Effects of dam operation and land use on stream channel

Geomorphology 82 (2006) 412 – 429
www.elsevier.com/locate/geomorph
Effects of dam operation and land use on stream
channel morphology and riparian vegetation
Eric Gordon a,⁎, Ross K. Meentemeyer b
a
Department of Geography and Global Studies, Sonoma State University, 1801 East Cotati Ave., Rohnert Park, CA 94928, USA
b
Department of Geography and Earth Sciences, University of North Carolina at Charlotte,
9201 University City Boulevard, Charlotte, NC 28223, USA
Received 11 January 2006; received in revised form 6 June 2006; accepted 6 June 2006
Available online 14 July 2006
Abstract
Dams are well known for influencing channel and vegetation dynamics downstream, but little work has focused on
distinguishing effects of land use and channel responses to the impoundment. In this paper, we examined interacting effects of a
dam and land use on downstream changes in channel morphology and riparian vegetation along an agricultural stream system in
northern California. Measurements of planform channel morphology, vegetation area, and land use were mapped along multiple
stream segments based on a chronological sequence of historical aerial photographs over a 34-yr period prior to operation of the
dam in 1983 and over a 17-yr period after dam operation, and compared to a nearby, undammed reference stream. A two-factor
analysis of covariance (ANCOVA) was used to examine the effect of the dam on changes in bankfull area, stream length, and
riparian vegetation area while accounting for the effect of land use and distance downstream. The dammed stream's bankfull area
contracted 94% after dam operation. Prior to dam operation, bankfull area decreased when land use area increased, but not after
operation of the dam. Stream length varied 64% less after dam operation as a consequence of less frequent episodic channel
migration and entrenchment. The area of riparian vegetation was decreasing during the pre-dam period, but then increased 72%
after operation of the dam. Across time periods, decreases in the area of riparian vegetation were also associated with increases in
land use area in both the dammed and reference stream. After operation of the dam, reduced peak discharges and sediment
reduction likely lead to channel incision and constrained channel migration, which allowed vegetation to increase 50% on less
accessible, abandoned banks. Rating curve and hydraulic exponent analyses based on stream gauge measurements corroborate
statistical analyses of the mapped changes. In conclusion, we found that operation of the dam and land use patterns together
influenced spatial and temporal changes in channel morphology and riparian vegetation. Use of a nearby undammed reference
stream in conjunction with multivariable analysis of spatially and temporally replicated observations provided an effective
framework for unraveling interacting effects of dams and land use activities on stream channel and vegetation dynamics.
© 2006 Elsevier B.V. All rights reserved.
Keywords: GIS; Channel morphology; Riparian vegetation; Land use; ANCOVA; Hydraulic exponent analysis
1. Introduction
⁎ Corresponding author.
E-mail addresses: [email protected] (E. Gordon),
[email protected] (R.K. Meentemeyer).
0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2006.06.001
Most river systems in the western United States are
currently impounded to provide societal services such as
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
hydropower, irrigation, flood control, and recreation
(Graf, 1999, 2001). By regulating natural flow regimes
and trapping sediment, an unfortunate trade off of dams
is their potential to change historical channel dynamics
and vegetation disturbances downstream (Dunne and
Leopold, 1978; Simons and Li, 1980; Petts, 1984;
Williams and Wolman, 1984; Chien, 1995; Brandt,
2000; Shields et al., 2000). Dams designed to actively
control discharge for flood control are particularly
effective at reducing peak discharge associated with
storm events and increasing discharge during dry periods
(Kondolf, 1997). The resultant loss of sediment load
impounded behind dams and reduced discharge during
storms can cause downstream channel incision and
entrenchment, which may also lead to contractions in
bankfull width and potential abandonment of floodplains
(Cleveland and Kelley, 1977; Gurnell et al., 1994;
Rosgen, 1996; Kondolf, 1997; Knighton, 1998; Brandt,
2000; Franklin et al., 2001). Accompanied by these
changes, riparian vegetation along channel banks
experiences less frequent flood disturbances, which can
lead to an encroachment of increased vegetation abundance on the floodplain but with lower species diversity
(Pelzman, 1973; Hupp, 1990; Hupp and Osterkamp,
1994; Friedman et al., 1998; Magilligan et al., 2003,
Marston et al., 2005). These changes affect ecological
processes in both aquatic and terrestrial riparian
environments and are becoming an increasing concern
in management and restoration of impounded river systems (Stevens et al., 2001; Thoms et al., 2005).
While dams play a critical role in stream channel
processes, human land use practices such as agriculture
and forest clearing can also impact fluvial geomorphic
systems and riparian vegetation (Murgatroyd and
Ternan, 1983; Osterkamp and Hupp, 1984; Mossa and
McLean, 1997). Elimination of riparian habitat for
agriculture can increase runoff, which can destabilize
channel banks and vegetation establishment, cause
channel aggradation, introduce fine-grained sediments
(< 1 mm) that inhibit fish spawning habitat (Everest et al.,
1987), and increase dissolved organic compounds
(Thoms et al., 2005). Changes to historical discharge
regimes and a channel's sediment transport capacity can
contribute to periods of sediment deficit or surplus.
During periods of surplus, aggradation may occur, resulting in increased bed eleation, bank narrowing, and/or
bed fining, whereas periods of sediment deficit, with
sufficient transport capacities, may lead to bed incision,
bank widening, and/or bed coarsening (Carson, 1984;
Bledsoe, 1999; Grams and Schmidt, 2005; Richard et al.,
2005). Riparian vegetation is also an important factor in
channel morphology and its distribution is affected by
413
both discharge regime and land use practices adjacent to
the channel (Hupp and Simon, 1991). Riparian vegetation influences channel adjustment processes by increasing bank stability, inhibiting erosion, and enhancing
sedimentation for floodplain formation (Friedman et al.,
1996). While the potential effect of land use on channel
and vegetation dynamics have been increasingly
explored (Knox, 1977, Martin and Johnson, 1987;
Knox, 2001; Urban and Rhoads, 2003; see also, U.S.
Bureau of Reclamation, 2005; U.S. Fish and Wildlife
Service, 2005), little is known about interacting effects
of land use and dam operation. Empirically based studies
are needed to differentiate effects of land use and dam
operation on stream channel dynamics (Grams and
Schmidt, 2005), but such data are typically difficult to
obtain with sufficient spatial and temporal replication for
multivariable analyses (Thoms et al., 2005). In addition,
few studies have statistically examined the effects of a
dam on channel and vegetation dynamics in relation to
an undammed reference stream, a control for potential
effects of climate change over the study period (but see,
Stover and Montgomery, 2001; Grams and Schmidt,
2002, 2005; Thompson, 2006).
In this paper, we examine interacting effects of a northern California dam and land use on downstream changes in
channel morphology and riparian vegetation that occurs
along the watercourse. Spatial and temporal changes in
planform channel morphology, riparian area, and land use
adjacent to the channel are measured at multiple sites from
a sequence of historical aerial photographs over a 34-yr
period prior to operation of Warm Springs Dam in 1983
and over a 17-yr period after dam operation. Changes
associated with the dam are also examined in relation to
distance downstream of the dam and are statistically compared to a nearby undammed reference stream with similar
climatic, geomorphic, and land use features. We corroborate these analyses using gauging station measurements
and developed rating curves and performed hydraulic
geometry exponent analyses to further examine stream
channel dynamics and validate channel and vegetation
changes interpreted from the air photos. Based on these
data, we examined the following three questions:
(i) Does operation of a dam alter the spatial and
temporal variability in channel morphology and
the amount of vegetation in the riparian corridor,
and to what degree does land use function as a
contributing factor?
(ii) Does the rate and direction of changes in channel
morphology and riparian vegetation differ after
dam operation and does land use influence these
changes?
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E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
(iii) Do field-measured hydraulics data correspond to
air photo interpreted changes in planform channel
morphology and riparian vegetation?
We hypothesize that the undammed reference stream
will experience similar variability in channel morphology
over the duration of the study period, while the dammed
stream (Dry Creek) will experience significant changes in
variability after operation of the dam. Specifically, we
expect bankfull width to decrease as a result of reduced
sediment loads and increased channel incision leading to
an entrenched channel following dam operation; stream
length will decrease from reduced episodic channel migration and less sinuous meandering in the entrenched
channel, and riparian vegetation will increase in area
because of less disturbance from reduced overbank
flooding. In both dammed and undammed systems, we
also expect agricultural land use along channels to
encroach upon and reduce the area of the riparian forest
vegetation during periods of reduced overbank flooding.
2. Study system
The Dry Creek watershed (672 km2), located approximately 120 km north of San Francisco, CA, is a major
tributary of the Russian River watershed (3846 km2)
(Fig. 1). Dry Creek has a Mediterranean-type climate, with
a cool wet season from December to April (92 cm/yr)
followed by a warm dry season with little to no precipitation. Substantial rain during the wet season in
conjunction with a primarily gravel bed stream of Quaternary alluvium historically contributed to considerable
flooding and sediment input to the Russian River. Dry
Creek's average pre-European (before 1850) channel bed
Fig. 1. California hydrologic basins with Russian River basin darkened and boxed. Enlargement of the Dry Creek subbasin and the reference stream in
the Maacama Creek subbasin, with confluences to the Russian River. Warm Springs Dam impounds Lake Sonoma. Coyote Dam is located 60 km
upstream on the Russian River.
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
Table 1
Dam and water flow characteristics of Warm Srpings Dam, established
1983, and Lake Sonoma
Dam height (m)
Dam volume (million cubic meters)
Drainage area (km2)
Capacity (ac-ft)
Water supply (ac-ft)
Flood pool (ac-ft)
Sediment pool (ac-ft)a
Fish maintenance (ac-ft)
97
23
337
381,000
212,000
130,000
26,000
13,000
a
Based on 100-yr economic model. Actual sediment trapped
>260 ac-ft/yr (USACOE, 1973).
grain size was 0.7 mm (Harvey and Schumm, 1987). From
1850 to 1900 over 40% of the forest in the upper watershed
was cleared and lead to accelerated slope erosion that
contributed to channel aggradation with an average channel
grain size of 5.6 mm (Harvey and Schumm, 1987).
Channel incision without accompanying channel widening
occurred from 1900 to 1940 from suspected gravel mining
near Dry Creek's mouth. From 1940 to 1984 channel
deepening and widening occurred accompanied by large
inputs of sediment as a result of unusually high annual runoff (Cleveland and Kelley, 1977) coincident with severe
fires in the upper watershed from the 1950s–early 1960s
(Simons and Li, 1980), as well as headward erosion into
Dry Creek's upper tributaries. This period lead to Dry
Creek being the highest sediment yielding tributary of the
Russian River (2222 t/km2/yr: Ritter and Brown, 1971;
Goudey et al., 2002). Harvey and Schumm (1987) found
grain size samples (d84 32–45 mm) in 1984/1985 (2 years
after dam closure) that suggested Dry Creek's channel bed
was armored for at least 3 km downstream from Warm
Springs Dam.
415
In 1983 the U.S. Army Corps of Engineers began
operation of Warm Springs Dam, which captures 362,000
metric tons of sediment annually (USACOE, 1997). Prior
to operation of the dam, Dry Creek functioned as an
ephemeral stream but now flows perennially with dry
season discharges averaging 3.4 m3/s and sustained above
2.4 m3/s since 1986 (Table 1). Discharges above 500 m3/s
were common before operation of the dam, but are now
reduced substantially. Peak discharges before the operation of the dam averaged 454 m3/s versus 117 m3/s after
dam operation (Fig. 2). Since operation of the dam, the
largest peak discharge occurred in February during the
1998 El Nino at 210 m3/s. After Warm Sprigs Dam was
operated in 1983, peak discharges for events with a return
period of 2 years, or greater, were reduced by a minimum
of 70% (Simons and Li, 1980).
Since European settlement in the 1700s, total riparian
area in the Russian River basin has declined 70–90%
with over 40% of this decline occurring since 1940
(Marcus et al., 2001; CDFG, 2002). Ninety percent of
the upper reaches of the Dry Creek subbasin redwood
and Douglas fir forests have been cleared for timber and
livestock grazing, while the valley floor's riparian
forests have been converted for crop cultivation since
the early 1900s (USACOE, 1973).
In this paper, Dry Creek is compared to a nearby
undammed reference stream (Maacama Creek) with
similar climatic, geomorphic, and historical land use charcteristics (Fig. 1). The reference stream's watershed is
smaller (117 km2) than Dry Creek's, but the proportion of
pre-dam peak annual discharge to drainage area contributing to each study reach is not different (Dry Creek measurements occurred between 1960 and 1982 and Maacama
Creek measurements occurred between 1960 and 1981;
Fig. 2. Annual peak discharge at the USGS gauge station (no. 11465200) located at Yoakim Bridge (see Fig. 1). Annual peak discharges dramatically
decline and decrease in variability after dam operation in 1983.
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E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
n = 23, df= 20, t = 2.09, P = 0.92). The riparian forest along
both streams is dominated by willow (Salix sp.), cottonwood (Populus trichocarpa), and alder (Alnus rhombifolia) near the channel and with coast live oak (Quercas
agrifolia) often occurring along the outer edges of riparian
forest areas. Land uses are predominantly agricultural
vineyards and orchards, as well as some livestock grazing.
3. Methods
We examined interacting effects of Warm Springs
Dam and land use on downstream changes in channel
morphology and riparian vegetation distribution on the
floodplain by using multivariable analysis. Changes in
channel features and riparian vegetation were assessed
from a series of historical aerial photographs from the
dammed and undammed streams taken both prior to, and
after the establishment of the Warm Springs Dam (1983).
Dry Creek's morphological channel changes were
corroborated using gauge station measurements from a
nearby USGS station analyzing changes in channel
geometry during the study period.
3.1. Data collection and mapping
3.1.1. Channel, vegetation, and land use mapping
The Dry Creek study reach (10.5 km) was divided into
20 equal length segments. The width of the segments was
2025 m, which is the buffer size that included all land
parcels adjacent to the channel. For analysis, 10 segments
were randomly selected from the Dry Creek study reach
(Fig. 3). The reference stream's study reach (3.6 km) was
divided into 18 equal segments with a total of 10 segments
randomly selected for analysis. For each stream segment,
we digitally scanned a historical sequence of panchromatic aerial photographs (1:4000–1:24,000) taken during
the dry season May to August (pre-dam: years 1942,
1961, 1976–1980; post-dam: years 1987, 1993, and 2000).
Each photo was georeferenced to match the 1993 USGS
digital orthophoto quarter quadrangle (DOQQ) and
integrated in the GIS for mapping and analysis. Four
planform variables were delineated in the GIS from the
imagery: thalweg length (center of the stream channel),
bankfull width, riparian vegetation area, and human land
use area (Fig. 4). Bankfull width measurements were
based on exposed depositional features (e.g., point and
side channel bars) created by bankfull discharge (Knighton, 1998; Kondolf et al., 2002). Bankfull width was
estimated by mapping the area of the exposed depositional features within each segment. When the vegetation
canopy obscured the bank, the high flow mark was estimated visually from the nearest visible upstream and
downstream banks which undoubtedly produced small
errors in some places. We defined riparian vegetation as
areas with an established tree canopy along the stream's
bank and floodplain. Finally, we mapped all areas with
visual evidence of human land use adjacent to the channel,
based on the presence of agriculture (e.g., crops, orchards,
grazing) or development (e.g., roads, structures, ponds).
3.1.2. Stream gauge measurements
Hydraulic measurements of discharge, velocity, watersurface elevation (gauge height), and channel geometry
were collected for Dry Creek from a USGS-monitored
gauge station at Yoakim Bridge, 300 m upstream of the
study reach (Fig. 1). Established in 1960, the station has
continuously measured velocity–area discharge profiles
to present. For the reference stream, however, USGS
stream gauge measurements were only available for the
pre-dam period (1960–1981), which therefore precluded
a comparison to Dry Creek.
The gauged discharges were used to measure channel
geometry, a means of summarizing interaction of morphologic and dynamic variables in natural streams and illustrative of channel variations between streams or reaches in
the same system (Rhodes, 1977; Huang and Nanson,
1997). Geometry based on discharge per unit channel
width can define temporal changes as a direct result of
channel adjustments to natural or anthropogenic disturbances (Williams, 1986). The analysis and comparison of
Fig. 3. The 10.5-km Dry Creek study reach shown bounded by the box to include adjacent land parcels. Numbered divisions are the 525 m randomly
selected segments used for analyses. Yoakim Bridge is discernible at left (upstream of the study reach) crossing Dry Creek. Shown are the 1976 aerial
photographs.
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
417
Fig. 4. Stream channel features manually digitized in GIS from historical black and white aerial photographs: (A) example of onscreen digitizing of a
streamline; selected years for (B) bankfull area, (C) streamline (thalweg), and (D) riparian area are shown below. Not shown: digitized polygons
representing land use areas adjacent to the channel. This is segment 2, Dry Creek (see Fig. 3).
hydraulic geometry for different time periods provides a
method to assess the impact of environmental change in
the stream (Ritter et al., 2002).
3.2. Statistical analyses
3.2.1. Channel, vegetation, and land-use effects
A two-factor analysis of covariance (ANCOVA) was
used to examine the effect of two categorical variables on
changes in bankfull area (BKFA), stream length (SL),
and riparian vegetation area (RA) while accounting for
the effect of potentially influential covariates (Quinn and
Keough, 2002). The two categorical explanatory variables were STREAM (Dry Creek versus the undammed
reference stream) and TIME (pre-versus post-dam
period) with individual stream segments (SEGMENTS)
included as a random variable nested in STREAM. The
covariates included land use area (LUA), distance downstream from the dam (DISTANCE), and bankfull area
(BKFA). The covariate bankfull area was only examined
in relation to changes in riparian vegetation to determine
if post-dam reductions in bankfull area lead to expansion
in riparian vegetation on the floodplain.
Each analysis included the 10 randomly selected
segments from Dry Creek and the reference stream over
the pre- and post-dam time periods (n = 40). Up to threeorder interaction terms were examined and, where significant, their main effects were also included. Covariates
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and interactions were included in models only if
statistically significant (P < 0.05). Covariates were standardized (standard score) when assessed as an interaction
to avoid intercorrelation with their lower order terms
(Quinn and Keough, 2002).
Two measures of variability were calculated for bankfull area, stream length, and riparian vegetation to examine their response to STREAM and TIME, yielding a
total of six ANCOVA analyses. The measures included
the coefficient of variation (CV) and the rate and direction
of changes during each time period. The coefficient of
variation was calculated as
CV ¼
s
100
Ȳ
ð1Þ
where s was the standard deviation and Ȳ was the mean
for a given segment in either the pre- or post-dam period.
CV standardizes differences in magnitudes of predictor
variables because of the different study area sizes between
Dry Creek and the reference stream while providing a
measure of variation that is independent of the measurement units (Quinn and Keough, 2002). CV values were
log10 transformed to provide normal distributions and
homogenous variances.
We examined the trajectory (rate and direction) of
changes in channel morphology and in riparian and land
use area as an indicator of stream adjustment to disturbance
by considering the magnitude and rate of changes over
time (Brooks and Shields, 1996; Downs and Thorne, 1996;
Rhoads and Herrick, 1996; Fryirs and Brierley, 2000;
Grams and Schmidt, 2002). The rate and direction of
changes that occurred during the pre-versus post-dam
periods were derived for each variable by regressing the
response (e.g., BKFA, SL, and RA) for each segment
against each year during the pre-dam (years 1942, 1961,
and 1976) and post-dam periods (years 1987, 1993, and
2000). The resulting slope coefficient indicated the rate
and direction of change of the response variable
experienced by a particular stream segment during one
time period. All slope coefficients were log10 transformed,
as were the coefficients of variation. A slope of zero
indicates stability during a particular time period.
The covariate land use area (LUA) was also examined
as a slope coefficient (computed as above) to give a measurement of the rate and direction of changes in land use
area during a particular time period. Distance downstream
from the dam to each segment (DISTANCE) was standardized for Dry Creek and the reference stream by dividing
distance downstream by the total length of the stream.
LUA and DISTANCE were used as covariates in all six
analyses.
3.2.2. Stream gauge changes
Changes in hydraulic geometry were examined using
two methods. First, we estimated changes in Dry Creek's
bed-level. We calculated rating curves derived from the
USGS gauge-height discharge records by regressing gauge
height against discharge. The measured gauge height corresponds to a discharge that represents bed level elevation
at that location. The change in gauge height with time
would indicate a proportional change in the same direction
of the elevation of the streambed. This method provided a
general trend for bed elevation changes, but water-surface
elevations can be affected by changes in channel shape,
roughness and other features even when width has remained approximately constant (Williams and Wolman,
1984). Although bed elevation is most accurately
determined using low discharge observations (Williams
and Wolman, 1984), we unavoidably used all discharge
observations because USGS primarily made observations
only during the rainy season preceding the 1990s, and only
a few times per year. A one-way ANOVA was then used to
test if mean bed-elevation was significantly lower during
the post-dam period than the pre-dam period by comparing
gauge heights. We compared responses (gauge heights)
between periods that shared the same range of discharges.
The second analysis focused on changes in hydraulic
geometry dimensions (width, depth, velocity) between
the pre- and post-dam periods by regressing the natural
logs of hydraulic geometry against discharge (Leopold
and Maddock, 1953). The equations for the dependent
variables are
w¼
aQ
b
ð2Þ
d ¼ cQ f
ð3Þ
v¼
ð4Þ
kQ
m
where Q is discharge, and w, d, and v are the dependent
variables channel width, depth, and velocity, and a, c, k,
b, f, and m are constants. The “hydraulic geometry exponents” b, f, and m are regressed slope coefficients,
which indicate the rate of change of the hydraulic dependent variable (w, d, or v) with increasing discharge.
Substituting these expressions into the continuity equation, Q = wdv, it follows that b + f + m = 1 and ack = 1
(Rhodes, 1977). First, a one-factor ANCOVA was used to
examine if the regressed slope coefficients of b, f, and m
differed between the pre- and post-dam periods, which
tested the significance of interactions between the response variables (w, d, v) and the two time periods.
Second, we used a one-way ANOVA to examine if the
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
Fig. 5. Bankfull area variation (BKFA) for Dry Creek and the reference
stream during pre- and post-dam periods. Even though bankfull area
had greater post-dam variation than during the pre-dam period or in the
reference stream, it was in a negative direction (see Fig. 6). The
coefficient of variation (CV) normalized values between the differing
feature magnitudes of the Dry Creek and Maacama Creek watersheds
while providing a measure of variation.
mean hydraulic dimensions changed between the pre- and
post-dam periods.
4. Results
4.1. Channel, vegetation and land use effects
4.1.1. Bankfull area
The ANCOVA analysis of bankfull area variability
(CV) shows that bankfull area varied similar amounts in
both Dry Creek and the undammed reference stream
during the pre-dam period (Fig. 5). Dry Creek's bankfull
area varied 60% more during the post-dam period than
during the pre-dam period (P = 0.0082; Table 2; Fig. 5),
whereas the reference stream experienced the same
419
Fig. 6. Bankfull area rate and direction (BKFA) of variation for Dry
Creek and the reference stream during pre- and post-dam periods. Only
Dry Creek's bankfull area significantly decreased during any time
period.
amount of variation during both the pre- and post-dam
periods.
The analysis of rate and direction of bankfull area
changes (BKFA) indicates that Dry Creek's bankfull area
was increasing slightly during the pre-dam period (mean
slope coefficient = 0.002 ± 0.001 S.E.) and then decreased considerably during the post-dam period (mean
slope coefficient = − 0.026 ± 0.001 S.E.), a 94% change
(P < 0.0001; Table 2; Fig. 6). In contrast, the reference
stream's bankfull area was decreasing slightly during the
pre-dam period (mean slope coefficient = − 0.003 ± S.E.
0.001) and then remained almost constant during the
post-dam period (mean slope coefficient = 0.0001 ± S.E.
0.001; Fig 6). Two covariates were also associated with
changes in Dry Creek's bankfull area. During the pre-dam
period, Dry Creek's bankfull area increased while
agricultural land use area decreased (est. = − 0.003 ±
0.001 S.E.; P < 0.0001; Fig. 7; Table 2, source of variation:
STREAM × TIME× LUA). Bankfull area increased with
Table 2
ANCOVA results for bankfull area variation (BKFA) for Dry Creek and the reference stream during the pre-dam and post-dam periods
Source of varitation
STREAM
SEGMENTS [STREAM ]
TIME
STREAM × TIME
Covariates
LUA
DISTANCE
STREAM × TIME × LUA
STREAM × TIME × DISTANCE
Error
⁎P < 0.05, ⁎⁎P < 0.01, ⁎⁎⁎P < 0.001.
Bankfull area variation (CV)
Bankfull area rate and direction
df
SS
F
df
SS
F
1
18
1
1
0.7720
2.9465
0.5934
0.9982
6.82⁎
1.45
5.25⁎
8.82⁎⁎
1
18
1
1
0.0012
0.0003
0.0015
0.0023
90.76⁎⁎⁎
1.40
118.23⁎⁎⁎
182.04⁎⁎⁎
1
1
1
1
12
0.0005
0.0001
0.0005
0.0002
0.0002
36.60⁎⁎⁎
7.15⁎⁎⁎
37.73⁎⁎⁎
13.39⁎⁎
18
–
–
–
–
2.0366
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E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
Fig. 7. Bankfull area rate and direction (BKFA) effected by the
covariate land use area (LUA) in Dry Creek during the pre- and postdam periods. Slope coefficients are standardized for each stream.
increasing distance downstream during the pre-dam period
(est. = 0.001 ± 0.0003; P < 0.0001; Fig. 8; Table 2, source
of variation: STREAM × TIME × DISTANCE), but decreased with increasing distance downstream during the
post-dam period (est. = −0.002 ± 0.0007; P = 0.003; Fig. 8;
Table 2, source of variation: STREAM × TIME × DISTANCE). Land use area in Dry Creek slightly decreased
during the pre-dam period (mean slope coefficient = −
0.0004) then increased at three times that rate during the
post-dam period (mean slope coefficient = 0.0012;
P = 0.04), while land use area in the reference stream
remained relatively constant during both periods (mean
slope coefficients = −0.00007 and −0.006, respectively;
P = 0.18).
dam period (P = 0.15), but this weak trend was not
distinguishable between Dry Creek and the reference
stream. The analysis of the rate and direction of riparian
area (RA) changes shows that Dry Creek's riparian area
was decreasing during the pre-dam period (mean slope
coefficient = − 0.002 ± 0.002 S.E.), but then increased
significantly (72%) during the post-dam period (mean
slope coefficient = 0.011 ± 0.002 S.E.; P = 0.003; Table 4;
Fig. 12). In contrast, the reference stream did not experience a reversal in the direction of riparian area
changes during the post-dam period (mean slope coefficient = 0.0026 ± 0.002 S.E. pre-dam; 0.0029 ± 0.002
S.E. post-dam; P = 0.91), but riparian area was slightly
increasing in the reference stream during both periods.
Two covariates were also associated with changes in
riparian area. Across time periods, increases in land use
area (LUA) were associated with decreases in riparian area
in both Dry Creek and the reference stream (est. =−0.006
± 0.002; P =0.006; Table 4; Fig. 13). Increases in BKFA
were also associated with decreases in riparian area,
but only in Dry Creek (est.= −0.004± 0.001; P = 0.003;
Table 4; Fig. 14).
4.2. Stream gauge measurements
4.2.1. Rating curves
The regression indicated that gauge height increased
with increasing discharge at a 50% greater rate during
the post-dam period (slope coefficient = 0.03) than the
pre-dam period (slope coefficient = 0.02; Fig. 15). The
4.1.2. Stream length
During the pre-dam period, stream length varied (CV)
similar amounts in both Dry Creek and the reference
stream (P = 0.74). During the post-dam period, the
reference stream maintained the same level of variation,
while Dry Creek experienced a 64% reduction in stream
length variation (P = 0.016; Table 3; Fig. 9). The analysis
of the rate and direction of stream length changes (SL)
indicates that stream length changes were not significantly different between Dry Creek and the reference
stream during either time period (Table 3; Fig. 10).
4.1.3. Riparian vegetation
During the pre- and post-dam periods, Dry Creek
experienced 13% more variability (CV) in riparian area
than the reference stream (P = 0.02) with individual
stream segments accounting for 40% of the variation
(P = 0.04; Table 4; Fig. 11). Riparian area varied slightly
more during the pre-dam period than during the post-
Fig. 8. Bankfull area rate and direction (BKFA) affected by the
covariate distance downstream (DISTANCE) during the pre- and postdam periods. Even though pre-dam bankfull area trajectory increased
as a function of distance downstream, average bankfull area remained
relatively constant.
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
421
Table 3
ANCOVA results for stream length variation (SL) for Dry Creek and the reference stream during the pre-dam and post-dam period
Source of variation
STREAM
SEGMENTS [STREAM]
TIME
STREAM × TIME
Covariates
LUA
DISTANCE
Error
Stream length variation (CV)
Stream length rate and direction
df
SS
F
df
SS
F
1
18
1
1
0.9803
2.9733
1.5428
0.7523
9.26⁎⁎
1.56
14.58⁎⁎
7.11⁎
1
18
1
1
0.0001
0.0002
0.0001
0.0001
0.098
1.379
0.412
0.399
18
–
–
1.9047
18
0.0001
⁎P < 0.05, ⁎⁎P < 0.01.
ANOVA analysis compared gauge heights between
the two periods for the same range of discharges where
the pre-dam period mean gauge height = 1.13 m, and the
post-dam period mean gauge height = 2.16 m (n = 122,
F = 19.87, P < 0.0001; Fig. 15). The combination of a
steeper post-dam slope coefficient than the pre-dam
slope coefficient and lower pre-dam mean gauge height
than post-dam, suggests that the pre-dam channel was
wider and shallower than the post-dam channel, which
had narrowed and deepened by as much as 1.03 m.
4.2.2. Hydraulic geometry exponent analyses
ANCOVA indicated that the rate width (b) increased
with discharge differed from the pre-dam to post-dam
periods (slope coefficients = 0.50 pre-dam and 0.42 postdam; n = 128, F = 4.44; P = 0.037; Table 5; Fig. 16A), a
24% decrease. The one-way ANOVA showed a difference
in mean width from the pre-dam to post-dam period (mean
Fig. 9. Stream length variation (SL) in Dry Creek and the reference
stream during the pre- and post-dam periods.
width = 45.26 m pre-dam and 21.1 m post-dam; n = 128,
F = 23.89, P < 0.0001). The rate depth (f) increased with
discharge and also differed from the pre- to post-dam
period (slope coefficients = 0.30 pre-dam and 0.37 postdam; n = 128, F = 3.83, P = 0.053; Table 5; Fig. 16B), a
23% increase. Mean depth was significantly different between time periods (mean depth = 0.94 m pre-dam and
0.51 m post-dam; n = 128, F = 23.76, P < 0.0001). The rate
velocity (m) increased with discharge and remained unchanged between the two periods (slope coefficients = 0.20
both periods; n = 128, F = 0.076, P = 0.078; Table 5;
Fig. 16C), although mean velocity decreased from the
pre- to post-dam periods (mean velocity = 1.28 m/s predam and 0.75 m/s post-dam; n = 128, F = 45.26,
P < 0.0001). As a consequence, pre-dam channel dimensions (1960–1986) were more wide than deep. Post-dam
dimensions (1987–2003) indicated that the channel
deepened in relation to width. Whereas the pre-dam
channel exhibited a braided pattern, including mid-channel
Fig. 10. Stream length (SL) rate and direction of variation in Dry Creek
and the reference stream during the pre- and post-dam periods. Stream
length neither increased nor decreased in either stream throughout the
study period.
422
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
Table 4
ANCOVA results for riparian area variation (RA) for Dry Creek and the reference stream during the pre-dam and post-dam periods
Source of variation
STREAM
SEGMENTS [STREAM]
TIME
STREAM × TIME
Covariates
BKFA
LUA
STREAM × BKFA
Error
Riparian area variation (CV)
Riparian area rate and direction
df
SS
F
df
SS
F
1
18
1
1
0.2366
1.5134
0.0810
0.0379
6.50⁎
2.31⁎
2.23
1.04
1
18
1
1
0.0001
0.0008
0.0004
0.0004
0.70
1.53
13.81⁎⁎
12.53⁎⁎
1
1
1
15
0.0004
0.0003
0.0004
0.0005
14.46⁎⁎
10.43⁎⁎
12.93⁎⁎
18
–
–
–
0.6545
⁎P < 0.05, ⁎⁎P < 0.01.
Prior to establishment of Warm Springs Dam, unregulated peak flows and agricultural land use practices,
including land clearing, promoted wide channels with a
moderate amount of riparian forest vegetation along Dry
Creek. Since operation of Warm Springs Dam, our data
show that regulation of peak flows lead to reduced variability in channel morphology and a contraction in bankfull width. With less frequent flood disturbance, riparian
forest vegetation expanded significantly along the chan-
nel's incised banks during the post-dam period, even after
accounting for increases in cultivated area on the floodplain adjacent to Dry Creek. In contrast, the undammed
reference stream experienced consistent variability in both
channel morphology and riparian vegetation. Here, there
was a constant increase in vegetated area while bankfull
area remained unchanged with little change in planform
dimensions throughout the entire 58-yr study period. In
addition, changes in riparian area were not associated with
changes in stream morphology, rather with changes in
land use area. Despite the similarities in climate, soils, and
land use between Dry Creek and the reference stream,
agricultural land use practices coupled with dam-induced
changes in stream morphology had a greater impact on the
riparian environment in Dry Creek than what occurred in
the reference stream.
Fig. 11. Riparian area variation (RA) in Dry Creek and the reference
stream during the pre- and post-dam periods. Even though Dry Creek's
riparian area varied the same between time periods, it actually was
decreasing during the pre-dam period, then increased during the postdam period (see Fig. 12).
Fig. 12. Riparian area rate and direction of variation (RA) in Dry Creek
and the reference stream during the pre- and post-dam periods.
Riparian area consistently decreased in Dry Creek from 1942 to 1976
(pre-dam), then consistently increased from 1987 to 2003 (post-dam).
The reference stream, Maacama Creek, increased in riparian area
during both periods.
bars and chute cutoffs (Figs. 17A,B), the post-dam channel
was a single-thread meandering stream.
5. Discussion
5.1. Channel, vegetation, and land use effects
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
Fig. 13. Riparian area rate and direction of variation (RA) affected by
the covariate bankfull area (BKFA) in Dry Creek and in the reference
stream. Although increases in land use area (LUA) were associated
with decreases in riparian area in both Dry Creek and the reference
stream, variation was high and the relationship was not strong. Slope
coefficients are standardized for each stream.
5.1.1. Bankfull area
The result that bankfull area decreased in variability
after operation of the dam suggests that the stream, as
expected, entrenched as a consequence of reduced sediment and overbank flooding and increased summer flows
with greater stream power. With no dam, the reference
stream experienced consistent variation across the entire
study period, and as expected it experienced the same
level of variability as did Dry Creek during its pre-dam
period.
Prior to dam operation, expansions in bankfull width
were also related to decreases in land use area. A couple
423
Fig. 15. Rating curve showing gauge height-discharge at the Yoakim
Bridge station from 1960–1986 versus 1987–2003. The steeper increase
in gauge height to increasing discharge during the post-dam period
indicates that the channel likely had incised from the earlier period.
of factors may underlie this pattern. First, the air photos
show that land users typically encroached as far as
possible toward the channel bank to cultivate their crops
(Fig. 18). Land use activity at the edge of the channel
banks, including crop cultivation practices and bank
stabilization structures, probably inhibited channel widening as well as promoted it through bank erosion (Simons
and Li, 1980; Harvey and Schumm, 1987). Conversely,
episodic flood events likely pushed land use back from the
channel's edge, decreasing total land use area. As a consequence, the combined effects of overbank flood flows
with large sediment inputs to the channel and agricultural
pressure to cultivate land near the active channel boundary
Table 5
Hydraulic geometry exponent analyses
Dry Creek
Yoakim Bridge guage
b (width)
f (depth)
m (velocity)
Total (b + f + m)
Fig. 14. Riparian area rate and direction of variation (RA) affected by
the covariate land use area (LUA) in Dry Creek and in the reference
stream. Slope coefficients are standardized for each stream.
1960–1986
1987–2003
0.50 [0.97]
0.30 [0.95]
0.20 [0.97]
1.00
0.42⁎ [0.82]
0.37⁎ [0.81]
0.20⁎ [0.42]
0.99
Guage measurements from the Yoakim Bridge station on Dry Creek
300 m upstream of the study reach. Response variables width, depth, and
velocity regressed as a power function against discharge. Slope coefficients indicate rate of increase of response with increasing discharge.
The pre-dam and post-dam periods are compared.
⁎ Indicates significant difference (P < 0.0001) between mean values of
the two periods for the same variable (width, depth, and velocity) at the
Yoakim Bridge guaging station on Dry Creek.
[] R2 of regressions.
424
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
dam operation; bankfull width consistently decreased
downstream and at nearly three times the rate it had increased during the pre-dam period. Decreasing channel
width likely lead to increased channel depth, which is
consistent with the Harvey and Schumm (1987) findings
that the channel bed near the dam was armored 1 year after
dam operation, as well as predictions that the channel bed
would continue to erode downstream and become increasingly armored in following years (Williams and
Wolman, 1984). In contrast, no relationship existed
between bankfull area and distance downstream in the
undammed reference stream.
5.1.2. Stream length
The result that Dry Creek's stream length varied 64%
less during the post-dam period supports the hypothesis
that flow regulated by dams reduces the frequency of
overbank flows and episodic channel migration within
the entrenched channel. This is further supported by the
result that the reference stream varied in length
consistently during both time periods and was not
different than Dry Creek's pre-dam variability. Once Dry
Creek's active channel became vertically and horizontally contained, it likely was unable to episodically
migrate as it had during pre-dam flood events. While
variability in Dry Creek's stream length differed between
the pre- and post-dam periods, we are not sure why actual
stream length was not shorter after dam operation as
hypothesized. However, an examination of Dry Creek's
entire 10.5-km study reach revealed that Dry Creek's
total stream length did decrease by ∼ 5% after dam
operation, and stream length in the reference stream was
unchanged between the pre- and post-dam study periods.
Fig. 16. Hydraulic exponent analyses from Yoakim Bridge gauging
station on Dry Creek. Response of (A) width, (B) depth, and (C)
velocity to increasing discharge from 1960–1986 versus 1987–2003.
The exponent (slope coefficient) indicates the rate that width, depth
and velocity increased with increasing discharge (see Table 5). Width
(A) increased more slowly from 1987 to 2003, likely from a deepening
channel relative to width. Depth (B) also increased more rapidly with
increasing discharge during the later period, whereas (C) velocity
remained unchanged.
may have lead to decreases in riparian vegetation. After
dam operation, this interaction between channel structure
and land use was no longer detectable.
Bankfull width of Dry Creek also gradually increased
downstream along the 10.5-km study area during the predam period, as would be expected as drainage area increases downstream. However, this trend reversed after
5.1.3. Riparian vegetation
Although we found that Dry Creek experienced similar
levels of variability in riparian forest area during the preand post-dam periods, the riparian environment actually
decreased in area by 21% over the pre-dam period, then
significantly increased and exceeded 1942 levels in 2000
by 5%. Changes in the distribution of riparian vegetation,
and the extent of the riparian corridor, were associated
with interactions in changes of stream channel dynamics
and land use activities directly adjacent to the channel.
Prior to dam operation, a pattern existed in Dry Creek in
which riparian area decreased in response to expansions in
land use and bankfull area. The historical air photos
indicate that land users typically encroach as far into the
riparian corridor as possible to cultivate crops during both
the pre- and post-dam periods. Land use practices to maximize floodplain cultivation in conjunction with periodic
pre-dam fluctuations in bankfull width (due to unregulated
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
425
Fig. 17. Example of two reaches in Dry Creek that exhibited a pre-dam multithread channel with wide banks: (A) Yoakim Bridge at the top of three
pre-dam aerial photographs where the USGS gauge station is located, and (B) randomly selected segment 6 (see Fig. 2) exhibits pre-dam braiding
pattern and post-dam, entrenched, single-thread channel.
storm discharges, including periods of channel bed
aggradation from increased sediment inputs contributed
by land clearing practices or fire scarred landscapes, as
well as channel bank scour) probably contributed to limited amounts of space and conditions for the establishment
of channel bank vegetation.
During the post-dam period riparian area likely increased primarily in response to contractions in the
bankfull area (a response to the new flow regulation
regime imposed by the dam that lead to channel incision
and steepened channel banks), and the inability for land
users to easily access and cultivate the abandoned
terrace. During the post-dam period agricultural activities continued to increase in area over the pre-dam
period, which probably prevented riparian vegetation
from expanding away from the stream. But, regulated
flows and absence of episodic channel migrations and
overbank flooding likely provided stable conditions for
vegetation growth on the now less accessible, steepened
banks and abandoned terrace that resulted in a greater
426
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
Fig. 18. Distribution of riparian vegetation in meander of segment 2 (see Fig. 2), Dry Creek, over the 58-yr study period. The 34-yr pre-dam period
from 1942 to 1976 shows a slight increase in vegetation for this reach. Vegetation rapidly colonized the banks and bars during the 13-yr post-dam
period from 1987–2000. After dam emplacement (1983), agriculture was able to move into previously flood-threatened areas near the channel.
E. Gordon, R.K. Meentemeyer / Geomorphology 82 (2006) 412–429
area of riparian vegetation than was present during the
pre-dam period.
In contrast to Dry Creek, the reference stream experienced steady increases in riparian area over the preand post-dam periods. Additionally, riparian area changes
were influenced only by changes in land use area and not
by fluctuations in bankfull area, regardless of time period.
Bankfull area remained unchanged during the entire
58-yr study period. Along the reference stream, the
riparian area apparently increased primarily in response to
contractions and abandonment of agriculture near the
channel boundary, which occurred over time. Once abandoned, riparian vegetation was able to re-establish along
the stream bank. Assessment of the air photos suggests that
the reference stream's riparian corridor has become
increasingly stable over time, which may be stabilizing
the banks against the scour of large storm discharges and
reducing the effect of peak flow events on riparian area
distribution along the channel. In contrast to Dry Creek, the
reference stream's riparian environment has experienced a
consistent morphological trend in channel and riparian
vegetation dynamics through a wholly different interaction
between land use and hydro-geomorphic effects.
5.2. Stream gauge measurements
The rating curves and hydraulic geometry exponent
analyses indicate significant changes in Dry Creek's
channel morphology between the pre-dam to post-dam
periods. Even though replication of stream gauge measurements was not high enough, the field data corroborated our multivariable analyses of air photo-mapped
changes in channel morphology and riparian vegetation.
Results indicated that bankfull width significantly
decreased with increasing discharges by nearly 25%,
whereas depth increased nearly 25% after operation of the
dam. The rate at which velocity increased with increasing
discharge was consistent between the 1960–1986 and
1987–2003 periods, although mean velocity was significantly less with increased discharges from 1987–
2003. This was especially true for 1987–2003 discharges
>30 m3/s, where water surface elevation was sufficiently
high enough to encounter post-dam vegetation that had
colonized the abandoned banks, which increased roughness and decreased velocities.
Harvey and Schumm (1987) indicated that the channel bed just below the dam was armored by 1984, and
stream power was sufficient to promote progressive
erosion of the channel bed farther downstream over
time. At Yoakim Bridge (5600 m downstream of Warm
Springs Dam and 300 m upstream of the study reach),
bed level had dropped a mean 1.03 m from 1987–2003,
427
indicating that the channel bed had not become fully
armored. The channel bed likely will continue to armor
itself over the future decades, with half of the total depth
change typically occurring in the first 7 years (Williams
and Wolman, 1984).
In conclusion, dam operation and land use patterns
together influenced spatial and temporal changes in
channel morphology and riparian vegetation. Disturbances observed at a particular reach may be a stream's
natural adjustment to larger scale impacts of dams. Stream
channel modifications that occur at segments downstream
of impoundment, whether in response to dam effects or as
a consequence of agricultural activities may exacerbate
disturbance effects up and down the stream rather than
contribute to a channel's natural adjustment to large-scale
impacts. Rigorous quantification with spatial and temporal replication and a control for environmental factors is
important to help discern causal-mechanistic processes
that lead to impacts within hydrogeomorphic systems
strongly impacted by human activities. Rehabilitation of
stream channels should consider relationships between
discharge, sediment load, channel geometry, land use, and
riparian vegetation. Our results suggest that managers
who seek to rehabilitate a channel and its ecological
function need to consider a stream system's reactions to
historical disturbances and current stream processes to
enhance geomorphic adjustment. We believe use of a
nearby reference in conjunction with multivariable
analysis of spatially and temporally replicated observations is an effective approach for understanding stream
channel and vegetation processes in response to operation
of a dam and land use pressures.
Acknowledgements
We gratefully acknowledge Dorothy Freidel for her
guidance in interpreting stream morphology. Nathan
Rank was invaluable for assisting with statistical design.
We sincerely thank J. Hall Cushman and David Stokes
for their detailed comments that greatly improved this
manuscript. Mathew Dietch was kind enough to provide
archived USGS stream gauge measurements not readily
available online.
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