems do not physically exclude fish from powerhouse intakes, but instead create hydraulic conditions in front of the structures to discourage fish from entering the intakes.

Appurtenant Facilities: Refers to the non-integral aspects of a hydropower facility such as substations and transmission lines.

Arch Dam: A concrete, masonry, or timber dam with the alignment curved upstream so as to transmit the major part of the water load to the abutments. This dam type is similar to a gravity structure however arch dams utilize a narrowing upstream curve which directs water pressure against the shoreline, providing the force to compress and maintain the strength of the dam.

Attraction Flow: Auxiliary water supplied at an upstream or downstream fishway entrance that creates hydraulic signals strong enough that fish will sense the fishway route and pass through it in a timely manner to avoid undue delay, fatigue, injury, and/or mortality.

Axial Flow Turbine: A turbine in which water flows through in a parallel direction to the turbine shaft.


Bascule Gate: An adjustable gate used to regulate water flow through a dam. This gate type lowers to open and raises to close.

Base Flow (Aquatic Base Flow): The term Aquatic Base Flow was coined by the US Fish and Wildlife Service to describe a set of chemical, physical and biological conditions that represent limiting conditions for aquatic life and wildlife in stream environments. In hydrological terms, it means median August flow as calculated by USFWS.

Basin-scale (or Basin-wide): Basin-scale or basin-wide encompasses the activities that occur within the area of land drained by a river and its tributaries.

Bathtub Spillway: Double side-channel spillway.

Berm: A nearly horizontal step in the sloping profile of an embankment dam. Also a step in a rock or earth cut.

Bifurcation: Division into two branches.

Broome Gate: A type of roller-mounted vertical lift gate supported along either side by stainless steel roller chains.

Brownfields: A property, the expansion, redevelopment, or reuse of which may be complicated by the presence or potential presence of a hazardous substance, pollutant, or contaminant.

Bulb Turbine: An axial flow type of propeller turbine in which the turbine and generator are a sealed unit placed directly in the water stream.

Butterfly Valve: A valve mechanism that regulates the flow of materials within large pipe diameters by the quarter-turn rotation of a disk. The butterfly valve is a quarter-turn valve. It works like a ball valve for quick shutoff, but unlike a ball valve, the butterfly valve can also open and close incrementally. This difference means that the disc is always present in the flow and allows for a drop in pressure.

Buttress Dam: Reinforced concrete buttresses proportioned to transfer the water load to the foundation rock through individual buttress footings, using the weight of the impounding water and the weight of the structure to provide resistance against sliding and overturning.

Bypass: A channel or conduit through or around a dam that provides river flow and a route for fish to pass without going through the turbine units.

Bypassed Reach: The area in a waterway between the initial point where water has been diverted through turbines or other mechanical means for the purpose of water-powered generation of electricity and the point at which water is released into the waterway below the turbines or other mechanical means of electricity generation.


Canal: A man-made open channel for conveying water typically from a reservoir to a powerhouse.

Catadromous: Fish that begin their life cycle in saltwater, then migrate as juveniles into freshwater, where they grow into adults before migrating back into the ocean to spawn. American eels are the only catadromous fish in North America.

CERCLA (Superfund Act): The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), commonly known as Superfund, was enacted by Congress on December 11, 1980. This law created a tax on the chemical and petroleum industries and provided broad Federal authority to respond directly to releases or threatened releases of hazardous substances that may endanger public health or the environment.

Conservation Easement: A voluntary legal agreement between a landowner and a land trust or government agency that permanently limits uses of the land in order to protect its conservation values. Landowners retain many of their rights, including the right to own and use the land, sell it and  pass it on to their heirs.

Consumptive Water Withdrawals: Quantity of water lost to evaporation, plant, and industrial process loss.

Crest Gate: A gate on the crest (top) of a spillway to control the discharge or reservoir water level. These gates are hinged at the bottom and open by rotating from vertical to horizontal.

Cross Flow Turbine: A cross-flow turbine is drum-shaped and uses an elongated, rectangular section nozzle directed against curved vanes on a cylindrically shaped runner. The cross-flow turbine allows water to flow through the blades twice. On the first pass, water flows from outside of the blades to the inside; the second pass goes from the inside back out. A guide vane at the entrance to the turbine directs the flow into a limited portion of the runner.


Dam: A barrier that impounds water, wastewater, or any liquid-borne material, for the purpose of storage or control of water.

DDT: Dichloro-diphenyl-trichloroethane was developed as the first of the modern synthetic insecticides in the 1940s. It was initially used with great effect to combat malaria, typhus, and the other insect-borne human diseases among both military and civilian populations...The US Department of Agriculture began regulatory actions in the late 1950s and 1960s to prohibit many of DDT's uses because of mounting evidence of the pesticide's declining benefits and environmental and toxicological effects. In 1972, EPA issued a cancellation order for DDT based on its adverse environmental effects, such as those to wildlife, as well as its potential human health risks. Since then, studies have continued, and a relationship between DDT exposure and reproductive effects in humans is suspected, based on studies in animals.

Delaware Style Eel Ladder: An upstream eel passage constructed by providing a hole through flashboards, surface gates, or other structures near the crest of the dam. By passing trawl netting or similar rope-like material though the hole (and optionally sheathed in a length of PVC pipe to train the flow), a roughened route for eels to ascend over the dam is created.

Denil Style Fishway: Denil designs are a family of baffled-chute ladders that utilize roughness elements (i.e., baffles) to dissipate the kinetic energy of water moving through a flume to create a low velocity zone of passage for migratory fish.

Diadromous: Diadromous is a general category describing fish that spend portions of their life cycles partially in fresh water and partially in salt water. These represent both anadromous and catadromous fish.

Dioxins: Dioxins refers to a group of toxic chemical compounds that share certain chemical structures and biological characteristics. Dioxins are highly toxic and can cause cancer, reproductive and developmental problems, damage to the immune system, and can interfere with hormones.

Diversion: The transfer of water from a stream, lake, aquifer, or other source of water by a canal, pipe, well, or other conduit to another watercourse or to the land, as in the case of an irrigation system.

Diversion Dam: A dam built to divert water from a waterway or stream into a different watercourse.

Double Curvature Arch Dam: An arch dam that is curved both vertically and horizontally.

Draft Tube: A conduit that carries water from a turbine runner to the tailrace. Designed to maximize head and power generation.


Earth Dam: An embankment dam in which more than 50% of the total volume is formed of compacted earth layers which are generally smaller than 3-inch size.

Eel Ladder/Eel Ramp/Eelway: An inclined trough over which water flows from above to below a dam that eels use to ascend the dam.

Embankment Dam: Massive dam created by the placement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock, with a typically dense impervious core.

Entrainment: Fish that are inadvertently drawn into a powerhouse intake structure and through a turbine.

Essential Fish Habitat: Under the Magnuson-Stevens Fishery Conservation and Management Act, EFH applies to federally managed species. EFH includes all types of aquatic habitat where these managed fish spawn, breed, feed, or grow to maturity.

Estuarine: Areas where freshwater meets saltwater. Highly productive ecosystems.

Exemption: An exemption is a small hydroelectric project having limited environmental impacts and meeting certain criteria so that it is exempt from licensing under Part I of the Federal Power Act. Exemptions are issued in perpetuity, and include mandatory environmental terms and conditions prescribed by state and federal fish and wildlife agencies.

Extirpated: To completely remove or locally eliminate a species population from the facility’s affected area.


False Attraction Flow: Water flows or turbulence that impede the ability of fish to locate an upstream or downstream fishway. False attraction flows can result from turbine discharge flows, spill flows over a dam, or through the operation of dam gates.

Fauna: Animal species of a region or habitat.

FERC: Federal Energy Regulatory Commission - The independent federal agency in charge of regulation and oversight of interstate transmission of natural gas, oil, and electricity as well as oversight of natural gas and hydropower projects.

Fish Passage: The ability for fish to move safely and effectively upstream and downstream of an artificial obstruction.

Fish Screen: A barrier installed to divert fish away from a canal or powerhouse intake or outlet.

Fishway: The combination of elements (structures, facilities, devices, operations, and measures) necessary for movement of fish past a barrier.

Flashboards: Structural members of timber, concrete, or steel placed in channels or on the crest of a spillway to raise the reservoir water level but intended to be quickly removed, tripped, or fail in the event of a flood.

Floodgate: A gate to control flood releases from a reservoir.

Floodplain Forest: Floodplain forests usually occur in the low, flood-prone areas along rivers, typically less than 20 feet above the river channel. They are often associated with oxbows (pools that have become separated from the river channel), temporary wetlands that dry up in summer (vernal pools), open meadows of grasses and wildflowers, and dense shrub thickets.

Floodplains: A floodplain is a generally flat area of land next to a river or stream.

Flora: Plant species of a particular region or habitat.

Flow Duration Curve: The flow duration curve is a plot that shows the percentage of time that flow in a stream is likely to equal or exceed some specified value of interest. For example, it can be used to show the percentage of time river flow can be expected to exceed a design flow of some specified value (e.g., 20 cfs), or to show the discharge of the stream that occurs or is exceeded some percent of the time (e.g., 80% of the time).

Flume: A man-made structure for conveying water in the form of an open, declined gravity chute with walls that are raised above the surrounding terrain.

Forebay: The impoundment immediately upstream of a dam or hydroelectric plant intake structure.

Francis Turbine: The Francis turbine is a reaction turbine, which means it develops power from the combined action of pressure and moving water. The runner is placed directly in the water stream so the water is flowing over the blades rather than striking each individually. A Francis turbine has a runner with fixed blades, usually nine or more. Water is introduced just above the runner and all around it which then falls through, causing the blades to spin. Besides the runner, the other major components include a scroll case, wicket gates, and a draft tube. The Francis turbine was the first modern hydropower turbine and was invented by British-American engineer James Francis in 1848 and first used at the Lowell Hydroelectric Project (LIHI #142) in Massachusetts.

Freshet: Spring thaw of snowpack and ice that runs off into a water body in a short period of time.


Gate: A structure, typically on a dam or canal, that allows passage of water when open.

Governor: The device which measures and regulates turbine speed by controlling wicket gate angle to adjust water flow to the turbine.

Gravity Dam: Constructed from concrete or stone masonry and designed to hold back water by primarily using the weight of the dam itself to resist the horizontal pressure of water pushing against it.


Head: Head is the difference between the water surface elevations upstream (the reservoir) and downstream of a project (the tailrace), providing the energy for the falling water.

Headgate: A gate that controls water flow into a canal, sluiceway or powerhouse intake.

Headwaters: The initial source of a river or stream.

Helical: Spiral.

Hollow Gravity Dam: A dam constructed of concrete and/or masonry on the outside but having a hollow interior and relying on its weight for stability.

Hydraulic Fill Dam: An earth dam constructed of materials, often dredged, which are conveyed and placed by suspension in flowing water.


Impingement: The physical contact of a fish with an intake structure (such as a trash rack or fish screen) due to water velocities that are too high to allow the fish to swim away.

Impoundment: A body of water created by erecting a barrier to flow, e.g., dam and diversion structures. Typically used in reference to run-of-river facilities.

Inflatable flashboards: An inflatable structure affixed to a dam crest used to manage impoundment levels.

Inflow: The rate or volume of water that flows into a reservoir or forebay during a specified period.

Instream Flow Incremental Methodology: A science-based habitat evaluation technique that is a quantitative environmental assessment procedure that describes how habitat quantity and quality change over a range of physical conditions, such as stream flows or impoundment surface elevations. The term refers to a relatively large number of assessment methods. The most important aspect of these techniques is that they involve an analytical framework that considers the tradeoffs between multiple ecological processes and resource management objectives and evaluates a range of flow options over a continuum of hydrologic conditions.

Instream Flows: Water volumes and levels in a stream needed to support aquatic life, water quality, consumptive uses or recreation, synonymous with other commonly used terms such as ‘environmental’ flow and ‘ecological’ flow.

Intake: The structure leading to a conduit/penstock. Can be integral to the powerhouse to divert water to the turbines.


Kaplan Turbine: A Kaplan turbine is a reaction propeller-type turbine which generates power from the combined forces of pressure and moving water. A runner is placed directly in the water stream, allowing water to flow over the blades rather than striking each individually. In a Kaplan turbine both the blades and the wicket gates are adjustable, allowing for a wider range of operation. This turbine was developed by Austrian inventor Viktor Kaplan in 1919.


Leakage Flow: Uncontrolled loss of water by flow through a hole or crack in a dam or diversion structure, or through flashboards.

Littoral: The area of a waterbody closest to the shore in which sunlight can reach sediment, supporting growth of aquatic flora.

Load Following: Power generation is adjusted throughout a day in response to demand for electricity.

Louver Racks: Angled bar racks and louvers are used to direct fish toward bypasses and sluiceways at hydropower plants. These structural guidance systems are devices that do not physically exclude fish from intakes, but instead create hydraulic conditions in front of the structures to divert fish away from the intakes.


Masonry Dam: A dam constructed mainly of stone, brick, or concrete blocks pointed with mortar. A dam having only a masonry facing should not be referred to as a masonry dam.

Minimum Flow: The minimum flow that must be released from a project in order to meet environmental or other non-power water requirements. Flows are released either to a bypassed reach or downstream of the project.

MOA: Memorandum of Agreement - a document written between parties to cooperatively work together on an agreed upon project or meet an agreed upon objective.

MOU: Memorandum of Understanding - a document describing a bilateral agreement between parties.

Multiple Arch Dam: A buttress dam comprised of a series of arches for the upstream face.


National Geodetic Vertical Datum: NGVD - A vertical datum is a surface of zero elevation to which heights of various points are referenced. The NVGD of 1929 was utilized until it was superceded in 1992 to be replaced with the North American Vertical Datum of 1988 for the contiguous US and Alaska.

National Register of Historic Places: The National Register of Historic Places is the official list of the Nation's historic places worthy of preservation. Authorized by the National Historic Preservation Act of 1966, the National Park Service's National Register of Historic Places is part of a national program to coordinate and support public and private efforts to identify, evaluate, and protect America's historic and archeological resources.

Nature-like Fishway: Nature-like fishways mimic natural side channels and are placed next to migration barriers. They include a wide variety of designs such as step‐pools, roughened ramps, rock‐arch rapids, rocky riffles, and cross vanes which are typically constructed of boulders, cobble, and other natural materials to create diverse physical and hydraulic conditions providing efficient passage to multiple species of migratory and resident fish.

NGVD: National Geodetic Vertical Datum - A vertical datum is a surface of zero elevation to which heights of various points are referenced. The NVGD of 1929 was utilized until it was superceded in 1992 to be replaced with the North American Vertical Datum of 1988 for the contiguous US and Alaska.

NPDES: The National Pollutant Discharge Elimination System permit program addresses water pollution by regulating point sources that discharge pollutants to waters of the United States. Hydropower projects often have NPDES permits for non-generation discharges from cooling water and industrial wastewater systems.

Nutrient Loading: The amount of nutrients entering a system in a period of time. The overloading of nutrients in a system leads to eutrophication, causing excessive growth of algae.


Ogee: A double curve shaped like an elongated S, usually associated with a spillway configuration designed with optimal overflow characteristics.

Outlet: An opening through which water can be freely discharged from a reservoir to the river for a particular purpose.

Outlet Gate: A gate controlling the flow of water through a reservoir outlet.

Overflow Dam: A dam designed to be overtopped.

Oxbow: An oxbow wetland is a meander of a stream, river or creek, that has become separated from the flow of water. Oxbow wetlands store excess water that might otherwise lead to flooding, filter water to improve water quality and provide habitat to a variety of wildlife.


PCBs: Polychlorinated Biphenyls are a group of man-made organic chemicals consisting of carbon, hydrogen and chlorine atoms. PCBs do not readily break down once in the environment. They can remain for long periods cycling between air, water, and soil. They are also taken up into the bodies of small organisms and fish. As a result, people who ingest fish may be exposed to PCBs that have bioaccumulated in the fish they are ingesting.

Peaking: A peaking project is one in which water can be stored in a reservoir until needed, and then released through turbines to generate power to help supply the peakload demand.

Pelton Turbine: The Pelton turbine is an impulse turbine, which means that it generally uses the velocity of the water to move the runner. The water stream hits each bucket on the runner. There is no suction on the down side of the turbine, and the water flows out the bottom of the turbine housing after hitting the runner. An impulse turbine is generally suitable for high head, low flow applications.

Penstock: An enclosed pipe-like structure that conveys water directly from a reservoir forebay to a powerhouse.

Plunge Pool: A natural or artificially created pool that dissipates the energy of free falling water.

Point Source: Any single identifiable source of pollution from which pollutants are discharged, such as a pipe, ditch, ship or factory smokestack.

Polycyclic Aromatic Hydrocarbons: Polycyclic aromatic hydrocarbons (PAHs) are a group of over 100 different chemicals that are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat.

Potamodromous: Potamodromous fishes move and complete their life cycle entirely within freshwater.

Powerhouse: A structure that contains turbines, generators, and associated equipment for the production of electrical power.

Primary Contact Recreation: Recreational activities in which quantities of water may be ingested. Examples include swimming, surfing, etc.

Programmable Logic Controller: A PLC (programmable logic controller) is a digital computer used for industrial automation to automate different electro-mechanical processes. At hydropower plants, PLC are often used to monitor and control key functions such as impoundment levels, minimum flow releases, turbine operation, and gate operation.

Propeller Turbine: A type of reaction turbine (generates power from the combined forces of pressure and moving water). Propeller turbines generally have a runner with three to six blades. Water contacts all of the blades constantly with constant pressure. The pitch of the blades may be fixed or adjustable. The major components besides the runner are a scroll case, wicket gates, and a draft tube. There are several different types of propeller turbines including Kaplan, bulb, straflo and tube.

PURPA: Public Utility Regulatory Policies Act of 1978, enacted following the energy crisis of the 1970s to encour­age cogeneration and renewable resources and promote competi­tion for electric generation. The statute imposes mandatory purchase obligations on electric utilities for power generated by cogeneration facilities and small power production facilities called "qualifying facilities."


Radial Flow Turbine: A type of turbine in which the water flows perpendicular to the shaft.

Radial Gate: A gate with a curved upstream plate and radial arms hinged to piers or other supporting structure.

REC: A renewable energy credit is a market-based instrument that represents the property rights to the environmental, social and other non-power attributes of renewable electricity generation.

Regulating Dam: A dam impounding a reservoir from which water is released in a controlled manner to regulate inflow that is then discharged downstream.

Reservoir: A body of water impounded by a dam and in which water can be stored. Typically used in reference to store-and-release facilities.

Riffle: Riffles are the shallower, faster moving sections of a stream or river.

Riparian: Riparian zones are the areas of land bordering streams and rivers, forming the transition area between aquatic and terrestrial systems. When riparian areas are intact and filled with trees, shrubs, grasses, and/or other vegetation, they act as buffers, protecting the river from the adverse effects of adjacent and nearby land use. Healthy riparian zones can capture and filter sediment, excess nutrients, and pollution from surface runoff. They can provide temperature regulation through shading to keep water cool. Plant roots hold the sponge-like soils in place providing bank stability and preventing erosion, and those soils can soak up and store excess water to reduce flooding during high flows and then release it slowly during periods of low flow. Riparian zones also offer habitat for a diverse community of plants and animals and provide critical inputs for the aquatic food web in the form of leaves, and other organic materials that drop from the vegetation into the water.

River Basin: The area drained by a river and all of its tributaries. Also referred to as the drainage area.

Rock Fill Dam: An embankment dam in which more than 50% of the total volume is comprised of compacted or dumped cobbles, boulders, rock fragments, or quarried rock generally larger than 3-inch size.

Roller Compacted Concrete Dam: A concrete gravity dam constructed by the use of a dry mix concrete transported by conventional construction equipment and compacted by rolling, usually with vibratory rollers.

Roller Gate: A dam gate designed to control flow through large waterway openings. They may be designed as either upward or downward (skimmer) opening. A roller or fixed wheel gate consists of a fabricated steel slide with cast iron rollers and rubber seals.

Rubber Dam: An inflatable structure affixed to a dam crest used to manage impoundment levels.

Rubble Dam: A stone masonry dam in which the stones are unshaped or uncoursed.

Rule Curve: A mathematical curve or family of curves indicating how a reservoir is to be operated under specific conditions to obtain best or predetermined results. Rule curves can be designated to regulate water storage for flood control, hydropower production, and other operating objectives, as well as combinations of objectives.

Run of River: A mode of hydropower operation where the outflow from a project matches the inflow to the impoundment. on an instantaneous, hourly, or other time basis.

Runner: The rotating part of the turbine that converts the energy of falling water into mechanical energy. The rotating shaft connects to an electrical generator.


Saddle Dam: A subsidiary dam of any type constructed across a saddle or low point on the perimeter of a reservoir.

Secondary Contact Recreation: Water contact is limited to the limbs, no ingestion of water. Boating, fishing, etc.

Section 106: Section 106 of the National Historic Preservation Act of 1966 requires federal agencies to consider the effects on historic properties of projects they carry out, assist, fund, permit, license, or approve throughout the country. If a federal or federally-assisted project has the potential to affect historic properties, a Section 106 review will take place.

Siphon: An inverted enclosed pipeline structure that conveys water under roads, drainage channels, rivers, etc.

Slab and Buttress: Reinforced concrete buttresses proportioned to transfer the water load to the foundation rock through individual buttress footings, using the weight of the impounding water and the weight of the structure to provide resistance against sliding and overturning.

Slide Gate: A hydraulic gate that operates in vertical guides and has no wheels, rollers, or other friction-reducing devices. Normally, such a gate must be opened or closed under balanced head conditions.

Sluice: An opening for releasing water from below the static head elevation of an impoundment. Usually equipped with a gate to control the flow and sometimes used for downstream fish passage. Can also be used like a skimmer gate to move debris downstream.

Spillway: A structure over or through which flow is discharged from a reservoir. If the rate of flow is controlled by mechanical means, such as gates, it is considered a controlled spillway. If the geometry of the spillway is the only control, it is considered an uncontrolled spillway.

Stanchion: A structure composed of vertical support bars or frames typically made of steel or concrete. A stanchion may be part of gate system and houses the mechanisms used to raise or lower a gate. A stanchion bay is a spillway section in which the frame creates a slot to hold stop logs or flashboards used to retain water in an impoundment. When stanchions are triggered by high flows and the logs or boards are released, flows discharge rapidly. In some cases, the logs or boards cannot be replaced until the impoundment level recedes below the elevation of the bay.

Stave: A narrow length of wood with a slightly bevelled edge to form the sides of barrels, tanks, tubs, vats and pipelines. In hydropower, wood stave pipelines or penstocks convey water from a dam to a powerhouse located at a distance.

Stilling Basin (or Pool): A basin constructed to dissipate the energy of rapidly flowing water, e.g., from a spillway or outlet, and to protect the riverbed from erosion or to support fish habitat below a dam.

Stop Logs: Large logs, timbers, or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit so as to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate.

Substation:  A high-voltage electric system facility. It is used to switch generators, equipment, and circuits or lines in and out of a system. It also is used to change AC voltages from one level to another, and/or change alternating current to direct current or direct current to alternating current.

Substructure: The lower part of a building which is constructed below the ground level. The function of substructure is the transfer of loads from the superstructure to the underlying soil.

Surge Tank: A hydraulic structure designed to control pressure and flow fluctuations in a penstock or tunnel. It functions as a reservoir that temporarily stores or releases water to the turbine.

Switchyard: A concentration of electrical equipment which connects two or more electric circuits through switches, selectively arranged in order to permit a circuit to be disconnected or to change the electric connection between the circuits. In a hydroelectric project, the switchyard is the point at which the energy generated at the project is connected to the transmission or distribution system.


Tailrace: The river channel into which water is discharged after passing through the outlet works or turbines.

Tainter Gate: A gate with a curved upstream plate and radial arms hinged to piers or other supporting structure.

Timber Crib: A dam constructed of timber cells filled with rock ballast and covered with sheathing on the water side to minimize leakage.

Timber Crib Dam: A gravity dam built up of boxes, crossed timbers or gabions (cages), filled with earth or rock.

TMDL: Total Maximum Daily Load - The calculation of the maximum amount of a pollutant allowed to enter a waterbody so that the waterbody will meet and continue to meet water quality standards for that particular pollutant.

Toe Drain: A system of pipe and/or pervious material along the downstream toe of a dam used to collect seepage from the foundation and embankment and convey it to a free outlet.

Transformer: An electromagnetic device used to change the voltage of AC electricity.

Trap and Haul (or Trap and Truck): At dams and places where it is not practical to install a fish ladder, a trap and haul operation can be used to move fish upstream of a dam. Fish are attracted to flow provided at the base of a fish ladder. They climb the ladder to a loading system where they wait in pools or tanks before transfer into specialized tankers or barges. These vehicles release the fish into the river on the other side of the dam.

Trashrack: A rack or screen of parallel bars installed at an intake to prevent debris from entering a canal or turbine.

Turbidity: The extent to which water has become clouded as a result of suspended sediments.


Water Classification: Categories of waters defined by states or tribes in their water quality standards that are based on the water's suitability for different uses such as irrigation, recreation, aquatic habitat, drinking water, or industrial, or other human uses.

Wetland: Lands including swamps. marshes, bogs, and similar areas such as wet meadows, river overflows, mud flats, and natural ponds. An area characterized by periodic inundation or saturation, hydric soils, and vegetation adapted for life in saturated soil conditions.

White Nose Syndrome: White-nose syndrome (WNS) is the fungal disease killing bats in North America. Research indicates the fungus that causes WNS, Pseudogymnoascus destructans, is likely exotic, introduced from Europe. WNS causes high death rates and fast population declines in the species affected by it, and scientists predict some regional extinction of bat species.

Wicket Gates: Adjustable vanes that control the flow of water to a turbine.

Wild and Scenic River: The National Wild and Scenic Rivers System was created by Congress in 1968 to preserve certain rivers with outstanding natural, cultural, and recreational values in a free-flowing condition for the enjoyment of present and future generations.


303(d) Impaired Water Listing: The term is short for a state’s list of impaired and threatened waters (e.g. stream/river segments, lakes). States are required to submit their list for EPA approval every two years. For each water on the list, the state identifies the pollutant causing the impairment, when known. States often submit an "integrated report" that includes section 305(b) water quality reports for all state waters.

4(d) Rule: Section 4(d) of the federal Endangered Species Act (ESA) directs the US Fish and Wildlife Service to issue regulations deemed “necessary and advisable to provide for the conservation of threatened species.” It allows the Service to promulgate special rules for species listed as threatened (not endangered) that provide flexibility in implementing the ESA. 4(d) rules are used to target the take prohibitions to those that provide conservation benefits for the species. This targeted approach can reduce ESA conflicts by allowing some activities that do not harm the species to continue, while focusing the Service's efforts on the threats that make a difference to the species’ recovery.