Glossary/water dictionary

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backwater effect

The rise in surface elevation of flowing water upstream from and as a result of an obstruction to flow, such as a narrow bridge opening, buildings or fill material that limits the area through which the water must flow. In stream gaging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by flooding of the stream below. The difference between the observed stage and that indicated by the stage-discharge relation is reported as backwater. Also referred to as 'heading up'. ^

backwater pools

A pool type formed by an eddy along channel margins downstream from obstructions such as bars, rootwads, or boulders, or resulting from backflooding upstream from an obstructional blockage. Backwater pools are sometimes separated from the channel by sand or gravel bars.^

balance of water resources and needs

The usable water resource of a certain water management unit in a given period of investigation, and the assessment and comparison of quantitative and qualitative characteristics of the water requirements to be supplied by this resource.^

balanced groundwater scenario (BGS)

A term referring to the development of a scenario exploring changes in cropping patterns such that long-term ground water withdrawals do not exceed long-term groundwater recharge rates. ^

balanced operation

Operation of a canal system where the water supply exactly matches the total flow demand.^

ball cock

A self-regulating device controlling the supply of water in a tank, cistern or toilet by means of a float connected to a valve that opens or closes with a change in water level.^

ball valve

A valve regulated by the position of a free-floating ball that moves in response to fluid or mechanical pressure.^


Heavy material, often seawater, placed in the hold of a ship to gain stability. Periodic discharges of this ballast water from oil tankers constitute a significant portion of the oil introduced into the oceans of the world each year.^


A major landform comprising distinctively round topped ridgeline remnants of fan alluvium. The ridge’s broadly rounded shoulders meet from either side to form a narrow crest and merge smoothly with the concave backslopes. In ideal examples, the slightly concave footslopes of adjacent ballenas merge to form a smoothly rounded drainage-way.^

bank and channel stabilisation

Implementation of structural features along a stream bank to prevent or reduce bank erosion and channel degradation.^

bank storage

The water absorbed into the banks of a stream, lake, or reservoir when the stage rises above the water table in the bank formations, then returns to the channel as effluent seepage when the stage falls below the water table. Bank storage may be returned in whole or in part as seepage back to the water body when the level of the surface water returns to a lower level.^

bank, banks

The slope of land adjoining a body of water, especially adjoining a river, lake or a channel. With respect to flowing waters, banks are either right or left as viewed facing in the direction of the flow. As 'banks', a large elevated area of a sea floor.^

bankfull channel

The stream channel that is formed by the dominant discharge, also referred to as the active channel, which meanders across the floodplain as it forms pools, riffles and point bars.^

bankfull stage

The stage at which a stream first begins overflows its natural banks. More precisely, an established river stage at a given location along a river which is intended to represent the maximum safe water level that will not overflow the river banks or cause any significant damage within the river reach. 'Bankfull stage' is a hydraulic term, whereas 'flood stage' implies resultant damage.^


(1) A Christian sacrament marked by ritual use of water and admitting the recipient to the Christian community. (2) A non-Christian rite using water for ritual purification.^


(1) A sand or gravel deposit found on the stream bed that is often exposed during low-water periods. (2) An elongated landform generated by waves and currents, usually running parallel to the shore, composed predominantly of unconsolidated sand, gravel, stones, cobbles or rubble and with water on two sides. (3) A component landform comprised of elongate, commonly curving, low ridges of well-sorted sand and gravel that stand above the general level of a 'bolson floor' and were built by the wave action of a Pleistocene lake. (4) A unit of pressure equal to 106 dynes per cm2 , 100 kilopascals, or 29.53 inches of mercury.^

bar racks

Closely spaced rods, often in the form of a screen, that remove large solids from the wastewater entering a sewage treatment plant. ^

bar screen

In wastewater treatment, a device used to remove large solid materials. ^


An instrument that records simultaneous barometric pressure and temperature on the same chart. ^


An artificial obstruction, such as a dam or an irrigation channel, built in a watercourse to increase its depth or to divert its flow either for navigation or irrigation. Sometimes the purpose is to control peak flow for later release.^


A dark volcanic rock composed of microscopic grains of augite, feldspar and olivine. Some basalts have many holes that give the rock a swiss-cheese-like appearance. As the lava cools, gases escape, leaving holes of different sizes. ^

basalt aquifers

Aquifers found in basalt rock in areas of past volcanic activity, particularly in the Pacific Northwest region of the United States and in Hawaii.^

base (1)

(1) Any of various typically water-soluble and bitter tasting compounds that in solution have a pH greater than 7, are capable of reacting with an acid to form a salt, and are molecules or ions able to take up a proton from an acid or able to give up an unshared pair of electrons to an acid. (2) Chemicals that release hydroxide ions (OH– ) in solution. Such solutions have a soapy feel, neutralise acids, and conduct electricity.^

base (2)

A substance that is neutral and has a pH of more than 7. A base has fewer free hydrogen ions (H+) than hydroxyl ions (OH-).^

base flow (1)

The sustained or dry weather flow of streams resulting from the outflow of permanent or perched groundwater, and from the drainage of lakes and swamps. Also included are waters from glaciers, snow and other possible sources not resulting from direct runoff.^

base flow (2)

Sustained flow of a stream in the absence of direct runoff. It includes natural and human-induced streamflows. Natural base flow is sustained largely by groundwater discharges.^

base flow (3)

(1) The flow characteristic to which a perennially flowing stream reduces during the dry season. It is supported by groundwater seepage into the channel. (2) The fair-weather or sustained flow of streams; that part of stream discharge not attributable to direct runoff from precipitation, snowmelt or a spring. Discharge entering streams channels as effluent from the groundwater reservoir. (3) The volume of flow in a stream channel that is not derived from surface runoff. ^

base level

(1) The elevation to which a stream-channel profile has developed. (2) The lowest level to which a land surface can be reduced by the action of running water.^

base period

A period of time specified for the selection of data for analysis. The base period should be sufficiently long to contain data representative of the averages and deviations from the averages that must be expected in other periods of similar and greater length. For example, the U.S. Weather Bureau computes values of average, heavy and light monthly precipitation from data observed during the base period of 1931–1960. For groundwater studies, the base period should both begin and end at the conclusion of a dry trend so that the difference between the amount of water in transit in the soil at the ends of the base period is minimal.^

base runoff

Sustained or fair weather runoff. In most streams, base runoff is composed largely of groundwater effluent. The term 'base flow' is often used in the same sense as base runoff. However, the distinction is the same as that between streamflow and runoff. When the concept in the terms base flow and base runoff is that of the natural flow in a stream, base runoff is the more appropriate term.^

base width

(1) The time interval between the beginning and end of the direct runoff produced by a storm. (2) The time period covered by a unit hydrograph. (Baseline) The condition that would prevail if no action were taken.^

baseline (data)

A quantitative level or value from which other data and observations of a comparable nature are referenced. Information accumulated concerning the state of a system, process, or activity before the initiation of actions that may result in changes. (Basic) Describing a solution, sediment, or other material that has a pH greater than 7.0.^

baseline study

Study of the environmental conditions and organisms existing in a region prior to unnatural disturbances. ^

basic hydrologic data

Includes inventories of features of land and water that vary only from place to place (e.g., topographic and geologic maps), and records of processes that vary with both place and time (e.g. records of precipitation, streamflow, groundwater, and quality-of-water analyses). 'Basic hydrologic information' is a broader term that includes surveys of the water resources of particular areas and a study of their physical and related economic processes, interrelations and mechanisms.^


(1) (Hydrology) A geographic area drained by a single major stream; consists of a drainage system comprised of streams and often natural or man-made lakes. Also referred to as 'drainage basin', 'watershed' or 'hydrographic region'. (2) (Irrigation) A level plot or field, surrounded by dikes, which may be flood irrigated. (3) (Erosion control) A catchment constructed to contain and slow runoff to permit the settling and collection of soil materials transported by overland and rill runoff flows. (4) (Nautical) A naturally or artificially enclosed harbor for small craft, such as a yacht basin.^

basin irrigation

An irrigation method in which crops are surrounded by a border to form a submersion check called basin of round, square or any other form. Irrigation water generally comes directly from the supply ditch/canal or from other basins.^


The solid rock beneath the soil and superficial rock. A general term for solid rock that lies beneath soil, loose sediments, or other unconsolidated material^


Substances that increase in concentration in living organisms as they take in contaminated air, water, or food because the substances are very slowly metabolised or excreted. ^


The increase in concentration of a chemical in organisms that reside in environments contaminated with low concentrations of various organic compounds. Also used to describe the progressive increase in the amount of a chemical in an organism resulting from rates of absorption of a substance in excess of its metabolism and excretion.^


A method for quantitatively determining the concentration of a substance by its effects on the growth of a suitable animal, plant or microorganism under controlled conditions.^

biochemical oxidation

The process by which bacteria and other micro-organisms feed on complex organic materials and decompose them. Self-purification of waterways and activated sludge and trickling filter wastewater treatment processes depend on this principle.^

biochemical oxygen demand (BOD)

(1) A measure of the quantity of dissolved oxygen, in milligrams per liter, necessary for the decomposition of organic matter by micro-organisms, such as bacteria. (2) A measure of the amount of oxygen removed from aquatic environments by aerobic micro-organisms for their metabolic requirements. Measurement of BOD is used to determine the level of organic pollution of a stream or lake. The greater the BOD, the greater the degree of water pollution. Also referred to as 'biological oxygen demand' (BOD).^

biochemical oxygen demand (BOD) loading

BOD content, commonly expressed in pounds/day, of wastewater passing into a waste treatment system or a body of water. The greater the BOD content, the greater the degree of pollution. ^


A chemical substance that kills living organisms. Typically used to include materials that can kill desirable as well as undesirable organisms^

bioclimatic zones

Also referred to as 'biomes', these constitute the earth’s 10 zones differentiated by climate, soil, water, and plant and animal life. ^


A community of animal and plant life. ^


(1) The process by which an individual organism directly concentrates a substance from the surrounding air, water, or soil. (2) The increase in concentration of a chemical in an organism resulting from absorption levels exceeding the rate of metabolism and excretion.^

bioconcentration factor (BCF)

Used to describe the accumulation of chemicals in aquatic organisms that live in contaminated environments.^

bioconcentration. bioconcentration potential (BCP)

The maximum concentration of a chemical in an organism resulting from the rate of absorption equaling the rate of metabolism and excretion.^


The conversion of organic materials, such as plant or animal waste, into usable products or energy sources by biological processes or agents, such as certain microorganisms.^


Capable of being decomposed by biological agents or microorganisms, especially bacteria. The property of a substance that permits it to be broken down by micro-organisms into simple, stable compounds such as carbon dioxide and water. ^


The metabolic breakdown of materials into simpler components by living organisms. ^


The controlled use of microbes, usually bacteria, to reduce level of nitrates (NO3 – ) and thereby reclaim contaminated water or wastewater. The process consists of several stages to decompose the nitrates first into nitrites and then into nitrogen gas, N2 . Upon entering the treatment process, sodium sulfite (Na2SO3 ) is added as a reducing agent to the wastewater to remove the oxygen from the water. To break down the nitrates, the bacteria must have a carbon food source, and typically ethanol is added for the bacteria to feed on. In order to survive, however, the bacteria need oxygen, which they obtain by breaking down the nitrate ions, first to nitrite and then to harmless nitrogen gas. Also referred to as 'endogenous respiration'. ^


A large, rotating cylinder possessing surface features that allow for the growth of attached micro-organisms. The cylinder revolves and contacts the wastewater along one side, while the other side is exposed to air, thereby maximising the oxygenation of the water and stimulating decomposition of dissolved or suspended organic material.^


The gradual accumulation of waterborne organisms (as bacteria and protozoa) on the surfaces of engineering structures in water that contributes to corrosion of the structures and to a decrease in the efficiency of moving parts. ^


Methane gas produced during the anaerobic decomposition of the remains of plants or animal wastes by bacteria. ^


A living organism that denotes the presence of a specific environmental condition. For example, the presence of coliform bacteria identifies water that is contaminated with human fecal material. ^

biological activated carbon (BAC) process

The combination of ozonation and granular activated carbon (GAC) for the removal of dissolved organics, particularly dissolved organic carbon (DOC) from drinking water. This water treatment method has seen more widespread use in Europe primarily due to: (1) the generally poorer quality of surface waters there; (2) the greater concern and more stringent standards for chlorination byproducts; and (3) strict aesthetic demands of European consumers. Also referred to as 'biologically enhanced activated carbon process'.^

biological additives

Cultures of bacteria, enzymes or nutrients that are introduced into an oil discharge or other wastes to promote decomposition.^

biological community

All of the living things in a given environment.^

biological control

The direct human introduction of living organisms – predators, parasites, or pathogens – to eliminate or control undesirable species. The practice is usually considered an ecologically sound alternative to the application of chemical pesticides. ^

biological diversity

The number and kinds of organisms per unit area of volume; the composition of species in a given area at a given time. ^

biological magnification

Refers to the process whereby certain substances such as pesticides or heavy metals move up the food chain, work their way into rivers or lakes, and are eaten by aquatic organisms such as fish, which in turn are eaten by large birds, animals or humans. The substances become concentrated in tissues or internal organs as they move up the chain. ^

biological oxidation

Decomposition of complex organic materials by micro-organisms. Occurs in the self-purification of water bodies and in activated sludge wastewater treatment processes.^

biological oxygen demand (BOD)

1) The amount of oxygen required to stabilise decomposable matter by aerobic action. (2) An indirect measure of the concentration of biologically degradable material present in organic wastes. It usually reflects the amount of oxygen consumed in five days by biological processes breaking down organic waste. Also referred to as 'biochemical oxygen demand' (BOD).^

biological processes

Processes characteristic of, or resulting from, the activities of living organisms.^

biological treatment

A treatment technology that uses bacteria to consume organic wastes. In biological wastewater treatment, the use of bacteria to degrade and decompose organic materials in wastewater.^


(1) The science of life and of living organisms, including their structure, function, growth, origin, evolution and distribution. It includes botany and zoology and all their subdivisions. (2) The life processes or characteristic phenomena of a group or category of living organisms. (3) The plant and animal life of a specific area or region. ^


The process by which a substance is passed up the food chain, resulting in an especially high level of the substance at upper levels of the food chain. A biological process wherein a contaminant’s concentration increases at each level up the food chain, including humans. Thus, the availability of such contaminants, even in the seemingly insignificant parts per trillion range, often are ecologically important.^


(1) The total mass of living matter within a given unit of environmental area, typically expressed as mass per unit area or volume of habitat. (2) Plant material, vegetation or agricultural waste used as a fuel or energy source.^


Water that contains animal, human or food wastes; wastewater from toilets, latrines and sinks used for food preparation or disposal of chemical or chemical-biological ingredients. ^


To scald or parboil in water or steam in order to remove the skin from, whiten, or stop enzymatic action (as with, for example, food for freezing).^

blue-green algae

A group of phytoplankton that often cause nuisance conditions in water, so called because they contain a blue pigment in addition to chlorophyll. ^

blue water

The open sea.^

bog hole

A hole containing soft mud or quicksand.^

boiling point

The temperature at which the vapour pressure of a liquid is equal to the pressure exerted on the liquid. (Water) When the atmospheric pressure is 86 centimeters of mercury (sea level), the boiling point of water is, by definition, 100 degrees Celsius, or 212 degrees Fahrenheit. The boiling point decreases with elevation. ^

boiling water reactor (BWR)

A nuclear reactor in which water, used as both coolant and moderator, is allowed to boil in the core. The resulting steam can be used directly to drive a turbine generating electric power. ^


The vaporisation of liquid. Bolson — An alluvium-floored basin, depression, or wide valley, mostly surrounded by mountains and drained by a system that has no surface outlet; an undrained or an internally drained intermontane basin. Bolson fill is the alluvial Detritus that fills a bolson; also commonly called bolson deposits. Also see Semi-Bolson.^


An alluvium-floored basin, depression, or wide valley, mostly surrounded by mountains and drained by a system that has no surface outlet; an undrained or an internally drained intermontane basin. Bolson fill is the alluvial detritus that fills a bolson; also commonly called 'bolson deposits'. ^


A floating device used to contain oil on a body of water. ^

border ditch

A ditch used as a border of an irrigated strip or plot, water being spread from one or both sides of the ditch along its entire length.^

border irrigation

A sub-system of controlled flood irrigation in which the land is divided into parallel border strips demarcated from one another by earth ridges. Water is successively delivered into each strip from a head or field ditch at its upper end. On the upstream part of each strip is a flat zone, the level portion from which the stream of water spreads evenly across the entire downstream portion.^


A solution that is resistant to pH changes, or a solution or liquid whose chemical makeup tends to neutralise acids or bases without a great change in pH. Surface waters and soils with chemical buffers are not as susceptible to acid deposition as those with poor buffering capacity.^

buffer strips

(1) Strips of grass or other erosion-resisting vegetation between or below cultivated strips or fields. (2) Grassed or planted zones that act as a protective barrier between an area which experiences livestock grazing or other activities and a water body. Also referred to as a 'buffer zone'. ^

buffer zone

(1) A protective, neutral area between distinct environments. (2) An area that acts to minimise the impact of pollutants on the environment or public welfare. For example, a buffer zone may be established between a compositng facility and nearby neighborhoods to minimise odor problems.^