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GABION—A wire cage, usually rectangular, filled with cobbles and used as a component for water control structures or for channel and bank protection.

GAC (GRANULAR ACTIVATED CARBON)—In water treatment, granular activated carbon has been used mainly for taste and odor control, with some special applications that remove Synthetic Organic Chemicals (SOCs) or Volatile Organic Chemicals (VOCs) from contaminated water. Two basic forms of GAC are typically used: (1) a coal-based carbon manufactured as an adsorbent; and (2) a wood-based carbon manufactured primarily as a substrate for biological activity. Also see Biological Activated Carbon (BAC) Process.

GAGE, or Gauge—(1) An instrument used to measure magnitude or position; gages may be used to measure the elevation of a water surface, the velocity of flowing water, the pressure of water, the amount of intensity of precipitation, the depth of snowfall, etc. (2) The act or operation of registering or measuring magnitude or position. (3) The operation, including both field and office work, of measuring the discharge of a stream of water in a waterway.

GAGE HEIGHT—The height of the water surface above the gage datum (reference level). Gage height is often used interchangeably with the more general term, Stage, although Gage Height is more appropriate when used with a gage reading.

GAGE ROD—A measuring device that shows the water level in the reservoir.

GAGING STATION—A particular site on a stream, canal, lake, or reservoir where systematic observations of Gage Height or discharge are obtained.

GAGING STATION NUMBER—A U.S. Geological Survey (USGS) numbering system consisting of an eight-digit number assigned to a Gaging Station which identifies the station in downstream order relative to other gaging stations and sites where streamflow data are collected. The first two digits designate the major drainage basin, the others the station.

GAINING STREAM—A stream or reach of a stream, the flow of which is being increased by the inflow of ground water seepage or from springs in, or alongside, the channel. Also referred to as an Effluent Stream. Also see Stream.

GALLERY—(1) A passageway within the body of a dam or abutment; hence the terms "grouting gallery," "inspection gallery," and "drainage gallery." (2) A long and rather narrow hall; hence the following terms for a power plant: "valve gallery," "transformer gallery," and "busbar gallery."

GALLON [Imperial]—A unit of capacity in Great Britain containing four quarts, is used for both liquid and dry commodities, and is defined as the volume occupied by ten imperial pounds weight of distilled water, as weighed in air against brass weights with both water and air at 62° Fahrenheit, and the barometer at 30 inches (atmospheric pressure). It is equivalent to 4.5460 liters (277.420 U.S. cubic inches), or to 1.2003 U.S. gallons (defined below).

GALLON [U.S.]—A unit of capacity, containing four quarts, used in the United States primarily for liquid measure. One U.S. gallon contains 231 cubic inches, 0.133 cubic feet, or 3.7853 liters. One U.S. gallon is equivalent to the volume of 8.3359 pounds av. (avoirdupois) of distilled water at its maximum density (32.9°F or 4°C), weighed in dry air at the same temperature against brass weight of 8.4 density and with the barometer at 30 inches. It takes approximately 325,851 gallons to make up 1 acre-foot (AF). [Historical Note: The U.S. gallon is the same as the old English wine gallon which was originally intended in England to be equivalent to a cylinder of seven inches in diameter and six inches in height.]

GALLONS PER CAPITA (GPC)—A term used relative to water use per person per specified time, usually a day.

GALLONS PER CAPITA PER DAY (GPCD)—An expression of the average rate of domestic and commercial water demand, usually computed for public water supply systems. Depending on the size of the system, the climate, whether the system is metered, the cost of water, and other factors, Public Water Supply Systems (PWSS) in the United States experience a demand rate of approximately 60 to 150 gallons per capita per day. Also see Gallons per Employee per Day (GED) for information on the application of this concept to commercial water use by Standard Industrial Classification (SIC) Code. [See Appendix G-1, Gallons Per Capita Per Day (GPCD), Water Used for Public Water Supplies by State.]

GALLONS PER EMPLOYEE PER DAY (GED)—A measure or coefficient expressing an area's commercial water use per worker (employee), typically for distinct industry sectors. It is based on an analytical technique for measuring and forecasting commercial water use in a service area based upon the unique, seasonal, business-related water use by specific industrial sectors. GED commercial water-use coefficients are typically developed based upon Standard Industrial Classifications (SIC) codes for which comparable commercial water use and employment data are available. For forecasting more frequently than annually, GED coefficients will incorporate seasonal patterns (monthly or quarterly) as well. By deriving forecasts of trends in industry sector employment and combining them with appropriate, industry-specific GED coefficients, relatively accurate forecasts of the corresponding commercial water use may be obtained.

GALLONS PER MINUTE—A unit expressing rate of discharge, used in measuring well capacity. Typically used for rates of flow less than a few cubic feet per second (cfs).

GAMMA RADIATION—High energy photons which are emitted by many radioactive substances.

GAME FISH—Those species of fish considered to possess sporting qualities on fishing tackle, such as salmon, trout, black bass, striped bass, etc.; usually more sensitive to environmental changes than Rough Fish.

GAP ANALYSIS—A method for determining spatial relationships between areas of high biological diversity and the boundaries of National Parks, National Wildlife Refuges (NWR), and other preserves. The primary goal of Gap Analysis is to prevent additional species from being listed as threatened or endangered. Analyses are made and displayed using a Geographic Information System (GIS). Estimates of diversity are often derived from known or hypothesized relationships between mapped plant communities and animal populations. In addition to the National Biological Survey, which serves as the primary coordinating agency, there are over 200 collaborating organizations involved in performing Gap Analysis on a state-by-state basis, including businesses, universities, and state, local, and federal government entities. [The term Gap originated from an initial Biodiversity study in Hawaii which showed that for certain sensitive animal species there existed a physical (geographic) gap between the species and its habitat and wildlife preserves (national parks, forests, wildlife protection areas, etc.), indicating potential limitations of species and habitat protection.]

GAS—A state of matter; a substance that generally exists in the gaseous phase at room temperature.

GAS CHROMATOGRAPH/MASS SPECTROMETER (GC/MS)—A highly sophisticated instrument that identifies the molecular composition and concentrations of various chemicals in water and soil samples.

GAS CHROMATOGRAPHY (GC)—A method of separating chemical components of a mixture which involves the passa