M&I (Municipal and Industrial) Water Withdrawals (Use)  Water supplied for municipal and industrial uses provided through a municipal distribution system.
Maar  A flat-bottomed, roughly circular volcanic crater of explosive origin that is often filled with water.
Macroclimate  The general large-scale climate of a large area or country, as distinguished from Mesoclimate and Microclimate.
Macroinvertebrate  An animal without a backbone, large enough to see without magnification.
Macronutrient  A chemical element necessary in relatively large amounts (usually more than one part per million (ppm) in the plant) for the growth of plants.
Macrophyte  (1) A member of the macroscopic plant life, especially of a body of water.  (2) The macroscopic plants in the aquatic environment.  The most common macrophytes are the rooted vascular plants that are usually arranged in zones in aquatic ecosystems and restricted in the area by the extent of illumination through the water and sediment deposition along the shoreline.
Macrophytic Algae  Algal plants large enough either as individuals or communities to be readily visible without the aid of optical magnification.
Maelstrom  A whirlpool of extraordinary size or violence.
Magma  (Geology) Molten rock found in the mantle, beneath the cruse of the earth.  When forced toward the surface, magma cools and solidifies to become Igneous rock.
Magmatic Water  Water driven out of Magma during crystallization.
Magnetic Separation  The use of magnets to separate ferrous materials from mixed municipal waste stream.
Main  A relatively large pipe in a distribution system for drinking water or in a collection system for municipal wastewater.  Of or relating to utility distribution mains for transferring water.  Often used in the plural, as in water mains.
Main Canal System  A canal that delivers water from a primary source of supply to several points of diversion or canal-side turnouts to smaller distribution systems.
Main Channel Pool (California)  A pool formed by mid-channel scour that encompasses greater than sixty percent of the wetted channel.
Mainstem  (1) The major reach of a river or stream formed by the smaller tributaries which flow into it.  (2) The principal watercourse of a river, excluding any tributaries.
Major Flooding  Flood conditions resulting in extensive inundation and property damage.  Typically characterized by the evacuation of people and livestock and the closure of both primary and secondary roads.  Also see Minor Flooding and Moderate Flooding.
Major Landform  A subdivision of the piedmont slope or basin floor major physiographic parts that reflects a major morphogenetic process operating through a long time, or that is the prominent result of a special erosional or depositional history.  Many major landforms are dissected and their original area now is occupied by Component Landforms.
Majors  Larger Publicly-Owned Treatment Works (POTWs) with flows equal to at least one million gallons per day (mgd) or servicing a population equivalent to 10,000 person, certain other POTWs having significant water quality impacts.  Contrast with Minors.
Make Call  See Call the River.
Makeup Water  Water added to the flow of water used to cool condensers in electric power plants.  This new water replaces condenser water lost during passage of the cooling water through cooling towers or discharged in blowdowns.
Malpais  (Geology) A Southwest United States term for rough country underlain by basaltic lava.
Management Indicator Species (MIS)  (Environmental) A species selected because its welfare is presumed to be an indicator of the welfare of other species in the habitat.  A species whose condition can be used to assess the impacts of management actions on a particular area.  Managing for these species usually requires significant allocations of land or resources.  Also see Indicator Species.
Management Scenario  (Ecology) A description of future conditions expected to result from the general implementation of a broad resource management strategy.  Management scenarios are developed to explore the biological and social implications, tradeoffs, and uncertainties of Ecosystem Management rather than present a range of options for site specific adoption (management alternatives).
Managerial Controls  (Irrigation) Methods of nonpoint source pollution control based on decisions about managing agricultural wastes or application times or rates for agrochemicals.
Mangrove  Tropical evergreen trees and shrubs that have stilt like roots and stems, and often form dense thickets along tidal shores.  Also see Mangrove Swamp.
Mangrove Swamp  A tidal swamp forest populated by plant species capable of growth and reproduction in areas that experience periodic tidal submergence in seawater with a resulting increase in saline conditions.  These forests develop along coastal regions in tropical climates.  Mangrove swamps are dominated by trees referred to as red mangrove, Rizophora mangle, black mangrove, Avicennia germinans, and white mangrove, Laguncularia racemosa.  Typically, these trees have large, exposed root systems.
Manmade Lake  Any manmade body of water, including lakes, ponds, lagoons, and reservoirs (excluding tank-type reservoirs which are fully enclosed and contained), that are filled or refilled with water or reclaimed wastewater from any source and used for recreational, scenic, or landscape purposes, except swimming pools.
Manometer  An instrument for measuring pressure which usually consists of a U-shaped tube containing a liquid, the surface of which in one end of the tube moves proportionally with changes in pressure on the liquid in the other end.  The term is also applied to a tube-type differential pressure gage.
Mantle  (Geology) The division of the earth's interior between the core and the crust.  It is composed mainly of silicate rock and is around 2,900 kilometers (1,800 miles) thick.
Mare Clausum  A navigable body of water, such as a sea, that is under the jurisdiction of one national and closed to all others.
Mare Liberum  A navigable body of water, such as a sea, that is open to navigation by vessels of all nations.
Mare Nostrum  A navigable body of water, such as a sea, that is under the jurisdiction of one nation or that is shared by two or more nations.
Mariculture  The cultivation of marine organisms for use as a food resource.  Compare to Aquiculture.
Marina  A water-based facility used for storage, service, launching, operation, or maintenance of watercraft.
Marine  (1) Of or pertaining to the sea; having to do with the ocean or the things peculiar to the ocean.  (2) A system within the Wetlands and Deepwater Habitat Classification System.  Also see Deepwater Habitats and Wetlands.  (See Appendix D-2 for an explanation of the Wetland and Deepwater Habitat Classification System according to USFWS criteria.)
Marine Life  Plants and animals of the sea, from the high-tide mark along the shore (also see Shore Life) to the depths of the ocean.  These organisms fall into three major groups:  (1) Benthos plants such as kelp and animals such as brittle stars that live on or depend on the bottom; (2) Nekton  swimming animals such as fishes and whales that move independently of water currents; and (3) Plankton  various small to microscopic organisms that are carried along by the currents .
Marine Protection, Research, and Sanctuaries Act (MPRSA)  A 1972 federal law that includes provisions requiring citizens of the United States to obtain a permit from the U.S. Environmental Protection Agency (EPA) before disposing of materials in the oceans.  Subsequent amendments to the act have limited the types of waste that may be permitted for ocean disposal.
Marine Sanitation Device  Any equipment or process installed on board a vessel to receive, retain, treat, or discharge sewage.
Marine (Nautical) Surveying  The branch of surveying that comprises a topographic survey of the coast and a hydrographic survey of adjacent waters.  Also see National Oceanic and Atmospheric Administration (NOAA).
Maritime Law  Branch of law relating to commerce and navigation on the high seas and on other navigable waters.  Specifically, the term refers to the body of customs, legislation, international treaties, and court decisions pertaining to ownership and operation of vessels, transportation of passengers and cargo on them, and rights and obligations of their crews while in transit.
Mark  (Nautical) A knot or piece of material placed at various measured lengths on a lead line to indicate the depth of the water, or, more generally, measurement indicators of water depth, e.g., a Plimsoll mark.
Marl  A mixture of clays, carbonates of calcium and magnesium, and remnants of shells, forming a loam useful as a fertilizer.
Marsh  A term frequently associated with Wetlands.  An area of soft, wet, low-lying land, characterized by grassy vegetation that does not accumulate appreciable peat deposits and often forming a transition zone between water and land.  A tract of wet or periodically inundated treeless land, usually characterized by grasses, cattails, or other monocotyledons (sedges, lilies, irises, orchids, palms, etc.).  Marshes may be either fresh or saltwater, tidal or non-tidal.
Marsh Gas  Gas produced during the decomposition of organic material buried in wetland soils.  The primary gas produced is Methane, CH4.
Marshland  Treeless land in which the water table is at, above, or just below the surface of the ground; it is dominated by grasses, reeds, sedges, and cattails.  These plants typify Emergent Vegetation, which has its roots in soil covered or saturated with water and its leaves held above water.
Marsh, Tidal  A low, flat area traversed by interlacing channels and tidal sloughs and periodically inundated by high tides.  Vegetation in such areas usually consists of salt-tolerant plants, or Halophytes.
Mass Curve  A graph of the cumulative values of a hydrologic quantity (such as precipitation or runoff), generally as the Ordinate (y-axis), plotted against time or date as the Abscissa (x-axis).  Mass curves may also be used to show the excavated or filled material per unit of distance for a canal or other earth structure.
Mass Movement  (Geology) The downslope movement of a portion of the land's surface (i.e., a single landslide or the gradual downhill movement of the whole mass of loose earth material) on a slope face.  All movement of soil and bedrock materials occurring below the soil surface such as landslips, landflows, rock slides, slumps, etc.
Mass Spectrometry  An analytical technique wherein ions are separate according to their ratio of charge to mass.  From the mass spectrum produced, the atomic weight of the particle can be deduced.
Mathematical Model  A representation of physical laws or processes expressed in terms of mathematical symbols and expressions (i.e., equations).  The model is used as a basis for computer programs for examining the effect of changing certain variables in the analysis of the effect of flow changes in a water delivery system, for example.  Also see Econometric Model Building.
Matric Potential  The work per unit quantity of pure water that has to be done to overcome the attractive forces of water molecules and the attraction of water to solid surfaces.  The matric potential is negative above a water table and zero below a free water table.
Matrix  (1) Solid framework of a porous material or system.  (2) The material in which an environmental sample is embedded or contained, whether it is soil, water, dried biomass, or other substance.
Matrix Interference  The adverse influence of the environmental sample Matrix on the ability to detect the presence or amount of a chemical substance in the sample.
Matter  Anything which is solid, liquid, or gas and has mass.
Mattress  (Environmental) A blanket of poles, brush, or other material interwoven or otherwise lashed together and weighted with rock, concrete blocks, or held in place to cover an area subject to scouring by flowing water.
Masonry Dam  A dam constructed mainly of stone, brick, or concrete blocks that may or may not be joined with mortar.  A dam having only a masonry facing should not be referred to as a masonry dam.  Also see Dam.
Maximum Acceptable Toxicant Concentration (MATC)  The highest concentration at which a pollutant can be present and not exert an adverse effect on the Biota, used to experimentally determine the toxicity of the chemical.
Maximum Contaminant Level (MCL)  (1) Legally enforceable standards regulating the maximum allowed amount of certain chemicals in drinking water.  MDLs must be met by the time water reaches an individual's property.  (2) The designation given by the U.S. Environmental Protection Agency (EPA) to water quality standards promulgated under the Safe Drinking Water Act (SDWA) (Public Law 93-523).  As prescribed by the EPA after research of a contaminant, the MCL is the greatest amount of a contaminant that can be present in drinking water without causing a risk to human health.  MCLs are set for certain inorganic and organic chemicals, turbidity, coliform bacteria, and certain radioactive materials.  Also see Drinking Water Standards and Drinking Water Standards (Nevada).
Maximum Contaminant Level Goal (MCLG)  The designation given by the U.S. Environmental Protection Agency (EPA) to water quality standards promulgated under the Safe Drinking Water Act (SDWA) (Public Law 93-523).  This is a number which is associated with no adverse health effects.  If someone drinks water for a lifetime containing the contaminant at this level, there should be no ill effects.  As implied by the name, this number is a goal, not an enforceable standard.  For chemicals which are believed to cause cancer (carcinogens), the MCLG is set at zero because there is no known safe level for this type of chemical.
Maximum Depth (Reservoir)  The greatest depth of the body of water measured in feet and 10ths of feet.
Maximum Holding Time  The longest time period that water samples can be retained between the taking of the sample and the laboratory analysis for a specific material before the results are considered invalid.  The times vary from none in the case of the test for residual chlorine levels to six months for the testing of radioactivity.  Some types of analyses require that preservatives be added to the sample, and some require storage of samples at refrigerated temperatures.
Maximum Probable Flood  The largest flood for which there is any reasonable expectancy.
Maximum Sustainable Yield  The greatest amount of a renewable natural resource (e.g., forests or wildlife) that can be removed without diminishing the continuing production and supply of the resource.
Maximum Thermometer  An instrument used to measure the highest temperature since its last setting.  A constriction near the bulb prevents the mercury from returning to the bulb as the temperature falls.
Maximum Water Surface (Reservoir)  The maximum water-surface elevation is the highest water surface elevation for which the dam is designed.  It is also the top of the surcharge capacity.
Mayordomo  A Spanish word used in the Southwestern United States to identify the individual responsible for overseeing water allocation and maintenance of the water conveyance systems.  Used synonymously for water commissioner or ditch rider.
MBAS (Methylene Blue Active Substances)  Generally interpreted as an indication of the presence of detergents in a solution.
MCL  See Maximum Contaminant Level.
MCLG  See Maximum Contaminant Level Goal.
Meadow  An area of moist low-lying grassland usually along a watercourse supporting a more dense stand of grasses and perhaps dwarf shrubs as compared to adjacent more arid uplands.
Meadow, Dry  An area where during the spring, early summer, and in some open winters there is a greenup of succulent vegetation.  These areas are relatively few in number and highly important for sustaining animal populations within whose habitat these meadows exist.  During the summer and fall there is normally dry vegetation.
Meadow, Wet  A perennial wet area where the water table is maintained at or close to the ground surface to maintain shallow rooted water-dependent vegetative complexes.
Mean  (Statistics) The sum of a set of observations divided by the number of observations.  Also referred to as Arithmetic Mean and Sample Mean.  Compare to Mode and Median.
Mean Annual Flood  The average of all the annual flood stages or discharges of record.  It may be estimated by regionalization, correlation, or any other process that can furnish a better estimate of the long-term average than can the observed data.  Some investigators arbitrarily define the mean annual flood as the stage or discharge having an exceedence interval of 2.33 years.
Mean Annual Precipitation  The average of all annual precipitation values known, or an estimated equivalent value derived by such methods as regional indexes or Isohyetal maps.
Mean Annual Runoff  The average value of all annual runoff amounts usually estimated from the period of record or during a specified base period from a specified area.
Mean Annual Temperature  The average of the daily maximum and minimum temperatures.
Mean Concentration of Suspended Sediment  The time-weighted concentration of suspended sediment passing a stream section during a 24-hour day.
Mean Depth  The average depth of water in a stream channel or conduit.  It is equal to the cross-sectional area divided by the surface width.
Mean Discharge  The arithmetic mean of individual daily mean discharges during a specific period.
Mean Free Path  The average distance that a molecule in a fluid (air or water) moves before colliding with another molecule.
Mean High Water (MHW)  The average height of the high water over 19 years.
Mean Higher High Tide  The average height of the higher of two unequal daily high tides over 19 years.
Mean Low Water (MLW)  The average height of the low water over 19 years.
Mean Lower Low Water  The average height of the lower of two unequal daily low tides over 19 years.  Tides of the northeastern Pacific Ocean are characterized as mixed, with two unequal highs and two unequal lows daily.  The plane of reference for navigation channels is the long term average of the daily lower lows, termed mean lower low water.
Mean Sea Level (MSL)  (1) The level of the surface of the sea between mean high and mean low tide; used as a reference point for measuring elevations.  (2) The average height of the sea for all stages of the tide over a nineteen year period, usually determined from hourly height observations on an open coast or in adjacent waters having free access to the sea.  (3) (FEMA) For purposes of the National Flood Insurance Program (NFIP), the National Geodetic Vertical Datum (NGVD) of 1929 or other datum, to which base flood elevations shown on a community's Flood Insurance Rate Map (FIRM) are referenced.
Mean Tide Level  A plane midway between mean high water and mean low water.
Meander  (1) The turn of a stream, either live or cut off.  The winding of a stream channel in the shape of a series of loop-like bends.  (2) A sinuous channel form in flatter river grades formed by the erosion on one side of the channel (pools) and deposition on the other side (point bars).
Meander Amplitude  The distance between points of maximum curvature of successive meanders of opposite phase in a direction normal to the general course of the Meander Belt, measured between centerlines of channels.
Meander Belt  The zone along a valley floor that encloses a meandering river.
Meander Breadth  The distance between the lines used to define the Meander Belt.
Meander Length  The distance in the general course of the meanders between corresponding points of successive meanders of the same phase.
Meander Line  A line delineated by government survey for the purpose of defining the bends or windings of the banks of a stream or the shore of a body of water, and as a means for ascertaining the quantity of land embraced by the survey.
Mean Monthly Temperature  The average of the mean monthly maximum and minimum temperatures.
Measurement Uncertainty  The estimated amount by which the measured quantity may depart from the true value.
Measuring Point (MP)  An arbitrary permanent reference point from which the distance to water surface in a well is measured to obtain water level.
Measuring Weir  A shaped notch, typically in rectangular, trapezoidal, or triangular shape, through which flowing water is measured.
Mechanical Aeration  The use of mechanical energy to inject air into water to cause a waste stream to absorb oxygen.
Mechanical Dispersion  Process whereby solutes are mechanically mixed during advective transport, caused by the velocity variations at the microscopic level.  Synonymous with Hydraulic Dispersion.  Also see Coefficient of Mechanical Dispersion.
Mechanical Turbulence  The erratic movement of air or water influenced by local obstructions.
Median  (Statistics) In a set of observations, the middle-most value with an equal number of observations lying above and below the median value.  Also see Mean and Mode.
Median Stream Flow (Median Hydro)  The rate of discharge of a stream for which there are equal numbers of greater and lesser flow occurrences during a specified period.
Median Tolerance Limit  The concentration of a test substance at which just 50 percent of the test animals are able to survive for a specified period of exposure.
Mediterranean  Surrounded nearly or completely by dry land.  Used of large bodies of water, such as lakes or seas.
Medithermal  (Climatology) The present period of climatological conditions, beginning approximately 4,500 years ago and following the warmer Altithermal period.  Also see Anathermal.
Medium-Size Water System  A water system that serves 3,300 to 50,000 customers.
Megawatt (MW)  A unit of electricity equivalent to 1 million watts or 1,000 kilowatts (KW).
Melt  To be changed from a solid to a liquid state by application of heat or pressure or both.
Melting  The changing of a solid into a liquid as in changing ice to water.
Melting Point  The temperature at which a solid changes to a liquid.  The temperature will vary, and is consistent at equal temperatures and pressures, for each element or solid.  At a standard barometric pressure of one atmosphere, water will change from a solid to a liquid at 0?C (32?F).
Meltwater  Water that comes from the melting ice of a glacier or a snow bank.
Membrane  A plastic material used in the electrodialysis and reverse osmosis processes.  Electric current is the driving force that moves salt ions through solution in electrodialysis, and hydraulic pressure the driving force in reverse osmosis.  More properly referred to as a semipermeable membrane.
Membrane Filter  (10 A thin microporous material of specific pore size used to filter bacteria, algae, and other very small particles from water.  (2) Filter made of plastic or modified cellulose and having a known pore diameter.  Such filters are used in the bacteriolog
Membrane Filter Method  A procedure used to recover and count bacteria in samples of liquid substances, such as water.  The liquid is drawn through a Membrane Filter using a slight vacuum, with the bacteria in the liquid being retained on the filter.  The filter disk is then transferred to a medium suitable for the growth and incubation of the bacteria.
Membrane Filtration  The use of a membrane, or more properly, a semipermeable membrane, to separate substances when a driving force is applied across the membrane.  Once considered a viable technology only for desalination, membrane processes are increasingly employed for the removal of bacteria and other microorganisms, particulate material, and natural organic material which can impart color, tastes, and odors to water and react with disinfectants to form Disinfection Byproducts (DBP).  Due to their greater effectiveness, waste stream disposal, at up to 15-25 percent (and higher for the RO process) of the total treated water volume, is a significant problem with membrane treatment systems.  Pressure-driven membrane filtration processes include:
(1) Microfiltration (MF) - Loosely defined as a membrane separation process using membranes with a pore size of approximately 0.03 to 10 micros, a molecular weights cutoff (MWCO) of greater than 100,000 daltons, and a relatively low feedwater operating pressure of approximately 100 to 400 kPa (15-60 psi).  Representative materials removed by MF include sand, silt, clays, Giardia lamblia and Cryptosporidium cysts, algae, and some bacterial species.  MF is not an absolute barrier to viruses; however, when used in combination with disinfection, MF appears to control these microorganisms in water.
(2) Ultrafiltration (UF) - Involves the pressure-driven separation of materials from water using a membrane pore size of approximately 10,000 to 100,000 daltons, and an operating pressure of approximately 200 to 700 kPa (30-100 psi).  UF will remove all microbiological species removed by MF (partial removal of bacteria), as well as some viruses (but not an absolute barrier to viruses) and humic materials.
(3) Nanofiltration (NF) - Membranes which have a nominal pore size of approximately 0.001 microns and an MWCO of 1,000 to 100,000 daltons.  Pushing water through these smaller membrane pores requires a higher operating pressure than either MF or UF.  Operating pressures are usually near 600 kPa (90 psi) and can be as high as 1,000 kPa (150 psi).  These systems can remove virtually all cysts, bacteria, viruses, and humic materials.  They provide excellent protection from DBP formation if the disinfectant residual is added after the membrane filtration step.  Because NF membranes also remove alkalinity, the product water can be corrosive and further treatment may be required.  NF also removes hardness from water, which accounts for NF membranes sometimes being called "softening membranes."
(4) Reverse Osmosis (RO) - This process removes contaminants from water using a semipermeable membrane that permits only water, and not dissolved ions (such as sodium and chloride), to pass through its pores.  Contaminated water is subject to a high pressure that forces pure water through the membrane, leaving contaminants behind in a brine solution.  RO can effectively remove nearly all inorganic contaminants from water.  RO can also effectively remove radium, natural organic substances, pesticides, cysts, bacteria, and viruses.  RO is particularly effective when used in series; water passing  through multiple units can achieve near zero effluent contaminant concentrations.
Meniscus  The curved surface of the liquid at the open end of a capillary column.
Mere  (Middle English, from Old English) A small lake, pond, or marsh.  Also, an expanse of standing water; a lake, pool.  (Also French for sea.)
Meromictic Lake  A lake in which some water remains partly or wholly unmixed with the main water mass at circulation periods.  The process leading to a meromictic state is called Meromixts.  The perennially stagnant deep layer of a meromictic lake is the Monimolimnion.  The part of the meromictic lake in which free circulation can occur is the Mixolimnion.  The boundary between the monimolimnion and the mixolimnion is the Chemocline.  Compare to Dimictic Lake.
Meromixis  A condition of permanent stratification of water masses in lakes.
Mesa  Table land, flat in nature, moderately elevated, and well drained.
Mesic  Refers to environmental conditions that have medium moisture supplies as compared to wet conditions (Hydric) or dry conditions (Xeric).
Mesoclimate  The climate of small areas of the earth's surface; it may not be representative of the general climate of the district; intermediate in scale between Macroclimate and Microclimate.  Places considered in mesoclimatology include small valleys, "frost hollows", forest clearings and open spaces in towns.
Mesohaline  Term to characterize waters with salinity of 5 to 18‰ (parts per thousand), due to ocean-derived salts.
Mesophyte  A plant that grows under medium or usual conditions of atmospheric moisture supply, as distinguished from one which grows under dry or desert conditions (Xerophyte) or very wet conditions (Hydrophyte).
Mesosaline  Term to characterize waters with salinity of 5 to 18‰ (parts per thousand), due to land-derived salts.
Mesosphere  The division of the Atmosphere above the Stratosphere.  The mesosphere begins about 50 kilometers (31 miles) in altitude and extends to about 80 kilometers (50 miles).
Mesotrophic (Water)  Pertaining to a lake or other body of water characterized by moderate nutrient concentrations such as nitrogen and phosphorous and resulting significant productivity.  Such waters are often shallow, with algal blooms and periods of oxygen deficiency.  Slightly or moderately eutrophic water can be healthful and support a complex web of plant and animal life.  However, such waters are generally undesirable for drinking water and other needs.  Degrees of Eutrophication typically range from Oligotrophic water (maximum transparency, minimum chlorophyll-a, minimum phosphorus) through Mesotrophic, Eutrophic, to Hypereutrophic water (minimum transparency, maximum chlorophyll-a, maximum phosphorus).  Also see Carlson's Trophic State Index (TSI) and (Mean) Trophic State Index (TSI).
Meta- or Met- (Prefix)   Derived from by loss of water, as meta phosphoric acid.
Metabolism  (Biology) The sum of the processes concerned in the building up of protoplasm and its destruction incidental to life; the chemical changes in living cells, by which the energy is provided for the vital processes and activities, and new material is assimilated to repair the waste.  Metabolism may be considered as including two aspects or processes: constructive metabolism (termed Anabolism or Assimilation) or destructive metabolism (termed Catabolism or Dissimilation).  Anabolism and Catabolism go on together, but one may predominate and obscure the other.  Also see Zone of Net Metabolic Production.
Metalimnion  The middle layer of a thermally stratified lake or reservoir.  In this layer there is a rapid decrease in temperature with depth.  Also referred to as Thermocline.
Metamorphic Rock  (Geology) A sedimentary or igneous rock that has been changed by pressure, heat, or chemical action.  For example, limestone, a sedimentary rock, is converted to marble, a metamorphic rock.
Metamorphism  A change in the constitution of rock; specifically a pronounced change effected by pressure, heat, and water that results in a more compact and more highly crystalline condition.
Meteoric Water  Groundwater derived primarily from precipitation and the atmosphere.
Meteorological Drought  A drought said to occur when annual rainfall (or precipitation) is less than the long-term average annual rainfall.  Compare to Hydrological Drought and Agricultural Drought.
Meteorology  The science that deals with the phenomenon of the atmosphere, especially weather and weather conditions.
Meter  A unit of length which constitutes the basis of the Metric System, was intended to be, and is very nearly, one ten-millionth part of the distance measured on a meridian of the earth from the equator to the pole, being equal to 39.37 U.S. inches or about 3 feet 3-3/8 inches.  See Metric System.
Methane  A colorless, nonpoisonous, flammable gas, CH4, created by Anaerobic decomposition of organic compounds.
Methemoglobinemia  A blood disorder that impairs the ability of the blood supply to carry oxygen throughout the body.  Also known as "blue baby syndrome", it is frequently caused by high concentrations of nitrate in drinking water supplies.  It primarily affects infants less than 6 months of age.  Most instances of the problem can be traced to babies drinking milk formula mixed in water with very high nitrate levels.
Method Blank  Laboratory grade water taken through the entire analytical procedure to determine if samples are being accidentally contaminated by chemicals in the lab.
Methyl Tertiary Butyl Ether (MTBE)  A oxygenate and gasoline additive used to improve the efficiency of combustion engines in order to enhance air quality and meet air pollution standards.  MTBE is a product of petroleum refining that has been added to gasoline nationwide since the late 19
Methylcellulose  Any of various gummy products of cellulose methylation that swell in water and are used especially as emulsifiers, adhesives, thickeners, and bulk laxatives.
Methylene Blue  A basic aniline dye, C16H18N3SCl·3H2O, that forms a deep blue solution when dissolved in water.  It is used as an antidote for cyanide poisoning and as a bacteriological stain.
Methylene Blue Active Substances (MBAS)  Any material which forms a blue colored salt with methylene blue, but generally interpreted as an indication of the presence of detergents in solution.
Methylmercury  An organic compound that has known neurological toxicity effects that tend to biomagnify up the food chain in aquatic environments..  Biomagnification is 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.  Typically, methylmercury is formed by the action of certain bacteria on available supplies of inorganic mercury in stream-bottom sediments containing low concentrations of dissolved oxygen.  However, the reverse process, or demethylation also is known to occur and this "detoxifying" of methylmercury is the subject of ongoing research.
Metric System  A decimal system of measures and weights with the meter and the gram as bases.  The units of the metric system at the outset were all derived from the unit of length, the Meter, which was intended to be, and is very nearly, one ten-millionth part of the
Standard Metric Tables
(1)  Length
Myriameter  10,000. meters  6.2137 miles
Kilometer  1,000. meters  0.62137 miles
Hectometer  100. meters  328. feet 1 inch
Decameter  10. meters  393.7 inches
Meter  1. meter  39.37 inches
Decimeter  0.1 meter  3.937 inches
Centimeter  0.01 meter  0.3937 inches
Millimeter  0.001 meter  0.03937 inches
(2)  Area
Hectare  10,000. square meters  2.471 acres
Are  100. square meters  119.6 square yards
Centiare  1. square meter  1,550. square inches
(3)  Volume
    Metric  United States  British
Name   Liters   Cubic Measure   Measure    Measure
Kiloliter  1,000.  1 cubic meter  1.308 cubic yard  1.308 cubic yard
Hectoliter  100.  0.1 cubic meter  2.838 bu./26.418 gal  2.75 bu./22.00 gal
Decaliter  10.  10 cubic dm.  1.135 pk./2.6418 gal  8.80 qt./2.200 gal
Liter  1.  1 cubic dm.  0.9081 qt./1.0567 liq qt.  0.880 quarts
Deciliter  0.1  0.1 cubic dm.  6.1025 cu. in./0.845 gill.  0.704 gill.
Centiliter  0.01  10 cubic cm.  0.6102 cu. in./0.338 fl. oz.  0.352 fl. oz.
Milliliter  0.001  1 cubic cm.  0.061 cu. in./0.27 fl. dram.  0.28 fl. dram.
Volume Table Notes:
dm. = decimeter = 1/10 meter
cm. = centimeter = 1/100 meter
bu. = bushel = 4 pecks = 32 quarts
pk. = peck = 1/4 bushel = 8 quarts
qt. = quart = 2 pints = 1/4 gallons
liq qt. = liquid quart = 1.1635 (dry) quarts
gill = 1/4 pint
fl. dram = fluid dram = 1/16 ounce = 27.34375 grains = 1.772 grams
(4)  Weight
    Comparable Quantity 
  Number of  of Water at  Avoirdupois
Name   Grams   Maximum Density   Weight
Metric Ton  1,000,000.  1 Cubic Meter  2204.6 lb.
Quintal  100,000.  1 hectoliter  220.46 lb.
Myriagram  10,000.  1 decaliter  22.046 lb.
Kilogram  1,000.  1 liter  2.2046 lb.
Hectogram  100.  1 deciliter  3.5274 oz.
Decagram  10.  10 cubic centimeters  0.3527 oz.
Gram  1.  1 cubic centimeter  15.432 grains
Decigram  0.1  0.1 cubic centimeter  1.5432 grains
Centigram  0.01  10 cubic millimeters  0.1543 grains
Milligram  0.001  1 cubic millimeter  0.0154 grains
MGD  Million gallons per day.  A unit of water usage used in many applications of water and wastewater treatment processes.
Microbe  Short for Microorganism.  Small organisms that can be seen only with the aid of a microscope.  The term encompasses viruses, bacteria, yeast, molds, protozoa, and small algae; however, microbe is used most frequently to refer to bacteria.  Microbes are important in the degradation and decomposition of organic materials added to the environment by natural and artificial mechanisms.  Also referred to as Germs.
Microbial Growth  The activity and growth of microorganisms such as bacteria, algae, diatoms, plankton, and fungi.
Microbial Load  The total number of bacteria and fungi in a given quantity of water or soil or on the surface of food.  The presence of the bacteria and fungi may not be related to the presence of disease-causing organisms.
Microbiological Anaerobic Degradation  The use of Microbes, either already present at a site or introduced for a specific treatment process, to degrade and render harmless hazardous wastes and toxic compounds in soil and water.  Under such conditions, the microbes are used to break down organic compounds in contaminated soil and groundwater in an environment of little or no oxygen.  Also see Attenuation and Natural Attenuation.
Microbiological Tuberculation  (Water Quality) A condition in older water distribution pipes characterized by reddish brown mounds of various heights attached to the interior of the pipe wall.  These mounds are the result of many years of iron and manganese bacterial growth that depos
Microbiology  The study of organisms that can be seen only with the aid of a microscope.  The science deals with the structure and chemical composition of various Microbes, the biochemical changes within the environment that are caused by members of this group, the diseases caused by microbes, and the reaction of animals, including humans, to their presence.
Microbiota  The plants, animals, and microorganisms that can only be seen with the aid of a microscope.
Microclimate  (1) The local climate conditions, brought about by the modification of general climatic conditions by local differences in elevation and exposure.  The detailed climate of a very small area of the earth's surface.  (2) Also, the localized climate conditions within an urban area or neighborhood.
Microcosm  A laboratory model of a natural Ecosystem in which certain environmental variables can be manipulated to observe the response.  The model test results are not always applicable to an actual ecosystem because the microcosm is, of necessity, a simplified collection of selected physical, chemical, and biological ecosystem components.
Microfauna  Animals invisible to the naked eye, such as copepods and mites.
Microfiltration
Microflora  Plants invisible to the naked eye, such as diatoms and algae.
Micrograms per Gram (?g/g)  A unit expressing the concentration of a chemical constituent as the mass (micrograms) of the element per unit mass (gram) of material analyzed.
Micrograms per Kilogram (?g/kg)  A unit expressing the concentration of a chemical constituent as the mass (micrograms) of the element per unit mass (kilogram) of material analyzed.  One microgram per kilogram is equivalent to 1 part per billion.
Micrograms per Liter (?g/l)  A unit expressing the concentration of a chemical constituents in water as the mass (micrograms) of the element per unit volume (liter) of material analyzed.  One thousand micrograms per liter is equivalent to one Milligram per Liter (mg/l).  This measure is also equivalent to Parts Per Billion (PPB).
Microsiemens per Centimeter (?S/cm)  A unit expressing the amount of electrical conductivity of a solution as measured between opposite faces of a centimeter cube of solution at a specified temperature.  Siemens is the International System of Units nomenclature.  It is synonymous with mhos and is the reciprocal of resistance measured in ohms.
Micron (?)  A unit of length equivalent to a micro-meter (?m), or one-millionth of a meter (10-6 meter).  Micro-meter is the preferred term.
Micronutrient  A chemical element required only in small amounts (usually less than one part per million (ppm) in the plant) for the growth of plants.
Microscopic Particulate Analysis (MPA)  (Water Quality) A process used to assess water treatment plant performance.  This form of analysis compares type, size, and quantities of Bioindicators, or microbiota (1-600 ?m) in particles found in Raw Water to those found in the Finished Water.  This method is particularly effective in evaluating filtration efficiencies, as log reduction, of conventional treatment systems, as well as for on-site evaluation of alternate filtration technologies.
Microsystem Irrigation  Method of precisely applying irrigation water to the immediate root zone of the target plant at very low rates.
Microwave Oven  An oven in which food is cooked by the heat produced by the absorption of microwave energy by water molecules in the food.
Mid-Seral Condition  Synonymous with fair ecological conditions.
Midstream  The middle part of a stream.
Migration  The movement of oil, gas, contaminants, water, or other liquids through porous and permeable rock.
Milldam  A dam constructed across a stream to raise the water level so that the overflow will have sufficient power to turn a mill wheel.
Milliequivalents per Liter (MEQ/L)  An expression of the concentration of a material dissolved in water, calculated by dividing the concentration, in milligrams per liter, by the Equivalent Weight of the dissolved material.  For example, the equivalent weight of aluminum is 9.0.  A water concentration of aluminum of 1.8 milligrams per liter equals an aluminum concentration of 0.2 milliequivalent per liter.
Milligram (MG)  One-thousandth of a gram.
Milligrams Per Liter (mg/l)  A unit of the concentration of a constituent in water or wastewater and expresses the concentration of chemical constituents in water as the mass (milligrams) of constituent per unit volume (liter) of water.  Concentration of suspended sediment also is expressed in mg/l and is based on the mass of dry sediment per liter of water-sediment mixture.  It represents 0.001 gram of a constituent in 1.000 milliliter (ml) of water.  It is approximately equal to one part per million (PPM).  The term has replaced parts per million in water quality management.
Million Gallons per Day (MGD)  A rate of flow of water equal to 133,680.56 cubic feet (cf) per day, or 1.5472 cubic feet per second (cfs), or 3.0689 acre-feet per day.  A flow of one million gallons per day (mgd) for one year equals 1,120 acre-feet (365 million gallons).
Millipore Filter  A thin membrane of modified cellulose that is used as a filter in the bacteriological examination of water or wastewater.  The filter is typically used to filter a given quantity of aqueous sample followed by transfer of the filter to the surface of a special medium to allow for the growth of the bacteria that have been retained by the filter.  At one time the only commercial source of these filters was the Millipore Corporation, although presently a variety of sources are now available.  Even so, the common name Millipore filter has been retained.
Millpond  A pond created by damming a stream to produce a head of water for operating a mill.
Millrace  (1) The fast-moving stream of water that drives a mill wheel.  (2) The channel for the water that drives a mill wheel.  Also referred to as Millrun.
Millstream  The rapid stream of water flowing in a Millrace.
Mill Wheel  A wheel, typically driven by water, that powers a mill.
Mine Drainage  Water pumped or flowing from a mine.
Mineral  Any naturally occurring inorganic material with an orderly internal arrangement of atoms and specific physical and chemical properties.
Mineralization  (1) The general process by which elements present in organic compounds are eventually converted into inorganic forms, ultimately to become available for a new cycle of plant growth.  (2) The process whereby concentrations of minerals, such as salts, increase in water, often as a natural process resulting from water dissolving minerals found in rocks and soils through which it flows.
Mineral Resource  Known mineral deposits of an area which have present or future utility.
Mineral Soil  Soil composed of predominantly mineral rather than organic materials.
Mineral Water  Naturally occurring or prepared water that contains dissolved mineral salts, elements, or gases, often used therapeutically.  Also see Bottled Water (General), Bottled Water (Food and Drug Administration), and Bottled Water (Nevada).
Miner's Inch (Western United States)  The rate of discharge through an orifice one inch square under a specific head.  An old term used in the western United States, now seldom used except where irrigation or mining water rights are so specified.  The equivalent flow in cubic feet per second is fixed by state statute.  One miner's inch is equivalent to 0.025 cubic foot per second (1.5 cubic feet per minute, equivalent to one-fortieth of a second-foot) in Arizona, California, Montana, Nevada, and Oregon; 0.020 cubic foot per second in Idaho, Kansas, Nebraska, New Mexico, North and South Dakota, and Utah; 0.026 cubic foot per second in Colorado; and 0.028 cubic foot per second in British Columbia.
Miner's Inch (Nevada)  Defined as a rate of flow or discharge equivalent to 1/40 of 1 (0.025) cubic foot per second (cfs).
Mine Wash  Water-deposited accumulation of sandy, silty, or clayey material recently eroded in mining operations.
Minim  A unit of fluid measure, in the United States equal to 1/60 of a fluid dram (0.0616 milliliters, or 0.00208 fluid ounces), and in Great Britain equal to 1/20 of a scruple (0.0592 milliliters or 0.00200 fluid ounces).
Minimal Flood Hazard Areas  Areas between the 100-year and the 500-year flood boundaries are termed Moderate Flood Hazard Areas.  The remaining areas are above the 500-year flood level and are termed Minimal Flood Hazard Areas.
Minimum Flow Appropriation  An appropriation designed to preserve a specified minimum flow in a stream.  When the flow in the stream drops to that which is specified in the appropriation, junior appropriations will be required to stop diverting water in order to maintain the minimum flow.  See (Prior) Appropriation Doctrine.
Minimum Instream Flow (Streamflow)  The specific amount of water required to support aquatic life, to minimize pollution, or for recreation.  It is subject to the priority system and does not affect water rights established prior to its institution.  Also referred to as Minimum Instream Flow.
Minimum Moisture Content  The amount of water in soil during the driest time of the year.
Minimum Pool  A term used to describe the lowest level of reservoir capacity safe for maintaining fish and aquatic life or for some other designated beneficial purpose.  This term differs from Dead Storage Capacity in that the reservoir level may still be reduced below minimum pool, whereas the dead storage capacity represents a level below the lowest outlet level.
Minimum Streamflow  See Minimum Instream Flow.
Minimum Thermometer  An instrument with an index which remains at the lowest temperature occurring since its last setting.
Minimum Tillage Farming  A farming technique that reduces the degree of soil disruption.  Crop residues are not plowed under after harvest, and special planters dig narrow furrows in the crop residue when new seeds are sown.