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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

gage

1. A device for indicating the magnitude or position of a thing in specific units, when such magnitude or position undergoes change. For example: the elevation of a water surface, the velocity of flowing water, the pressure of water, the amount or intensity of precipitation, the depth of snowfall, etc.
2. The act or operation of registering or measuring the magnitude or position of a thing when these characteristics are undergoing change.
3. The operation, including both field and office work, of measuring the discharge of a stream of water in a waterway.

gage datum

The arbitrary "zero plane" from which all stage measurements are taken from. Usually set below the natural bottom of the channel so all stage height readings will be greater than zero.

gage height

The water-surface elevation referred to some arbitrary gage datum. Gage height is often used interchangeably with the more general term stage, although gage height is more appropriate when used with a reading on a gage.

gage zero

Same as gage datum.

gaging station

A particular site on a stream, canal, lake, or reservoir where systematic observations of stage or streamflow are obtained.

gallery

A passageway within the body of a dam or abutment.

gate

A device in which a leaf or member is moved across the waterway from an external position to control or stop flow. There are many different kinds of gates used on a dam. Some of the more common types are bulkhead, crest (or spillway), emergency, fixed wheel, flap, flood, guard, outlet, radial, regulating, and slide gates.

geohydrology

That branch of hydrology relating to subsurface, or subterranean waters.

geophysics

The study of the physical characteristics and properties of the earth; including geodesy, seismology, meteorology, oceanography, atmospheric electricity, terrestrial magnetism, and tidal phenomena.

Geostationary Orbiting Environmental Satellite (GOES)

A series of artificial satellites orbiting 22,300 miles (35,900 km) above the earth's surface which remains above the same locations on the earth. DCPs transmit river and rainfall data to the GOES for relay to a ground-receive site in Wallops Island, VA.

glacier

Bodies of land ice that consist of recrystallized snow accumulated on the surface of the ground, and that move slowly downslope.

glacier-dammed lake

The lake formed when a glacier flows across the mouth of an adjoining valley and forms an ice dam.

GOES DCS (Data Collection System)

A data collection system under NESDIS which is comprised of the DCPs, and the NESDIS Command and Data Acquisition (CDA) System components. This satellite-based system collects a variety of environmental data from locations in the western hemisphere. The system is a data relay network for more than 10,000 DCPs which transmits data to one of two GOES satellites (East and West). These data are relayed to the NESDIS CDA ground station located at Wallops Island, VA. The data are then relayed over to Silver Springs, MD, where the data is then distributed to the appropriate recipients.

gravity dam

A concrete structure proportioned so that its own weight provides the major resistance to the forces exerted on it.

Ground receive sites

A satellite dish and associated computer which receives signals from the GOES satellite, decodes the information, and transmits it to a another site for further processing. The GOES satellite ground-receive site is located at Wallops Island, VA; and the information is relayed to a mainframe computer at NWS Headquarters for processing.

Grounded ice

Ice which has run aground or is contact with the ground underneath it.

groundwater

Water found in that portion of the soil which is always saturated or below the water table. Also called "phreatic water."

groundwater divide

A line on a water table where 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.

groundwater hydrology

The branch of hydrology that specializes in groundwater, its occurrence and movements, its replenishment and depletion, the properties of rocks that control groundwater movement and storage, and the methods of investigation and utilization of groundwater

groundwater flow

Streamflow which results from precipitation which infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as baseflow, or dry-weather flow.

groundwater mining

Pumping groundwater from a basin where the safe yield is very small, thereby extracting groundwater which had accumulated over a long period of time.

groundwater outflow

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

groundwater overdraft

Pumpage of groundwater in excess of safe yield.

groundwater runoff

That part of runoff which has passed into the ground as precipitation or snowmelt, has become groundwater, and has been discharged into a stream channel as spring or seepage water.

grout curtain

A barrier produced by injecting grout into a vertical zone, usually narrow (horizontally), and in the foundation to reduce seepage under a dam.