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

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.

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.

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."

GATE — (1) (Irrigation) Structure or device for controlling the rate of water flow into or from a canal, ditch, or pipe. (2) (Dam) A device in which a leaf or member is moved across the waterway from an external position to control or stop the flow. The following types of gates apply to dams and other such structures:
[1] Bulkhead Gate — A gate used either for temporary closure of a channel or conduit to empty it for inspection or maintenance or for closure against flowing water when the head differential is small, e.g., a diversion tunnel closure. Although a bulkhead gate is usually opened and closed under nearly balanced pressures, it nevertheless may be capable of withstanding a high pressure differential when in the closed position.
[2] Crest Gate (Spillway Gate) — A gate on the crest of a spillway to control overflow or reservoir water level.
[3] Emergency Gate — A standby or reserve gate used only when the normal means of water control is not available.
[4] Fixed Wheel Gate (Fixed Roller Gate, Fixed Axle Gate) — A gate having wheels or rollers mounted on the end posts of the gate. The wheels bear against rails fixed in side grooves or gate guides.
[5] Flap Gate — A gate hinged along one edge usually either the top or bottom edge. Examples of bottom-hinged flap gates are tilting gates and fish belly gates, so-called due to their shape in cross section.
[6] Flood Gate — A gate to control flood release from a reservoir.
[7] Guard Gate (Guard Valve) — A gate or valve that operates fully open or closed. It may function as a secondary device for shutting off the flow of water in case the primary closure device becomes inoperable, but is usually operated under balanced pressure, no-flow conditions.
[8] Outlet Gate — A gate controlling the outflow of water from a reservoir.
[9] Radial Gate (Tainter Gate) — A gate with a curved upstream plate and radial arms hinged to piers or other supporting structures.
[10] Regulating Gate (Regulating Valve) — A gate or valve that operates under full pressure and flow conditions to throttle and vary the rate of discharge.
[11] Slide Gate (Sluice Gate) — A gate that can be opened or closed by sliding it in supporting guides.

GATED PIPE — (Irrigation) Portable pipe with small gates installed along one side for distributing water to corrugations or furrows.

GENERATOR — A machine that changes water power, steam power, or other kinds of mechanical energy into electricity.

GEOGRAPHIC INFORMATION SYSTEM (GIS) — A computer information system that can input, store, manipulate, analyze, and display geographically referenced data to support the decision-making processes of an organization. A map based on a database or databases. System plots locations of information on maps using latitude and longitude.

GEOHYDROLOGY — A term which denotes the branch of Hydrology relating to subsurface or subterranean waters; that is, to all waters below the surface.

GEOMORPHOLOGY (Geomorphic) — That branch of both physiography and geology that deals with the form of the earth, the general configuration of its surface, and the changes that take place in the evolution of land forms. The term usually applies to the origins and dynamic morphology (changing structure and form) of the earth's land surfaces, but it can also include the morphology of the sea floor and the analysis of extraterrestrial terrains. Sometimes included in the field of physical geography, geomorphology is really the geological aspect of the visible landscape.

GEOMORPHOLOGY, HISTORICAL — Historical geomorphology represents one branch of Geomorphology which provides the means to analyze the long-term change in landforms through the concept of cyclic change. The concepts evolved at the turn of the 20th century and were put forward by the American geologist William Morris Davis. The theory stated that every landform could be analyzed in terms of structure, process, and stage. Structure and process are treated by the science of geomorphology. However, the concept of stage introduced the element of time, and is subject to a far greater degree of interpretation. As postulated by Davis, every landform underwent development through a predictable, cyclic sequence: i.e., youth, maturity, and old age. Historical geomorphology relies on various chronological analyses, notably those provided by stratigraphic studies of the last 2 million years, known as the Quaternary Period. The relative chronology usually may be worked out by observation of stratigraphic relationships, with the time intervals involved established more precisely by dating methods such as historical records, radiocarbon analysis, tree-ring counting (Dendrochronology), and paleomagnetic studies. By applying such methods to stratigraphic data, a quantitative chronology of events is constructed that provides a means for calculating long-term rates of change.

GEOMORPHOLOGY, PROCESS — The second branch of Geomorphology, process geomorphology analyzes contemporary dynamic processes at work in landscapes. The mechanisms involved are weathering and erosion and combine processes that are in some respects destructive and in others constructive. The bedrock and soil provide the passive material, whereas the climatic regime and crustal dynamics together provide the principal active variables.

GEOTHERMAL — Terrestrial heat, usually associated with water as around hot springs.

GEOTHERMAL ENERGY — The heat energy available in the earth's subsurface, extracted from three basic sources: (1) steam; (2) hot water; and (3) hot rocks or near surface intrusions of volcanic molten rock. The normal thermal gradient of the earth's crust is such that the temperature in a deep well or mine typically increases by about 1°F (0.56°C) for each 100 feet of depth.

GEOTHERMICS — The science pertaining to the earth's interior heat. Its main practical application is in finding natural concentrations of hot water, the source of Geothermal Energy, for use in electric power generation and direct heat applications such as space heating and industrial drying processes. Heat is produced within the crust and upper mantle of the earth primarily by decay of radioactive elements. This geothermal energy is transferred to the earth's surface by diffusion and by convection movement of magma (molten rock) and deep-lying circulating water. Surface hydrothermal manifestations include hot springs, geysers, and Fumaroles.

GEYSER — A periodic thermal spring that results from the expansive force of super heated steam. Also, a special type of thermal spring which intermittently ejects a column of water and steam into the air with considerable force.

GIGAWATT HOUR (GWh) — One billion Watt-hours (Wh).

GLACIER — A huge mass of ice, formed on land by the compaction and recrystallization of snow, that moves very slowly downslope or outward due to its own weight.

GLACIER MEAL — Finely ground rock particles produced by glacial abrasion.

GLACIOLOGY — Collectively, the branches of science concerned with the causes and modes of ice accumulation and with ice action, on the earth's surface. Specifically, the branch of geology which studies the effects of glacial epochs, glaciation, and ice in modifying the earth's surface and in affecting the life and distribution of plants and animals.

GLADE — An open, spacious Wetland, as in the Everglades.

GRADE — (Hydraulics) The slope of a stream bed.

GRADED STREAM — A stream in which, over a period of years, the slope is delicately adjusted to provide, with available discharge and with prevailing channel characteristics, just the velocity required for transportation of the sediment load supplied from the drainage basin. Also, a stream in which most irregularities, such as waterfalls and cascades, are absent. Streams tend to cut their channels lower at a very slow rate after they become graded.

GRADE STABILIZATION STRUCTURE — A structure for the purpose of stabilizing the grade of a gully or other watercourse, thereby preventing further head-cutting or lowering of the channel grade.

GRADIENT — Degree of incline; slope of a stream bed. The vertical distance that water falls while traveling a horizontal distance downstream.

GRADUALLY VARIED FLOW — (Hydraulics) Non-uniform flow in which depth of flow changes gradually through a reach. Typical of normal natural valley and channel flow, which can be either steady or unsteady flows.

GRASSED WATERWAY OR OUTLET — A natural or constructed waterway, usually broad and shallow and covered with erosion-resistant grasses, suitable to resist potential damages resulting from runoff.

GRAVEL — A mixture composed primarily of rock fragments 2 mm (0.08 inch) to 7.6 cm (3 inches) in diameter. Usually contains much sand.

GRAVEL ENVELOPE — In well construction, a several-inch thickness of uniform gravel poured into the annular space between the well casing and the drilled hole.

GRAVITATIONAL HEAD — Component of total Hydraulic Head related to the position of a given mass of water relative to an arbitrary datum.

GRAVITATIONAL WATER — Water that moves into, through, or out of a soil or rock mass under the influence of gravity.

GRAVITY DAM — A dam constructed of concrete and/or masonry that relies on its weight for stability.

GRAVITY FLOW — The downhill flow of water through a system of pipes, generated by the force of gravity.

GRAVITY IRRIGATION — (1) Irrigation in which the water is not pumped but flows and is distributed by gravity, includes sprinkler systems when gravity furnishes the desired head (pressure). (2) Irrigation method that applies irrigation water to fields by letting it flow from a higher level supply canal through ditches or furrows to fields at a lower level.

GRAYWATER — Waste water from a household or small commercial establishment which specifically excludes water from a toilet, kitchen sink, dishwasher, or water used for washing diapers.

GREYWATER — Wastewater from clothes washing machines, showers, bathtubs, hand washing, lavatories and sinks that are not used for disposal of chemicals or chemical-biological ingredients.

GROSS RESERVOIR CAPACITY — The total amount of storage capacity available in a reservoir for all purposes, from the streambed to the normal maximum operating level. It does not include surcharge (water temporarily stored above the elevation of the top of the spillway), but does include dead (or inactive) storage.

GROSS DUTY OF WATER — (Irrigation) The irrigation water diverted at the intake of a canal system, usually expressed in depth on the irrigable area under the system.

GROSS WATER REQUIREMENT (FARM) — The Farm Delivery Requirement plus the seepage losses in the canal system from the headworks to the farm unit plus the waste of water due to poor operation.

GROSS WATER YIELD — The available water runoff, both surface and subsurface, prior to use by man's activities, use by phreatophytes, or evaporation from free water surfaces.

GROUND — (1) The solid surface of the earth. (2) The floor of a body of water, especially the sea.

GROUND WATER, also Groundwater — (1) Water that flows or seeps downward and saturates soil or rock, supplying springs and wells. The upper level of the saturate zone is called the Water Table. (2) Water stored underground in rock crevices and in the pores of geologic materials that make up the earth's crust. Ground water lies under the surface in the ground's Zone of Saturation.

GROUND WATER BARRIER — Rock, clay, or other natural or artificial materials with a relatively low permeability that occurs (or is placed) below ground surface, where it impedes the movement of ground water and thus causes a pronounced difference in the heads on opposite sides of the barrier.

GROUND WATER BASIN — A ground-water reservoir together with all the overlying land surface and the underlying aquifers that contribute water to the reservoir. In some cases, the boundaries of successively deeper aquifers may differ in a way that creates difficulty in defining the limits of the basin. A ground-water basin could be separated from adjacent basins by geologic boundaries or by hydrologic boundaries.

GROUND WATER, CONFINED — Ground water under pressure significantly greater than atmospheric, with its upper limit the bottom of a bed with hydraulic conductivity distinctly lower than that of the material in which the confined water occurs.

GROUND WATER DISCHARGE — (1) The flow of water from the Zone of Saturation. (2) (Water Quality) Ground water entering near coastal waters which has been contaminated by landfill leachate, deep well injection of hazardous wastes, septic tanks, etc.

GROUND WATER DISPOSAL — Refers to wastewater that is disposed of through the ground either by injection or seepage. This includes the following discharge methods: absorption beds, injection wells, drain fields, percolation ponds, rapid infiltration basins, and spray fields (land application). Land application systems (reuse systems) are considered a groundwater disposal method as the wastewater used to irrigate turf or crops is generally intended to filter down through the soil.

GROUND WATER DIVIDE — A line on a water table on either side of which the water table slopes downward. It is analogous to a drainage divide between two drainage basins on a land surface. It is also the line of highest Hydraulic Head in the water table or Potentiometric Surface.

GROUND WATER FLOW — The movement of water through openings in sediment and rock that occurs in the Zone of Saturation.

GROUND WATER FLOW MODEL — (1) A digital computer model that calculates a hydraulic head field for the modeling domain using numerical methods to arrive at an approximate solution to the differential equation of ground-water flow. (2) Any representation, typically using plastic or glass cross-sectional viewing boxes, with representative soil samples, depicting ground-water flows and frequently used for educational purposes.

GROUND WATER, FREE — Unconfined ground water whose upper boundary is a free water table.

GROUND WATER HYDRAULICS — The study of the movement of water, especially water under pressure and water's movement through various soil medium.

GROUND WATER HYDROLOGY — The branch of Hydrology that deals with ground water; its occurrence and movements, its replenishment and depletion, the properties of rocks that control ground water movement and storage, and the methods of investigation and utilization of ground water. Also referred to as Ground Water Hydraulics, although this term pertains more to the study of the motion of water.

GROUND WATER LAW — The common law doctrine of Riparian Rights and the doctrine of prior appropriation (Appropriative Rights) as applied to ground water. See Appropriation Doctrine and Riparian Doctrine.

GROUND WATER MINING — The withdrawal of water from an aquifer in excess of recharge which, if continued over time, would eventually cause the underground supply to be exhausted or the water table could drop below economically feasible pumping lifts.

GROUND WATER MOUND — Raised area in a water table or other Potentiometric Surface, created by Ground Water Recharge.

GROUND WATER OUTFLOW — That part of the discharge from a drainage basin that occurs through the ground water. The term "underflow" is often used to describe the ground water outflow that takes place in valley alluvium (instead of the surface channel) and thus is not measured at a gaging station.

GROUND WATER OVERDRAFT — The condition of a ground water basin in which the amount of water withdrawn by pumping exceeds the amount of water that recharges the basin over a period of years during which water supply conditions approximate average. Sometimes used interchangeably with Ground Water Mining.

GROUND WATER, PERCHED — Ground water that is separated from the main body of ground water by an impermeable (unsaturated) layer.

GROUND WATER PLUME — A volume of contaminated groundwater that extends downward and outward from a specific source; the shape and movement of the mass of the contaminated water is affected by the local geology, materials present in the plume, and the flow characteristics of the area groundwater.

GROUND WATER PRIME SUPPLY — The long-term average annual percolation to the major ground water basins from precipitation falling on the land and from flows in rivers and streams. Also includes recharge from local sources that have been enhanced by construction of spreading ground or other means. Recharge of imported and reclaimed water is not included nor is recharge using applied irrigation water.

GROUND WATER RECHARGE — Inflow of water to a ground water reservoir (Zone of Saturation) from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Also, the volume of water added by this process.

GROUND WATER REGISTRATION — A statement made by a well owner registering the Beneficial Use of ground water.

GROUND WATER RESERVOIR — An aquifer or aquifer system in which ground water is stored. The water may be introduced into the aquifer by artificial or natural means.

GROUND WATER RESERVOIR STORAGE — The amount of water in storage within the defined limit of the aquifer.

GROUND WATER RUNOFF — A portion of runoff which has passed into the ground, has become ground water, and has been discharged into a stream channel as spring or seepage water.

GROUND WATER STORAGE — The storage of water in ground water reservoirs.

GROUND WATER STORAGE CAPACITY — The space or voids contained in a given volume of soil and rock deposits. Also, the reservoir space contained in a given volume of deposits. Under optimum conditions of use, the usable ground water storage capacity volume of water that can be alternately extracted and replaced in the deposit, within specified economic limitations.

GROUND WATER SYSTEM — All the components of subsurface materials that relate to water, including Aquifers (confined and unconfined), Zones of Saturation, and Water Tables.

GROUND WATER TABLE — The upper surface of the Zone of Saturation for underground water. It is an irregular surface with a slope or shape determined by the quantity of ground water and the permeability of the earth materials. In general, it is highest beneath hills and lowest beneath valleys. Also referred to as the Water Table.

GROUND WATER, UNCONFINED — Water in an aquifer that has a water table.

GROUND WATER UNDER THE DIRECT INFLUENCE (UDI) OF SURFACE WATER — Any water beneath the surface of the ground with: (1) a significant occurrence of insects or other microorganisms, algae, or large-diameter Pathogens; or (2) significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity, or pH which closely correlate to climatological or surface water conditions. Under direct influence conditions are determined for individual sources in accordance with criteria established by the state.

GROUND WATER VELOCITY — The rate of water movement through openings in rock or sediment.

GROWING SEASON — (1) The period and/or number of days between the last freeze in the spring and the first frost in the fall for the freeze threshold temperature of the crop or other designated temperature threshold. (2) Also, the average number of days exceeding 32°F (0°C).

GROWTH MANAGEMENT PROGRAM — A program comprised of several techniques to coordinate public and private decisions about the location and timing of development in order to best utilize environmental and physical resources.

GROWING SEASON — (1) The period and/or number of days between the last freeze in the spring and the first frost in the fall for the freeze threshold temperature of the crop or other designated temperature threshold. (2) Also, the average number of days exceeding 32°F (0°C).

GROWTH MANAGEMENT PROGRAM — A program comprised of several techniques to coordinate public and private decisions about the location and timing of development in order to best utilize environmental and physical resources.