Channel Bed Mobility Downstream from the Elwha Dams,
Washington*
Molly Pohl
San Diego State University
Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been
conducted on the environmental impacts of large structures in the United States, smaller dams have received less
attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream
channel system using field data collection at river cross-sections. The relationship of average boundary shear
stress (to) to critical shear stress (tcr) served as the basis for determining channel bed material mobility under the
two-year and ten-year flood events. The channel had the greatest channel bed mobility at the natural crosssection upstream from the dams, low bed mobility between the structures, and an increase in channel bed
mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated
with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their
associated young riparian communities. Although these run-of-the-river dams do not modify streamflow
greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River. Key Words: dams, channel change, bed mobility, dam removal,
river restoration.
Introduction
R
ivers are one of the most dramatically
modified elements of the world’s natural
environment. The majority of large river systems in the Northern Hemisphere are strongly
or moderately affected by dams and flow regulation (Dynesius and Nilsson 1994). In the
United States, more than 75,000 dams fragment
our rivers, leaving less than 2% of the total length
of our streams in natural conditions (Echeverria,
Barrow, and Ross-Collins 1989; Graf 1999). Although dams provide important services such as
flood protection and power generation, the environmental costs of these structures is becoming apparent. Dams typically modify the stream
hydrology and sediment dynamics, which cause
adjustments in the downstream channel system
( Petts 1979; Andrews 1986; Schmidt and Graf
1990; Everitt 1993; Topping, Rubin, and Vierra
2000). Since the river channel and floodplain
system are the foundation for aquatic and riparian biological processes, these physical adjustments affect the ecological integrity of river
communities (Rood and Mahoney 1990; Nils-
son et al. 1991; Johnson 1994; Ligon, Dietrich,
and Trush 1995; Stevens et al. 1997; Jansson
et al. 2000). The nature and magnitude of the
environmental response of a river to dam imposition in the United States is variable, determined by the structural characteristics of the
dam, the operational rules, and the environmental context (see reviews by Williams and Wolman 1984; Collier, Webb, and Schmidt 2000).
Research addressing the impacts of dams on
downstream river systems often emphasizes
large, multipurpose dams such as Glen Canyon
Dam on the Colorado River (Graf 1980;
Andrews 1991; Kearsley, Schmidt, and Warren
1994; Topping, Rubin, and Vierra 2000). However, large structures (reservoir capacities
! 1.2 " 109 m3) represent only 3% of American dams (Graf 1999). Smaller structures,
representing the vast majority of American dams,
have received less scientific attention.
In this study, I investigated the impacts of
two dams of moderate size (reservoir capacities # 50 " 106 m3) on the channel system of
the coarse-bed Elwha River, Washington. The
primary research question addressed is: How
* This research was supported by a National Science Foundation Graduate Research Fellowship, a Geological Society of America Graduate Research
Grant, and an Arizona State University Graduate Research Support Grant. This study would not have been possible without the invaluable field
assistance of the K’lallam Tribal Fisheries Office, and support from a number of other federal agencies including the National Park Service, the Bureau
of Reclamation, the Army Corps of Engineers, and the Geological Survey. The author is grateful to Kent and Rosemary Brauninger and Paul
Crawford for their instrumental role in the logistical arrangements for the field season.
The Professional Geographer, 56(3) 2004, pages 422–431 r Copyright 2004 by Association of American Geographers.
Initial submission, August 2002; revised submissions, February and August 2003; final acceptance, September 2003.
Published by Blackwell Publishing, 350 Main Street, Malden, MA 02148, and 9600 Garsington Road, Oxford OX4 2DQ, U.K.
Channel Bed Mobility Downstream from the Elwha Dams
423
uing research is to investigate how a river channel adjusts to dam installation and operations,
then responds to the removal of the structures.
The study presented here provides an assessment of the mobility of the materials comprising
the current channel bed and, to the extent possible, the role of the Elwha River dams in influencing bed mobility. Since the channel provides
the physical foundation of the riverine ecosystem, an evaluation of the current channel system
also offers an important baseline from which to
evaluate the river response to dam removal.
have the Elwha River dams influenced the bed
mobility of the river? In the discussion section,
this study also briefly addresses a related question: What is the relationship between geomorphic surfaces and riparian vegetation and how
have they been affected by bed mobility? Finally, in conclusion, these issues are synthesized to
answer the question: What do these changes intimate about how the lower Elwha River channel
system is functioning with the dams in place?
The study site has particular significance
since the two dams that fragment the Elwha
River will be removed in the near future to restore the river and its salmon runs. The concept
of dismantling dams as an option for environmental restoration has been considered the radical fringe of politics until the last decade ( Pyle
1995). While there has been a growing social
and political agenda to restore rivers by razing
dams, research on the impacts of dam removal is
limited. Thus, a long-term goal of this contin-
Study Area
The Elwha River is a coarse-bed mountain
stream located in the northern Olympic Peninsula of Washington (Figure 1). From its headwaters in the Olympic Mountains at 2,100 meters above sea level, the river flows northward
for over 70 kilometers and discharges into the
Str ait
of
J uan
RS-6
RS-15
RS-21
de
F uc a
Port Angeles
Elwha Dam
MacDonald Bridge
stream gage
RS-29
OLYMPIC
RS-34
Glines Canyon Dam
NATIONAL
RS-35
RS-37
watershed
boundary
E
lw
a
h
Elwha River Basin
R
N
iv
0
5 miles
5 kilometers
PARK
er
0
Washington
Figure 1 Map of the Elwha River drainage basin with stream gage and cross-section locations.
424
Volume 56, Number 3, August 2004
Strait of Juan de Fuca. Although the headwaters
are steep, most of the length of the mainstem has
an average gradient of 1% and is characterized
by alternating wide, low-gradient alluvial reaches and narrow, steep canyon reaches.
The drainage basin of the Elwha River is
narrow, with steep tributaries and a total area
of 833 km2. The upper 83% of the watershed
is protected in Olympic National Park and remains largely in a natural condition (Wunderlich, Winter, and Meyer 1994). Downstream
from the national park, the river meanders
though a mixture of private, federal, and state
lands. The watershed vegetation is primarily
conifer forest dominated by Douglas fir ( Pseudotsuga menziesii), grand fir (Abies grandies), and
western hemlock (Tsuga heterophylla). Riparian
trees are principally willow (Salix spp.) and red
alder (Alnus rubra), with some black cottonwood ( Populus trichocarpa).
The Elwha River maintains an average daily
discharge of approximately 42 cubic meters per
second (m3/s). Low mean daily flows of 8.5 m3/s
occur in the late summer or early fall and high
mean daily flows of approximately 340 m3/s are
associated with winter frontal storms and spring
snowmelt ( Pohl 1999). The largest instantaneous peak flow of the approximately 81-year
record, occurring in 1898, was 1,178 m3/s.
Two dams fragment the lower one-third of
the river. Elwha Dam, completed in 1914, is located 8 km from the mouth of the river and
Glines Canyon Dam, completed in 1927, is another 14 km upstream and within Olympic National Park (Figure 1). Glines Canyon Dam is
a 64-meter high single-arch concrete dam, 82.3
meters wide at its crest, with a reservoir capacity
of 49.4 " 106 m3. Elwha Dam is a 32-meter
high gravity concrete structure, 137 meters
wide at its crest, with a reservoir capacity of
10 " 106 m3. The impact of these hydroelectric
dams on Elwha River hydrology was substantial
prior to 1945, with a 10–12% reduction in flood
flows and a dramatic decrease in low flows of up
to 96% for minimum daily flows. Since 1945,
the dams have operated as run-of-the-river
structures, with a minor attenuation of flood
flows and moderate depression of low flows
( Pohl 1999).
Although the hydrology has not been greatly altered in the past five decades, the sediment discharge to the Elwha River delta has
decreased 98% since dam installation (U.S.
National Park Service 1995). Three small
tributaries, erosion of the glacial bluffs and minor reworking of alluvium contribute to the
current sediment discharge of 4,500 cubic meters per year at its mouth. Anadromous fish runs
have declined precipitously in response to the
fragmentation of their habitat, loss of spawning
gravel (due to channel armoring), and altered
water temperatures, prompting Congressional
action and a mandate to remove the dams and
restore the river system ( Public Law 102–495).
Channel Stability
The channel bed material mobility of coarsebed rivers such as the Elwha River can be examined by evaluating the relationship between
the shear stress imposed on the bed materials by
the flowing water and the critical shear stress,
which accounts for the resistance of the particles. In general, if average boundary shear stress
(to) exceeds the critical shear stress (tcr), particle
entrainment will occur, and erosion begins.
Thus, for this study, bed mobility is associated
with the initiation of sediment entrainment
and transport, an important threshold that can
lead to adjustments in the downstream ecosystem. This section briefly describes the equations that provided the basis for the bed mobility
calculations.
In alluvial channels, sediment entrainment is
partly a function of average boundary shear
stress (to), which is the downslope component
of the fluid weight of water exerted on a unit area
of the channel bed, estimated by the duBoys
equation:
to ¼ gRS
ð1Þ
where to is average boundary shear stress (Nm–2),
g is specific gravity of water (Nm'3), R is hydraulic radius (m), and S is slope or energy
gradient (m/m). This equation assumes a steady
uniform streamflow and a regular channel
cross-section shape with a width at least 10
times greater than depth. In wide, shallow channels, R is approximated by mean depth, D.
Thus, as mean depth of flow or channel gradient
increase, the average boundary shear stress on
the channel bed becomes greater.
While a higher shear stress increases stream
competence, particle motion will not be induced until the stress exceeds the resistance
of the channel materials, measured by critical
Channel Bed Mobility Downstream from the Elwha Dams
ð2Þ
where the units for tcr are Nm'2, r and rs are the
densities of water and sediment respectively (kg/m3), g is the gravitational acceleration
(m/s2) and D is the diameter of the grain size of
interest (m). Using standard estimates for r
(1000 kg/m3), rs (2650 kg/m3), and g (9.8 m/s2),
and a reasonable estimate for t*cr (0.05) (see
Buffington and Montgomery 1997), this equation can be simplified to:
tcr ¼ 808:5 D
ð3Þ
Although particle size, packing, shape, density,
and angle of repose can affect critical shear
stress (Leopold, Wolman, and Miller 1964; Gomez 1994), the Elwha River channel is composed of coarse and loosely organized bed
materials, and critical shear stress may be reasonably estimated with Equation 3. The representative particle size (D) selected for the
critical shear stress calculations is the median
particle size, D50. The assumption is that if the
average boundary shear stress exceeds the D50
critical shear stress, entrainment and transport
of the median particle size can occur. However,
this is an expression of mean particle motion
and channel morphology. Hydraulic conditions
such as turbulence, as well as sediment character, affect entrainment.
100
50
0
-50
Elwha Dam
150
River mouth
Elevation (meters)
200
Lake Aldwell Delta
250
Seven cross-sections along the lower Elwha
River were selected for this study. The 30-kilometer long study area begins about 10 km upstream from Glines Canyon Dam and extends to
the outlet of the river at the Strait of Juan de
Fuca (Figures 1 and 2). Site selection for this
study was based on: (1) obtaining reasonable
geographic coverage of the lower Elwha River
(spacing of sites), (2) including canyon and valley settings (confined and unconfined reaches),
and (3) ability to locate existing survey monuments (National Park Service research permit
restricted number of cross-sections and dictated
use of existing monuments where possible). Site
nomenclature is consistent with existing monument labels. Table 1 provides descriptions of
the cross-section sites.
Data collection at each site included channel
gradient, particle size, cross-sectional geometry, and Manning’s n estimations. Channel gradients for sites RS-6 to RS-34 were calculated
from Global Positioning System (GPS) coordinates, and gradients at RS-35 and RS-37 were
based on interpretation of 7.5-minute series
topographic maps. Particle size data were collected using the Wolman pebble count methodology (Wolman 1954), and the median particle
size was determined using a cumulative frequency diagram. This data was complemented by
sediment composition analysis conducted by the
U. S. Bureau of Reclamation (1996). Channel
geometry was obtained by surveying the channel, perpendicular to flow. Field observations
RS-29
Lake Mills
Delta
tcr* ¼ tcr =ðrs 'rÞ g D
Methodology
Glines
Canyon
Dam
shear stress (tcr). Critical shear stress in this
study is estimated using the dimensionless critical shear stress (t*cr) equation:
425
RS-37
RS-35
RS-34
RS-21
RS-6 RS-15
-100
-150
0
5
10
15
20
25
30
Distance from river mouth (km)
Figure 2 Longitudinal profile of the lower Elwha River with cross-section locations. Figure adapted from
the U.S. Federal Energy Regulatory Commission (1993).
426
Volume 56, Number 3, August 2004
Table 1 Elwha River Cross-Section Information
Cross-section
River
km*
Slope
(m/m)
downstream from both dams
RS-6
2
0.005
Top Width*
(m)
Mean Depth*
(m)
140
1.6
RS-15
5
0.006
54
1.3
RS-21
8
0.007
59
3.4
between the dams
RS-29
17.5
0.011
165
4.3
RS-34
0.011
84
2.4
upstream from both dams
RS-35
25.5
0.008
138
0.6
26
1.7
RS-37
22
31.5
0.009
Geomorphic Context
Unconfined valley of Quaternary alluvium and glacial
deposits.
Same as RS-6, but located immediately downstream
from a local channel confinement of Tertiary massive
basalt and breccia
Confined canyon of Tertiary sedimentary deposits; river
bounded by conglomerate bedrock (river right) and an
alluvial/talus slope (river left)
Meander bend in semiconfined valley. River bounded
by Tertiary basalt (river right), with Quaternary
alluvium and glacial deposits on river left.
Downstream from Glines Canyon powerhouse; narrow
valley (0.5 km wide); alluvial terraces on both sides of
river bounded by Tertiary bedrock.
At upstream end of Lake Mills delta. Unconfined valley
of low alluvial terraces bounded by Tertiary sandstone
and slate (river right) and Quaternary alpine glacial
deposits overlying Tertiary metamorphics (river left).
Unconfined valley of Quaternary glacial deposits
and alluvium.
*River km is the approximate distance from the mouth of the river in kilometers.
*Top width and mean depth for cross-sections under bankfull conditions.
guided by Chow (1959) and Acrement and
Schneider (1989) provided estimations of Manning’s n. In addition, channel morphology was
mapped, and riparian vegetation composition,
age structure, and cover were evaluated along
each of the cross-sections. This information
provides the environmental context for the hydraulic conditions and is used to discuss the results of the hydraulic calculations. Quantitative
data on geomorphology and vegetation are presented elsewhere ( Pohl 1999).
Critical shear stress (tcr) was calculated for
each site using the median particle size (D50) of
the channel bed. This study evaluated the average boundary shear stress (to) imposed on the
channel bed for two different flow events,
the two and ten-year recurrence interval events.
These events were calculated using discharge
data from the U. S. Geological Survey stream
gauge at MacDonald Bridge (Figure 1). Future
studies would benefit by collecting field measurements of discharge at each site and adjusting
streamflow data for each cross-section based on
this information. Stream Channel Cross-Section
Analysis (XSPRO), a software package designed for the U.S. Forest Service, was used to
assess the shear stress at each cross-section for
the different floods. XSPRO uses basic conti-
nuity, momentum, and energy equations of fluid
mechanics to evaluate channel geometry and
hydraulics (U.S. Forest Service 1996). This
program was developed for mountain streams
with large roughness factors. After providing
position-elevation data (channel geometry),
channel gradient, and Manning’s n, the resulting mean shear stress on the channel bed is
calculated. Average boundary shear stress and
critical shear stress were compared for each site
under the two high-flow events to assess bed
mobility. The relative bed mobility of each
cross-section was examined using a ratio (to/
tcr). A ratio41 indicates that the median particle size is likely to be entrained.
Results
The lower Elwha River is characterized by substantial spatial variability in the size of bed
materials, critical shear stress, and average
boundary shear stress (Table 2). The bed materials of the two cross-sections closest to the
mouth of the river were mobile under the twoyear (362 m3/s) and ten-year daily average flows
(535 m3/s). Upstream at RS-21, the ten-year
flow was sufficient to mobilize the median
particle size, but the two-year flow was not.
Channel Bed Mobility Downstream from the Elwha Dams
427
Table 2 Channel Cross-section Bed Material Mobility for the Median Particle Size under Two-Year and
Ten-Year Flood Conditions
Cross-sections
RS-6
RS-15
RS-21
RS-29
RS-34
RS-35
RS-37
Median
Particle Size
(D50) in m
0.073
0.084
0.201
0.225
0.358
0.108
0.070
Critical
Shear, tcr
(N m'2)
59.0
67.9
162.5
181.9
289.4
87.3
56.6
2-yr. Shear
Stress*, t0
(N m'2)
83.4
80.6
145.9
100.9
183.0
69.5
209.9
2-yr.
Mobility Ratio
(to/tcr)
1.4
1.2
0.9
0.6
0.6
0.8
3.7
10-yr.
Shear
Stress*, t0 (N m'2)
74.4
99.5
179.5
120.0
220.0
93.7
260.5
10-yr.
Mobility Ratio
(to/tcr)
1.3
1.5
1.1
0.7
0.8
1.1
4.6
*The two-year flood (daily average flow) is 362 m3/s (2,800 f 3/s) and the ten-year flood is 535 m3/s (18,900 f 3/s).
Between the dams (RS-29 and RS-34), neither
high-flow condition was sufficient to entrain the
median particle size. RS-35, immediately upstream from Lake Mills delta, was mobilized
only under the ten-year flow, but the smaller
particle sizes and higher shear stress at RS-37
resulted in channel bed mobility for both floods.
Discussion
These cross-sections provide a broad, albeit incomplete, picture of the fragmented fluvial system of the lower Elwha River. Upstream from
the dams in Olympic National Park (RS-37), the
Elwha River transports a cobble bedload mixed
with moderate amounts of gravel and fines. Although this reach had one of the steeper channel
gradients (Table 1), it had the smallest median
particle size (70 mm) (Table 2). Site RS-37 reflects a natural sediment composition that is no
longer evident in the degraded and armored
reaches downstream from the dams. The channel system is active, with scoured bars and visible bank erosion providing visual confirmation
of the bedload transport under two-year and
ten-year flood events (Table 2). Several of the
bars and alluvial terraces are established by riparian communities with mixed age classes and
visible signs of seedling establishment.
Site RS-35, at the upstream end of the Lake
Mills delta, is much wider and shallower than
the natural reach (RS-37). The median particle
size (108 mm) is slightly larger than at RS-37,
reasonable since the river segment between
these two cross-sections is Rica Canyon, a narrow gorge cut into Tertiary micaceous sandstone and slate. The higher velocities and shear
stress of the Elwha River in this canyon facilitate
entrainment and transport of larger particles
that are deposited as velocity diminished near
RS-35. While the median particle size at RS-35
was slightly greater than the values upstream
at site RS-37, shear stress at this shallow, wide
cross-section tended to be about one-third the
magnitude of shear stress the upstream site (Table 2). Consequently, the channel bed at RS-35
was less mobile under the two-year and ten-year
flood conditions than the channel at RS-37.
Although the bed mobility is lower, RS-35
is characterized by a geomorphic complexity
that reflects this natural segment of the river
upstream from the dams. Multiple channels,
bars, and terraces offer varied surfaces for the
establishment and growth of riparian vegetation
(Figure 3).
Downstream from the dams, the channel bed
became armored over the years as relatively
sediment-free water entrained and transported
the fine sediments. Currently, the remaining
coarse particles of the channel bed are too large
to be entrained under most flow scenarios, leading to low bed material mobility. The river segment from Glines Canyon Dam (RS-34) to
Elwha Dam (RS-21) had the lowest bed mobility, as measured by the ratio of shear stress
to critical shear stress (Table 2). In addition to
channel armoring and localized bedrock control, the steep gradient between RS-34 and
RS-21 supports the transmission of sediment
through the supply-limited system, rather than
particle deposition and the creation or maintenance of channel forms. Visual inspection
indicates that these reaches have fewer bars
and alluvial terraces relative to sites upstream
from the dams. Riparian trees are aging on the
remaining terraces and high bars, while seedling establishment and success is limited due
to channel degradation and reduced access to
428
Volume 56, Number 3, August 2004
deposition of gravel and fines from tributary
inputs and internal channel adjustments related
to the reduction in gradient. The median
particle sizes at these cross-sections, 84 mm
and 73 mm respectively, reflect this hydraulic
change. Given the smaller particle sizes and reduced resistance to erosion in both the channel
bed and banks, the regulated river is most dynamic in this river segment. Bed mobility ratios
for cross-sections in this segment (RS-15 and
RS-6) indicated erosion of the channel beds
under both flow conditions. Physical evidence
suggests that the Elwha River in this segment is
active. Terrace banks are steep or undercut, bars
are commonly scoured or occupied by young
Height above
thalweg (m)
streamflow. Consequently, future quantitative
analysis may indicate that riparian communities
downstream from the dams tend to be evenaged and mature, in contrast to the upstream
Elwha River and to historical photographs of
these downstream reaches. The simplification
of the channel system is illustrated with a crosssectional representation of RS-34 (Figure 3).
Downstream from Elwha Dam canyon (RS21), the channel gradient declines as the Elwha
River meanders through a wide floodplain of
glacial and alluvial deposits and approaches its
termination at the Strait of Juan de Fuca. The
bankfull depth at RS-15 and RS-6 is less than
the upstream canyon sites, primarily due to
RS-6
4
terrace
2
0
levee
upper bar
channel
bar
25
0
high flow
channel
lower bar
75
50
100
Horizontal position (m)
Height above
thalweg (m)
RS-34
4
0
terrace
terrace
2
channel
boulder bar
0
boulder bar
50
25
100
75
low terrace
Height above
thalweg (m)
RS-35
4
2
0
0
channel
25
mid-channel bar
50
75
channel
100
low terrace
hillslope
high flow channel
Horizontal position (m)
low bar
125
150
Horizontal position (m)
Figure 3 Cross-section diagrams depicting channel geometry, geomorphology, and riparian vegetation for
representative cross-sections downstream from the dams (RS-6), between the dams (RS-34) and upstream
from the dams (RS-35).
Channel Bed Mobility Downstream from the Elwha Dams
riparian seedlings and saplings, and channel
widening and migration are visible.
RS-15 and RS-6 are most similar to the natural cross-section well upstream from the dams
(RS-37) in median particle size, bankfull depth,
and tendency for bed mobility under flood conditions. This comparison highlights the impact
of the dams on the river system. In a natural
river, a steep reduction in gradient near the
mouth of the river would tend to be associated
with deposition of sediments, assuming a comparison of similar reaches (as in this study).
Thus, the median bed particle size would be
expected to be smaller than that of an upstream
reach with a similar cross-sectional geometry
and reach scale morphology but high gradient.
However, the Elwha River dams block most of
the sediment supply to the lower reaches of the
river and have little impact on the moderate to
high flows of the river ( Pohl 1999). The resultant winnowing of fine sediment and gravel has
left the downstream reaches with a paucity of
these particle sizes. Thus, the low-gradient river
segments of the Elwha River channel below the
dams (RS-15 and RS-6) have a median bed particle size similar to the steep reach of the Elwha
River upstream from the dams (RS-37). Like the
more natural reaches upstream from the dams,
the river downstream from the dams has a
greater geomorphic complexity relative to the
section of the river between the dams. Varied
bars, terraces, and high-flow channels are characteristic of the lower Elwha River, providing
surfaces for seedling establishment and development of different vegetation age classes (Figures 3 and 4).
Figure 4 Photograph of RS-6,
taken from river right looking upstream from the cross-section.
The reach in this photo is characterized by geomorphic and biotic
complexity, including bars (note
vegetated midchannel bar in
background) and low terraces, as
well as a mixed-age class riparian
community (note visible seedlings on the bar in the foreground).
429
Conclusions
The lower Elwha River has been adjusting to
the installation and operation of Elwha Dam
since 1914 and Glines Canyon Dam since 1927.
These dams have altered streamflow, water
quality, and sediment yield in the river. The full
extent of these impacts on the physical, chemical, and biologic components of the river ecosystem has yet to be evaluated. This study
investigated bed material mobility, which provides the physical foundation for river processes
and its relationship to channel character, complexity, and function.
Bed mobility was lower where channel armoring was considerable, while river segments with
finer sediments, regardless of gradient and shear
stress, had higher bed material mobility. Geographically, the channel had the greatest bed
mobility at the natural cross-section upstream
from the dams (high ratio of shear stress to critical shear stress), low bed mobility between the
structures, and an increase in bed mobility in
the low gradient river segment downstream from
Elwha Dam.
Reaches with low bed material mobility were
typically associated with a lack of channel system complexity, including reduction or loss of
bars and low terraces and their associated young
riparian communities. Although the run-ofthe-river dams are not altering streamflow
greatly, the loss of sediment from the channel
system has had a substantial impact on downstream channel bed mobility and, consequently,
on the geomorphic and biotic complexity of the
river. The dams have created three segments
430
Volume 56, Number 3, August 2004
of lower Elwha River channel that function
differently and thus have distinct channel
morphologies.
Large dams often alter streamflow substantially, and the combined effect of modified
hydrology and paucity of sediment can have a
significant impact on downstream environments. The Elwha dams reduced bed mobility
by trapping sediment, which resulted in larger
mean particle sizes (and thus critical shear
stress) and resulted in a loss of channel system
complexity. This study confirms that dams with
a limited capacity to modify hydrology can still
have substantial impact on the downstream ecosystem. However, additional investigation of
cross-sectional geometry and hydraulics should
be undertaken to fully explore the influences of
the dams on the channel system.
Management of the modified and artificially
segmented Elwha River requires an understanding of how the river is functioning with
the dams in place. The channel system is particularly important to this evaluation since it
provides the physical foundation for ecological
process. In addition, the current management
goal is ‘‘full restoration’’ of the Elwha River ecosystem by dam removal, authorized by Congress in 1992 ( Public Law 102-495). However,
full restoration is not feasible without an understanding of the impact on the river by the
dams, and by implication, how it might function
without them. This study provides insight into
the influence of the dams on the channel system
and establishes a baseline of physical data from
which to evaluate geomorphic change in response to dam removal. Since the number of
dams being removed for environmental reasons
is escalating ( Pohl 2002), scientific studies are
increasingly needed to support river management decisions.’
Literature Cited
Acrement, G. J. Jr., and V. R. Schneider. 1989. Guide
for selecting roughness coefficients of natural
channels and flood plains. U.S. Geological Survey
Water-Supply Paper 2339. Washington, DC: U.S.
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MOLLY POHL is an assistant professor in the Department of Geography, San Diego State University,
San Diego, CA 92182-4493. E-mail: mpohl@mail.
sdsu.edu. Her research interests include fluvial geomorphology, the environmental impacts of dam operation and dam removal, and public land and water
policy.