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Relative size of fluvial and glaciated valleys in central Idaho
Byron E. Amerson
a, , David R. Montgomery
b
, Grant Meyer
c
a
Stillwater Watershed, Ecosystem, and Riverine Sciences, Inc., Seattle, WA, 98105, USA
b
Department of Earth and Space Sciences, University of Washington, Seattle WA 98195, USA
c
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA
Received 28 March 2007; accepted 2 April 2007
Available online 6 April 2007
Abstract
Quantitative comparisons of the morphometry of glaciated and fluvial valleys in central Idaho were used to investigate the
differences in valley relief and width in otherwise similar geologic and geomorphic settings. The local relief, width, and cross-
sectional area of valleys were measured using GIS software to extract information from USGS digital elevation models. Hillslope
gradients were also measured using GIS software. Power-law relationships for local valley relief, width, and cross-sectional area as
a function of drainage area were developed. Local valley relief in glaciated valleys relates to drainage area with a power-law
exponent similar to fluvial valleys, but glaciated valleys are deeper for a given drainage area. Local valley width in glaciated
valleys is greater than in fluvial valleys, but the exponent of the power-law relationship to drainage area is similar in both valley
types. Local valley cross-sectional area in glaciated valleys increases with drainage area with a power-law exponent similar to
fluvial valleys, however, glacial valleys have roughly 80% greater cross-sectional area. Steep valley walls in glaciated basins
increase the potential for bedrock landsliding relative to fluvial basins. Both the Olympic Mountains of Washington and valleys in
central Idaho show relationships in which glaciated valleys are up to 30% deeper than fluvial valleys despite differences in
lithology, tectonic setting, and climate.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Idaho; Glacial; Fluvial; Erosion; Morphometry; Relief; Valley
1. Introduction
Understanding the physical form of glaciated valleys
is necessarily rooted in theories on how glacial erosion
functions. The physical form of a glaciated valley is the
product of the interaction between the effectiveness of
erosion by glacier ice and the resistance to erosion and
the structure of the country rock in which the glaciated
valley resides. There are three broad aspects of glaciated
valley morphometry that have been the focus of
research: rates of sediment evacuation and relief
development (
Small and Anderson, 1998; Whipple et
al., 1999; Brocklehurst and Whipple, 2002, Montgom-
ery, 2002b, Koppes and Hallet, 2006; Mitchell and
Montgomery, 2006; Oskin and Burbank, 2005
), devel-
opment of the longitudinal profile (e.g.
MacGregor
et al., 2000
), and development of cross-sectional form
(e.g.
Harbor, 1995; Augustinus, 1995
).
Studies on the cross-sectional form of glaciated
valleys have used
form parameters including depth-
to-width ratio (form ratio) and the value of coefficients
of quadratic or power-law equations fitted to valley
wall profiles to empirically describe glacial valley
Available online at www.sciencedirect.com
Geomorphology 93 (2008) 537
547
www.elsevier.com/locate/geomorph
Corresponding author.
E-mail address:
byron@stillwatersci.com
(B.E. Amerson).
0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:
10.1016/j.geomorph.2007.04.001 Author's personal copy
morphology and regional distinctions (e.g.
Harbor,
1992
). The use of form parameters has allowed com-
parison between valley morphologies to help under-
stand erosional histories and investigate ways in which
variations in climate, erosional processes, and regional
lithology influence the character of particular terrain.
The prior erosional history of a valley has important
implications for the pattern of erosion by a glacier. For
instance,
Harbor (1992)
has shown that a valley that is shaped in profile (ostensibly generated by fluvial
processes) is initially subjected to greater erosion
along its margins than along its bottom when occupied
by a glacier than in a valley that is already
[ shaped. The
volume of sediment present on the valley floor has an
influence on subsequent erosion by either glaciers or
rivers because it retards bedrock erosion and thus bed
lowering. Valley bottom sediment volume also can
affect the shape of valley cross-sectional profiles.
Seismic data from Yosemite Valley in the Sierra Nevada
show that glaciated valleys can be filled with substantial
deposits of sediment, which can mask the total relief of
the bedrock surface (
Gutenberg et al., 1956
).
The volume of sediment stored in the valley bottom
can affect analysis of form parameters as well. The ex-
ponent of a power-law function fit to cross-sectional pro-
files has been shown to be sensitive to the bedrock datum
used to fit them, and so knowing the true bedrock profile
that is buried by valley bottom sediment is paramount if
meaningful comparisons between studies are to be made
when considering form parameters (
Harbor, 1992
).
Here we quantitatively analyze and compare the
relief and width of glaciated and fluvial valleys in
central Idaho to define morphometric relationships
among valley characteristics. As relief has been
described in various ways in the literature (
Montgomery,
2002a
), we consider local valley relief measured at
specific sites in a valley to consist of the elevation
difference between the valley floor and the mean
elevation of the subtending ridgelines. We define
glaciated valleys as valleys that were occupied by
glaciers during the last glaciation, as shown by little-
modified lateral and terminal moraines along many
lower margins, and by a large proportion of valley area
above the regional late Pleistocene equilibrium line
altitude (ELA) (
Meyer et al., 2004
). It is probable that
the drainage network and density of our glaciated
valleys were initially set by fluvial processes prior to the
onset of Quaternary glaciation, and that following
deglaciation, fluvial processes have again become the
dominant form of bed erosion in glaciated valleys, so
that the present form of these valleys is the result of a
complex erosional history. Hence, we seek to evaluate
the degree to which glacial erosion modified valley size.
The specific aim of the research is to investigate the
relationship of valley morphometry (local relief, width,
and cross-sectional area) with drainage area in glaciated
and fluvial basins. This work focuses not on specific
mechanisms of valley formation, but on the net result of
these erosional processes.
2. Glacial erosion processes and valley formation
Glacial erosion is considered to consist of four
components: abrasion, plucking, dissolution, and scour
by subglacial meltwater. Mechanistic models for the first
two components of glacial erosion are relatively advanced
(e.g.,
Hallet, 1979, 1996
), but erosion by subglacial
meltwater and dissolution are less well understood
(
Goudie, 2002
). In general, all of these processes are
influenced by whether the glacier is frozen to its bed
in
which case the ice moves by internal deformation alone,
and the bed remains unaffected. Large, alpine glaciers in
temperate latitudes are generally wet-based and therefore
not frozen to their beds because of moderate climate and
pressure melting of ice at the bed of the glacier. In contrast,
in polar latitudes, or during periods of cold climate,
freezing of glaciers to their beds is possible. In both cases,
local variation in temperature profiles at the glacier bed
are driven by seasonal subglacial water fluxes and heat
flow feedbacks between the ice surface slope and the ice
bed, particularly in areas of overdeepening (
Alley et al.,
1999, 2003
).
Hallet (1979, 1996)
proposed mechanical
models for glacial abrasion and quarrying (plucking) that
apply to glaciers whose basal ice has a relatively sparse
load of rock fragments. As more material is excavated and
entrained into the basal ice, the effective erosion slows
because interactions between the particles in the foot of
the glacier begin to dominate. Further erosion is limited by
the capacity of subglacial water flux to transport abraded
and plucked sediment out of the system so that fresh
bedrock is continually exposed to further erosion. The
integrated effects of these elements of glacial erosion give
rise to glaciated valley morphometry.
In alpine terrain, local differences between abrasion
and plucking combine with regional lithologic properties
to form characteristic long profiles of glaciated valleys
(
Hooke, 1991; Augustinus, 1995; MacGregor et al., 2000;
Brook et al., 2004
). In general, long profiles of glaciated
valleys are less concave than the profiles of fluvial valleys
and are frequently stepped. Cirque headwalls and over-
deepenings are genetically similar and result primarily by
plucking of bedrock rather than abrasion (
Hooke, 1991;
Oskin and Burbank, 2005
). The plucking of bedrock in
overdeepenings is facilitated, in part, by the addition of
538
B.E. Amerson et al. / Geomorphology 93 (2008) 537
547 Author's personal copy
meltwater via fractures in the ice, which contributes to
variability in glacier velocity patterns and subsequent
freeze
thaw dyna