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Unformatted text preview: e of arête is col, which is a
pass or saddle generally separating two glaciated peaks and bounded on both sides by cirques.
Arid refers to a climatic zone that receives average precipitation of less than 250 mm annually.
5 Armoring, as a fluvial-geomorphic process, is the winnowing of fine particles from the
uppermost bed sediment of a stream channel, resulting in a bed-surface layer of generally
gravel to boulder sizes that are resistant to scour; because armoring occurs at specific flow
rates, the armor layer may be susceptible to removal by higher flow and sedimentation during
lower flow. Armoring occurs on hillslopes by similar winnowing processes of eolian removal of
fine sediment, or by a variety of related processes, such as gully gravure, that result in the
concentration of coarse sediment or rock particles as a surface veneer.
Arroyo is a gully or small channel, generally in arid and semiarid areas of northern Mexico or
the southwestern United States, in which streamflow occurs inconsistently or infrequently and,
except during periods of streamflow, is directly underlain by unsaturated alluvium; arroyos
typically have a rectangular to steeply sided trapezoidal cross section, banks a meter or more in
height formed of fine-grained, poorly consolidated over-bank sediment, and a nearly flat, sandy
bed. Synonyms are ephemeral-stream channel, dry wash, and wadi (southwestern Asia,
Arabian Peninsula, and northern Africa)
Aspect, when used in a geomorphic context, refers to orientation or the compass direction in
which a landform or surface faces (the north-facing slope of a mountain has a northerly aspect).
Association, as applied to ecology, is a plant community, or an assemblage, the plants of which
exhibit similar environmental requirements; an association generally includes one or more
dominant species. As applied to soils and soil science in the United States, an association of an
area refers to two or more soils that are closely related, and are mapped as contiguous
features, owing to similar conditions of climate, topography, and vegetation; if the soils have
similar characteristics owing to common parent material from which they have developed, they
represent a catena.
Avalanche is a volume of snow, ice, soil, or rock fragments, or more commonly a mixture of
them, that moves very rapidly down a slope by gravity; the term is most typically applied to the
sudden movement of snow and ice in mountainous areas, but very poorly sorted deposits
resulting from similar slope failures of de-stabilized soil and near-surface rock material, as a
debris avalanche, are widespread in high-relief areas.
Avulsion, as applied to fluvial processes, is a rapid change in the course or position of a stream
channel, especially by incision (erosion) of lowland alluvium, to bypass a meander and thereby
shorten channel length and increase channel gradient; avulsion commonly occurs during floods
but also can occur by normal processes of lateral migration of a stream channel during nonflood discharges. For legal purposes, bottomland areas, including channel islands, repositioned
relative to the prior channel by avulsion belong to the previous owner and remain in the
political jurisdiction (state or county) to which they had formerly belonged. 6 B
Backwater, referring generally to natural hydrologic systems, is any volume of water that is
backed up or prevented from moving downslope or downstream by any barrier obstructing
movement; in hydrology, backwater often is the slowing or reversal of flow in a stream or
tributary upstream from its confluence with another stream that is at flood stage.
Badlands is a semi-technical term referring to a very rugged topography of badly eroded, soft,
relatively flat-lying sedimentary rocks that are deeply incised by rills and gullies; badlands, such
as the prototype example of western South Dakota, generally occur in areas of arid or semiarid
climate that sustain short-duration but high-intensity thundershowers, have steep side slopes
that support little or no vegetation, exhibit virtually no soil development, and yield copies
amounts of sediment to runoff and streamflow following rainfall.
Bajada is a group of coalesced alluvial fans, generally deposits of numerous streams draining
and eroding parts of the same mountain range, that form a broad band along and downslope
from a mountain front; because bajadas are composites of many alluvial fans, their surfaces are
of irregular shape. Bajadas are most common in arid and semiarid areas and were named for
landforms adjacent to mountain ranges of the southwestern United States and northern
Bank, as a geomorphic concept, typically refers to a sloping margin of a natural, stream-formed,
alluvial channel that confines discharge during non-flood flow; within the earth sciences,
designation of a right or left bank is done when looking in the downstream direction.
Bankfull discharge, a hydrologic term, is the flow rate (m3 s-1) when the stage (height) of a
stream is coincident with the uppermost level of the banks -- the water level at channel
capacity, or bankfull stage. Thus, the conce...
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