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Unformatted text preview: resh water, in contrast to brackish water or salt water, is water (or ice) on or beneath a land
(or water) surface that accumulates as a result of natural processes of precipitation; fresh water
contains dissolved solids (mostly salts) of insufficient concentration to cause deleterious effects
by ingestion of living organisms other than those adapted to sea water or similar supplies of
water with high concentrations of dissolved solids.
Froude number is a dimensionless index to characterize the type of flow, or tranquility of flow,
in a channel or similar hydraulic structure; it is defined mathematically as:
v/√gd, in which, in consistent units, v is water velocity, g is the acceleration due to gravity, and
d is depth of flow. G
Gage height, as determined for a gaging station, is the water-surface elevation, or stage,
relative to an arbitrary datum.
Gaging station, also referred to as a streamgage, is a specified site on a stream, channel, canal,
lake, or reservoir where systematic observations of streamflow or related hydrologic data are
Generalized skew coefficient is a skew coefficient, determined by the integration of many
values of skew coefficient within a specified area, that is assumed applicable to all streams of
the area. 22 Geomorphic threshold is a critical characteristic or condition of a landscape or geomorphic
system that, if rendered unstable by some measure of disturbance exceeding the critical level
of stability, change within the system, often as a sequence of responses, is induced;
geomorphic responses to an external stress exceeding a threshold are termed extrinsic,
whereas those occurring because on-going change within a system has caused a threshold to be
exceeded are termed intrinsic.
Geomorphology, a composite science, is the study of landforms including, in recent times
especially, investigations into the processes that cause and alter the landforms.
Glacial refers to the conditions, processes, features, and landforms of those areas with
adequate precipitation and sufficiently low temperatures that snow and ice accumulates, or
formerly accumulated, to a thickness that deformation and the flow, or movement, of glacial
Glacier is a large mass of ice formed wholly or mostly on land by the compaction and
recrystallization of snow, possibly with ice additions from the freezing of meltwater or rainfall,
that creeps slowly downslope by gravity-induced deformational processes; glaciers have
permanence measured in periods exceeding decades or centuries and range in size and
environment from small alpine glaciers of high-elevation or high-relief areas to flowing ice
sheets that are sub-continental in areal scale and that can move on very low slopes.
Gleying is a process of soil genesis, commonly of well weathered clay horizons, in which the soil
becomes mottled, generally in a tightly layered manner, caused by partial oxidation and
reduction of ferric-iron compounds due to fluctuating ground-water levels (intermittent
episodes of saturation and aeration). The amount of time indicated by gleying is indefinite but
often it is interpreted to suggest that the gleyed zone dates from sediment deposited in a
slough or similar depression of a flood-plain surface of a paleo-landscape.
Gradient, as applied to stream channels, is the rate of elevation change between two specified
sites of horizontal distance measured along the thalweg of the channel; it is generally
expressed as a non-dimensional number (m m-1).
Gravel, as fluvial sediment, is sediment defined to be of particle diameter between 2 and 64
mm in diameter (b-axis).
Ground water refers to water in the subsurface that saturates the rocks and sediment in which
it occurs; the upper surface of ground-water saturation is commonly termed the water table.
Ground-water reservoir is a saturated body of ground water having loosely definable spatial
limits; among the goals of ground-water hydrology is the objective of determining the volumes
of water in ground-water reservoirs.
Gully is a small hollow or channel worn in earth or unconsolidated material, as on a hillside, by
running water and through which water runs only after a rain or the melting of ice or snow; it is
larger than a rill and smaller than a stream channel.
Gully erosion is the displacement of soil or soft rock particles by running water that forms
distinct, narrow incisements that are larger and deeper than rills and that usually carry water
only during and immediately after heavy rain or the melting of ice or snow.
23 Gully gravure, a French term meaning to dig or engrave, refers to a sequence of slope-retreat
processes in which a hilltop capping of coarse, erosion-resistant rock debris concentrates in
channels or depressions formed by erosion on a hillslope below of less resistant geologic
materials. The coarse rock debris gradually entraps interstitial finer-grained erosion products,
either derived from the surrounding material or weathered from the deposited debris. Th...
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