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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.

Gaging Station — A particular site on a stream, canal, lake, or reservoir where systematic observations of Gage Height or discharge are obtained through mechanical or electrical means. When used in connection with a discharge record, the term is applied only to those gaging stations where a continuous record of discharge is computed. Also referred to as a Gage.

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.

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).

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.]

General Stream Adjudication — A judicial proceeding to determine the extent and validity of all water rights within a given geographic area, for example, along a given river reach, throughout one or more river basins, or within a state.

Geographic Information System (GIS) — A computer information system that can input, store, manipulate, analyze, and display geographically referenced (spatial) 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. Related terms include Geohydrologic and Geohydrologist.

Geologic Time (History) — Geologic history can be divided into five great Eras of recorded time. These Eras and approximate time periods include:
[1] Archeozoic — 4,500 million years ago (MYA) to 3,500 MYA;
[2] Proterozoic (or Prepaleozoic) — 3,500 MYA to 586 MYA;
[3] Paleozoic — 570 MYA to 230 MYA;
[4] Mesozoic — 230 MYA to 65 MYA; and
[5] Cenozoic — 65 MYA to present.
Each time Era (except the first) is divided into Periods (e.g., the Cenozoic into the Quaternary and the Tertiary) and Periods are further divided into Epochs (e.g., the Tertiary into the Pliocene, Miocene, Oligocene, Eocene, and the Paleocene). For each time period, whether an Era, Period, or Epoch, there is a corresponding rock formation by which the time period has been dated. Rock formations constituting a specific (time) Era form a Group of rocks; those rocks having been formed during a specific (time) Period constitute a rock System; and those rock formations originating during a specific (time) Epoch are said to belong to a particular Series of rocks. Series of rock formations are further subdivided into Formations, Stages, etc.

(United States) Geological Survey (USGS) — An agency of the U.S. Department of Interior responsible for providing extensive earth-science studies of the Nation’s land, water, and mineral resources. The USGS was established by an act of Congress on March 3, 1879, to provide a permanent federal agency to conduct the systematic and scientific “classification of the public lands, and examination of the geological structure, mineral resources, and products of national domain.” An integral part of that original mission is to publish and distribute the earth-science
information needed to understand, plan the use of, and manage the nation’s energy, land, mineral, and water resources. Since 1879, the research and fact-finding role of the USGS has grown and been modified to meet the changing needs of the nation it serves. As part of that evolution, the USGS has become the map-making agency for the federal government, the primary source of data on surface- and ground-water resources of the nation, and the employer of the largest number of professional earth scientists. The USGS is organized into three operational
Divisions: the National Mapping Division (NMD), charged with development and application of mapping and Geographic Information System (GIS) technology; the Geologic Division (GD), which conducts geologic mapping and research; and the Water Resources Division (WRD). The mission of the Water Resources Division of the USGS is to provide the hydrologic information and understanding needed to manage the nation’s water resources to benefit its residents.

Geomorphic Surface — A mappable area of the land surface formed during a defined time period by deposition or erosion (or both, in different parts) of at least a thickness of material sufficient to accommodate a pedogenic soil. Its age (i.e., period of formation) ordinarily is defined by relations to other geomorphic surfaces, or by the soils or sediments that form or underlie 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.

Giardia Lamblia — A flagellate protozoan that causes the severe gastrointestinal illness Giardiasis, when it contaminates drinking water.

Giardiasis — A disease that results from an infection by the protozoan parasite Giardia Intestinalis, caused by drinking water that is either not filtered or not chlorinated. The disorder is more prevalent in children than in adults and is characterized by abdominal discomfort, nausea, and alternating constipation and diarrhea.

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.

Ground water — In general, any water that exists beneath the land surface, but more commonly applied to water in fully saturated soils and geologic formations.