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Unformatted text preview: wth, a concept applied to landforms and especially stream systems, is derived
from the study of allometry, the recognition that the proportional sizes of rates of growth of
parts of organisms are comparable and often quantifiable; as an example of its application to
fluvial geomorphology, it includes the measurement of increases in stream-channel width and
depth with increases in discharge of the stream. Hydraulic-geometry relations are based on an
inference of allometric growth.
Alluvial aquifer is a partially saturated deposit of alluvium that yields water to wells; most
alluvial aquifers are unconfined and are composed of channel and flood-plain sediment; the
water that partially saturates an alluvial aquifer is largely derived from the stream that
transported and deposited the sediment.
Alluvial fan is a wedge-shaped deposit of recent stream alluvium (erosion products) or poorly
consolidated rock debris that radiates outward and downslope as, in plan view, an open fan
from a site draining an area of high relief or topography, such as the mouth of a mountain
valley, onto a gentler slope, typically a pediment or an alluvial plain; the deposit is thickest at
the fan apex, near the valley mouth, and thins to a feather edge at the distal edge of the fan.
Active alluvial fans are surfaces of net deposition whereas inactive alluvial fans generally exhibit
erosion and stream incision at the apex, the depth of incision decreasing with distance
downslope to the distal edges of the fan.
Alluvium is a general term for sediment deposited in a streambed, on a flood plain or other
bottomland feature, delta, or at the base of a mountain during comparatively recent geologic
time. 4 Anabranch of a stream is a separate channel that has diverged from the main channel and
rejoins the stream at some downstream site; an anabranch is a discrete, semi-permanent
channel that may be of equal or smaller size as the main channel, thereby distinguishing it from
channel braids that are not discrete and may be highly ephemeral.
Angle of repose is the maximum departure from horizontal, expressed as an angle, at which a
slope formed of loose, cohesionless sediment retains stability. The angle of repose is a function
of the frictional properties of the sediment and the angularity of the sediment grains; it varies
from about 30 degrees for coarse, rounded particles to about 39 degrees for angular particles
of sand, and is typically 32 to 34 degrees for dry sand of dunes.
Annual flood is the maximum instantaneous discharge, typically expressed in cubic meters per
second (m3 s-1), that occurs at a stream site in a water year (October 1 through September 30).
Annual flood series is a list of annual floods measured at a streamflow gage site for the period
of record or a selected part of the period of record.
Antidune, in a fluvial setting or on a stream bed, is a transient sand wave or dune that moves
upstream by processes in which erosion of sand particles occurs on the downstream slope of
the bedform followed by deposition of the sand particles on the next upstream slope.
Aquiclude is a nearly impermeable rock body that can absorb water very slowly and hence can
only release the absorbed water to springs or seeps very slowly; where an aquiclude is at or
near a lowland surface, swampy conditions may occur owing to the inability of surface water to
Aquifer is any rock body or geologic deposit of alluvium or similar rock debris that is partially or
fully saturated with ground water and has properties of permeability (transmissivity) and
porosity that enable it to yield the ground water to a well or spring at a rate significantly high to
fulfill a specified purpose; aquifers are grouped as unconfined, those controlled by near-surface
gravitational and atmospheric-pressure conditions, and artesian, those that are poorly
connected to the land surface due to an impermeable layer separating it from the land surface.
Aquifuge is an impermeable rock body that cannot absorb water and hence cannot release
water to springs or seeps very slowly; where an aquifuge is at or near a lowland surface,
swampy conditions may occur owing to the inability of surface water to infiltrate.
Archaeology is a composite science involving the study and understanding of both recent-past
and ancient peoples and cultures through the excavation of sites of habitation and the recovery
of artifacts; the practice of archaeology typically requires the application of disciplines including
soil science, paleontology, hydrology, geomorphology, chemistry, and sedimentology, and the
use of analytical techniques such as the identification of clays and rock fragments, particle-size
analysis, and dating by radiocarbon analysis.
Arête (from the French term for fishbone) is a sharp-edged, serrated ridgeline feature of a high
alpine area sculpted by progressive back erosion of steep bedrock walls above two or more
cirques formed by the glaciers occupying the cirques. A specific typ...
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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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- The Adventures of Tom Sawyer