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Unformatted text preview: die, ed., 2003), The Encyclopedic Dictionary of Geography (Goudie,
ed., 1994), A Dictionary of the Natural Environment (Monkhouse and Small, 1978), Limited
Glossary of Selected Terms (MacArthur and Hall, 2008), Glossary of terms relating to the
phreatophyte problem (Phreatophyte Subcommittee PSIAC, 1962), and Bulletin 17B of the
Hydrology Subcommittee, Interagency Advisory Committee on Water Data (1982). Appendix 1
lists references for the information sources cited above; others were used less extensively and are
not listed. 2 Definition and description of selected terms:
Ablation, as applied to geomorphology, is the wasting and removal from a rock mass of
material by physical processes such as wind erosion or by chemical processes, including
dissolution of cementing agents followed by failure of rock material at the chemically altered
surface. As applied to glaciology, ablation is the removal of snow and ice from any mass of
frozen water through processes of melting, sublimation and evaporation, wind erosion, and
calving (such as failure at the snout of a glacier).
Abrasion, a form of mechanical weathering, is the reduction of rock fragments or rock surfaces
by the wearing, grinding, or rubbing action of other rock particles or of the transport medium,
particularly ice in glacial or periglacial environments. Particle-to-particle impact of the
sediment load is the principal process of abrasion in streams and the cause of rounding of the
sand and coarser stream sediment that comprises bed material. Similar but less dynamic
abrasion occurs in glaciers.
Accelerated erosion is erosion that occurs at a more rapid rate than is typical for a specified site
or area. The term generally refers to human-induced land-surface disturbance, especially
disruption of soil structure and destruction of natural soil cover by rock fragments and
vegetation, that reduces the ability of soil to resist the erosive effects of raindrop impact,
overland flow, concentrated flow in rills and gullies, and wind velocities great enough to detach
and entrain soil particles. Most accelerated erosion is the result of human activity (agricultural,
grazing, logging, surface mining, urban construction), but it also occurs naturally when a
geomorphic threshold is exceeded by processes such as slope failure, avulsion, the effects of
high-magnitude floods, fire, avalanche, or plant disease.
Accretion, relative to fluvial geomorphology, is a natural process of gradual sedimentation on
channel features, especially channel bed and banks, or bottomland surfaces, including the flood
plain and low-lying terraces, that extends steep surfaces channelward and slowly increases the
elevation of near-horizontal bottomland surfaces through the deposition of fine fluvial
Active channel of an alluvial stream is a short-term geomorphic feature subject to change by
prevailing discharges; its upper limit is defined by a break in the relatively steep bank slope of
the active channel to a more gently sloping surface beyond the channel edge. The break in
slope normally coincides with lower limit of perennial vegetation so that the two features,
individually or in combination, define the active-channel reference level.
Active-channel shelf is a gently sloping riparian surface of an adjusted alluvial bottomland that
normally extends from the break in slope of the channel banks that marks the active-channel
edge to the higher bank slope that rises to the edge of the flood plain; the active-channel shelf
typically corresponds to a stage approximating mean discharge of perennial streams and is
inundated between 5 and 20 of the time.
Actual evapotranspiration is the actual loss of water to the atmosphere, as a rate or volume,
from a land surface through combined evaporation from the soil and transpiration of plants. 3 Adjustment, as applied to geomorphology in general and to fluvial systems in particular, is the
tendency of non-rigid landforms, such as stream channels, to change in size and shape in
response to the changing effects (mostly fluxes) of water, sediment, dissolved solids, and
organic matter that alter them or pass through them.
Adventitious describes a plant part, generally a limb or root of a tree, that grows from the tree
following damage or disturbance; adventitious limbs and trunks are common on willow or
cottonwood trees that have been damaged during a flood, the date of which often can be
determined by applying techniques of dendrochronology to the adventitious limb or trunk.
Aggradation is the raising or elevating of a bottomland surface through the process of alluvial
deposition; conceptually it is the vertical component of accretion and is most frequently
applied to sediment deposition on a channel bed, bar or other near-channel surfaces, flood
plain, or, less often, low-lying alluvial terrace.
Alkalinity of a naturally occurring water is a measure of its capacity to neutralize acids and is
primarily the result of dissolved salts of weak acids; bicarbonate is a principal form of alkalinity
owing to chemical reactions of carbon dioxide dissolved in water with calcium carbonate and
other soil components.
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