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Unformatted text preview: edmont is a gently sloping surface extending from the base of a mountain or mountain range
toward the valley center area; an extensive piedmont of the eastern United States lies along the
eastern flank of the Appalachian Mountains and in the western United States numerous
discontinuous piedmont areas are at the bases of block-fault mountains of the Basin and Range
Pinyon-juniper refers to an open-woodland plant community, ecosystem, or habitat, of
semiarid parts of North America (especially piedmont areas of New Mexico, Arizona, and Utah)
that are dominated by pinyon pine (Pinus edulis and/or P. cembroides) and various species of
juniper (especially oneseed juniper, Juniperus monosperma, and Rocky Mountain juniper, J.
scopulorum); a pinyon-juniper community is comprised of one or more indicator species of
pinyon and juniper genera, and generally occurs on well drained sandy to gravelly soils of
moderately to steeply sloping pediments and alluvial fans that have mean-annual precipitation
range of 250 to 400 mm,
Piping is an erosional process, by water percolating through unsaturated soil or subsoil, that
results in removal of generally fine-grained sediment particles by downward and lateral
migration and the formation of small conduits, tunnels, or pipes through which the water and
entrained sediment moves; the conduits created by the piping process may cause collapse of
overlying sediment and surface expression as small depressions and gullies. 32 Plant community is a dynamic group or association of complementary plant species that
compete for the same water, nutrient, and mineral resources of a limited area with specific
ranges of physical characteristics.
Plant density, a measure of plant abundance, is the number of individual plants, or stems, of a
species, a group of species, or of all species, per unit area; the concept of plant density is often
extended to express how the plants are distributed, such as random, clumped, or organized
according to land-surface gradients. The concept of plant density is complicated by the
difficulty in defining area; for example, what is the area of a steep slope?
Plant cover, a measure of plant abundance, is the amount, generally as a percent, of a unit area
that is visually covered when viewed from above; plant cover can be applied to all species, to
tree canopy for forests, or to ground cover when limited to smaller species; if observations
suggest a plant cover of 50 percent, 50 percent of the area is also bare and subject to erosion
by raindrop impact. The concept of plant cover is complicated by issues of area (what is the
area of a steep slope?), amount of cover (are there multiple leaf layers within a plant, between
plants, or between groups of vegetation?), and completeness of cover (are there holes between
the leaves or branches of a plant through which light can penetrate?). Because plant cover
reduces the potential erosive effect of raindrop impact, it is a critical consideration in the
studies of erosion, geomorphology, and sedimentology.
Playa is an ephemeral lake of an arid or semiarid area, the floor of which supports sparse to
seasonal vegetation and is underlain by fine-grained deposits washed to the lake bed by
infrequent precipitation events. Sediment of the playa floor typically is mostly silt and clay with
abundant organic material and salts that precipitate as water evaporates following runoff to
the basin interior. Sediment underlying many playas of the Basin and Range Province is
saturated at shallow depth; playa floors of the Southern High Plains in northern Texas and
western New Mexico generally lie well above the zone of saturation. A playa basin, or bolson,
has interior drainage toward the playa and is the surface area that contributes excess
precipitation, as runoff, to the playa.
Pluvial, from the Latin word for rain, connotes rain or rainy. As used in the earth sciences, the
word signifies a period of abundant rainfall and runoff and the hydrologic and earth-surface
processes effects of that rainfall and runoff; a principal geomorphic use of the term pluvial is to
distinguish parts of Pleistocene time when rainfall rates, that were greater than present rates in
much of the warmer-climate areas of North America, and runoff resulting from abundant
snowfall in mountainous areas generated high water levels in pluvial lakes, such as Lake
Bonneville and Searles Lake of the Basin and Range Province.
Point (or meander) bar is bed sediment, generally sand and gravel, that is deposited on the
inside part of a meander curve as part of the normal process of fluvial sorting of sediment.
Pool, as applied to alluvial stream channels, is a relatively deep, low velocity reach of quiescent
flow between upstream and downstream riffles, or rapids, at which the flows are ordinarily
more rapid and turbulent. 33 Pool-riffle sequence in alluvial stream channel refers to a succession of one or more
combinations of pools and riffles along the channel in the downstream direction; during flood
the normally low water velocities in pools and higher water velocities at riffles are reversed,
causing scour and removal of accumulated sediment from pooled reaches and deposition of
bed sediment on riffles.
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This document was uploaded on 02/26/2014 for the course ES 322 at Western Oregon University.
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