Riparian reforestation and channel change: A case study of two small tributaries to
Sleepers River, northeastern Vermont, USA
Maeve McBride a,
, W. Cully Hession b
, Donna M. Rizzo a
a
Civil & Environmental Engineering, University of Vermont, 213 Votey Building, Burlington, Vermont, 05405, USA
b
Biological Systems Engineering, Virginia Tech, Blacksburg, Virginia, USA
A B S T R A C TA R T I C L E I N F O
Article history:
Received 28 August 2007
Received in revised form 3 May 2008
Accepted 5 May 2008
Available online 16 May 2008
Keywords:
Riparian reforestation
Stream widening
Channel morphology
Large woody debris
Conceptual model
Measurements of two small streams in northeastern Vermont, collected in 1966 and 2004­2005, document
considerable change in channel width following a period of passive reforestation. Channel widths of several
tributaries to Sleepers River in Danville, VT, USA, were previously measured in 1966 when the area had a
diverse patchwork of forested and nonforested riparian vegetation. Nearly 40 years later, we remeasured bed
widths and surveyed large woody debris (LWD) in two of these tributaries, along 500 m of upper Pope Brook
and along nearly the entire length (3 km) of an unnamed tributary (W12). Following the longitudinal survey,
we collected detailed channel and riparian information for nine reaches along the same two streams. Four
reaches had reforested since 1966; two reaches remained nonforested. The other three reaches have been
forested since at least the 1940s. Results show that reforested reaches were significantly wider than as
measured in 1966, and they are more incised than all other forested and nonforested reaches. Visual
observations, cross-sectional surveys, and LWD characteristics indicate that reforested reaches continue to
change in response to riparian reforestation. The three reaches with the oldest forest were widest for a given
drainage area, and the nonforested reaches were substantially narrower. Our observations culminated in a
conceptual model that describes a multiphase process of incision, widening, and recovery following riparian
reforestation of nonforested areas. Results from this case study may help inform stream restoration efforts by
providing insight into potentially unanticipated changes in channel size associated with the replanting of
forested riparian buffers adjacent to small streams.
 2008 Elsevier B.V. All rights reserved.
1. Introduction
Restoration of streamside forests is a major focus of watershed
initiatives throughout the United States (National Reseach Council,
1992; U.S. Environmental Protection Agency, 1999) and commonly
accompanies in-stream restoration efforts. One example is the
Chesapeake Bay Program's initiative to restore riparian forests along
3234 km (2010 mi) of stream by the year 2010 (Palone and Todd,
1997). When the Bay Program achieved this goal ahead of schedule in
2002, the goal was increased to 16,093 km (10,000 mi) by 2010
(Chesapeake Bay Program, 2007). Similarly, the federal Conservation
Reserve Enhancement Program (CREP), a voluntary agricultural land
retirement program, has provided incentives leading to over
3200 km2
of replanted riparian buffers nationwide since 1997 (U.S.
Department of Agriculture, 2007). Stream restoration projects aimed
at reconfiguring channels, protecting streambanks and infrastructure,
or in-stream habitat modifications often include riparian plantings to
supplement these activities (Bernhardt et al., 2005). Indeed, riparian
forest restoration has been identified as a successful tool for
improving stream ecosystems through filtering of pollutants (Herson-Jones
et al., 1995; Berg et al., 2003); regulating nutrients, light,
and temperature; and providing both physical habitat and the food/
energy base (Gregory et al., 1991; Sweeney, 1992; Berg et al., 2003;
Sweeney et al., 2004). Despite these benefits, the best treatment for
streambanks remains unresolved because of the uncertain role of
riparian vegetation in affecting streambank erosion rates and
processes (Montgomery, 1997; Trimble, 1997; Lyons et al., 2000).
Riparian vegetation exerts important influences on stream channel
morphology (Thorne, 1990; Montgomery, 1997; Anderson et al., 2004).
Riparian vegetation is highly varied, hindering simple quantification;
however, many studies have shown distinct differences in morphology
resulting from two broad types of vegetation: forests and grasslands.
Most studies from widely different geographic locations indicate that
stream reaches with riparian forests are wider than those with adjacent
grassy vegetation (Zimmerman et al., 1967; Murgatroyd and Ternan,
1983; Clifton,1989; Sweeney,1992; Peterson,1993; Davies-Colley,1997;
Trimble,1997; Scarsbrook and Halliday,1999; Hession et al., 2000, 2003;
Sweeney et al., 2004; Allmendinger et al., 2005; Roy et al., 2005). In
contrast, a few other studies suggest that widths of streams through
grassland are generally greater than those through forest (Charlton et al.,
Geomorphology 102 (2008) 445­459
 Corresponding author. Tel.: +1 802 658 9404; fax: +1 802 656 8446.
E-mail address: mmcbride@cems.uvm.edu (M. McBride).
0169-555X/$ ­ see front matter  2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2008.05.008
Contents lists available at ScienceDirect
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journal homepage: www.elsevier.com/locate/geomorph
1978; Hey and Thorne, 1986; Gregory and Gurnell, 1988; Rosgen, 1996).
The aforementioned studies have categorized riparian vegetation in
unique ways, so it can be challenging to make generalized conclusions.
These apparently contrary findings may be partially explained by a
scale-dependent effect of riparian vegetation. Small forested streams are
wider than small nonforested streams; whereas in large streams with
drainages greater than 10 to 100 km2
, an opposite effect is observed such
that channel widths are narrower when thick woody vegetation is
present (Anderson et al., 2004).
Ideas about the geomorphic processes responsible for the observed
channel width differences are varied and remain untested. Zimmerman
et al. (1967) attributed the wider channels in forested stream
reaches to local scouring around LWD, debris dams, or at streamside
tree-throw locations. They surmised that grass roots in nonforested
reaches reinforced streambanks and encroached more rapidly on the
channel during periods of low flow than roots from trees or
understory plants in the forest (Zimmerman et al., 1967). Subsequently,
Murgatroyd and Ternan (1983) found more active bank
erosion in forested reaches than in nonforested reaches, which they
attributed primarily to the loss of a thick grass turf and secondly to
scour around log jams and debris dams. Davies-Colley (1997)
hypothesized that when a riparian forest is cleared, grasses are able
to grow on the gravel bars in the channel, making the bars more stable
and narrowing the channel. In the same year, Trimble (1997) proposed
that wider forested channels were unstable and that narrower
nonforested channels more efficiently trapped sediment causing
channel narrowing. The influence of riparian vegetation on streambank
stability is complex, highly variable, and a broader topic with its
own body of literature (Gray and MacDonald, 1989; Thorne, 1990;
Dunaway et al., 1994; Montgomery, 1997; Stott, 1997; Abernethy and
Rutherfurd, 1998; Simon and Collison, 2002; Pollen et al., 2004;
Rutherfurd and Grove, 2004; Wynn and Mostaghimi, 2006). Recently,
Allmendinger et al. (2005) found higher rates of deposition and lateral
migration in nonforested reaches than in forested reaches, and they
suggested that the differences in width between forested and
nonforested reaches were related to a balance between rates of erosion
and deposition on active floodplains. In addition, results from a flume
study suggest that near-bank turbulence during overbank flows may be
an important process in channel widening (McBride et al., 2007).
Clear evidence for channel widening or narrowing with a change in
riparian vegetation is difficult to obtain as such processes likely
operate on a timescale greater than the length of a typical research
study, thus limiting field-based research opportunities. Long-term
channel change in response to riparian vegetation change in small
streams has been documented in a few cases (Clifton, 1989; Parkyn
et al., 2003; McBride et al., 2005). Most studies must rely on the spacefor-time
substitution and attempt to find paired sites that have similar
background characteristics (Davies-Colley, 1997; Huang and Nanson,
1997; Stott, 1997; Trimble, 1997; Scarsbrook and Halliday, 1999;
Hession et al., 2003; Parkyn et al., 2003; Sweeney et al., 2004;
Allmendinger et al., 2005; McBride et al., 2005).
In this paper we expand on a preliminary effort to compare historic
and current channel-size data from the Sleepers River Research
Watershed (SRRW) in Danville, VT (McBride et al., 2005). One of the
first studies to document the influence of riparian forests on channel
form was completed within the SRRW by Zimmerman et al. (1967).
We revisited two of the streams described in the Zimmerman et al.
(1967) study to assess potential changes in channel dimensions and
LWD characteristics in response to riparian reforestation. Our
objectives were (i) to evaluate the extent of riparian reforestation
along the two study streams; (ii) to determine differences in bed
widths since 1967; and (iii) to investigate differences in channel
dimensions and LWD as related to current riparian vegetation. Based
on our results, we present a conceptual model that describes the
process of channel adjustment following the introduction of forested
riparian vegetation over time.
2. Study area
The SRRW is one of the longest-running, cold-region research
watersheds in the United States (Fig. 1); hydrologic data have been
collected continuously since 1958 (Pionke et al., 1986). The site is
currently administered by the U.S. Geological Survey (USGS). Sleepers
River drains 111 km2
that is predominantly forested (~67%); agriculture
and rural residences are the other common land uses (Shanley
et al., 1995). The SRRW contains rolling topography at elevations
between 201 to 780 m, and it is underlain with 1 to 20 m of calcareous
till atop the Waits River Formation bedrock, a metamorphosed limestone
(Shanley et al., 1995). The mean annual temperature is 6 °C, and
mean annual precipitation is 90 cm (Shanley et al., 1995). We studied
two streams: (i) stream W12 is an unnamed tributary to Sleepers River
that drains a 2.1-km2
mixed land-use drainage; and (ii) upper Pope
Brook, another tributary, is a headwater stream that drains a 1.1-km2
forested area within State of Vermont forest lands. Forests are
predominantly mixed with both coniferous and deciduous trees. The
most common species are northern white cedar (Thuja occidentalis),
yellow birch (Betula alleghaniensis), white spruce (Picea glauca),
balsam fir (Abies balsamea), and sugar maple (Acer saccharum).
3. Methods
3.1. Field methods
We conducted a longitudinal survey of W12 and portions of
upper Pope Brook, revisiting the field sites of Zimmerman et al.
(1967) during the summer of 2004. We excluded two segments of
W12 where cattle had access to the stream and riparian areas. We
measured bed widths every 8 m to replicate the measurements made
by Zimmerman et al. (1967). In that study, widths were measured
"between breaks-in-slope between bed and bank" (Zimmerman
et al., 1967); likewise, we measured widths at the edge of the bed,
essentially capturing the active channel width, or bed width. We also
collected information on LWD during the longitudinal survey. For
each piece of LWD N10 cm in diameter and 1 m in length
(Montgomery et al., 1995; Jackson and Sturm, 2002), we recorded
its location, diameter, length, and decay class. Decay class describes
the age of the LWD on a scale from 1 to 6, where 1 is a recently
recruited piece of LWD with leaves or needles still present, and 6 is
an old, weathered piece of LWD (Martin and Benda, 2001). Debris
dams were also located and tallied.
In the summer of 2005, we conducted detailed channel and
riparian surveys at nine reaches within the stream sections previously
surveyed. Seven of the reaches were in W12: two nonforested sites
(NF1 and NF2); one forested site (F1); and four reforested sites (R1­R4).
Reforested sites were located in segments of W12 that were
identified as nonforested in 1966 but have reforested in the last
40 years. The other two forested reaches were in upper Pope Brook
(F2 and F3). All reaches were 75 m long, a length equivalent to
approximately 20 bankfull widths. In each reach we completed a
detailed cross-sectional survey of the channel at a characteristic riffle
feature, extending to the floodplain or valley slope adjacent to each
streambank. Bankfull elevations were determined considering a
combination of the presence or absence of perennial vegetation,
topographic breaks in the bank, and a change in sediment texture or
size (Dunne and Leopold, 1978). We randomly chose a 10-m2
plot, as
recommended by Kent and Coker (1992), along either the right or left
bank of each reach, within which we measured the diameter at breast
height (DBH) of every tree stem N1.0 cm in diameter. Within the plot,
we also tallied and measured the diameter and length of each woody
debris piece on the floodplain using the same size criteria as the instream
LWD. To estimate forest age at reaches F1, F2, and F3, we cored
the largest tree within the riparian zone of the reach with an increment
tree borer.
446 M. McBride et al. / Geomorphology 102 (2008) 445­459
3.2. Spatial data methods
To assess quantitatively the change in riparian vegetation, we
assembled, scanned, and georeferenced two historic aerial photographs.
The earliest aerial photograph available was from 1943
(1:1000). An aerial photograph from 1965 (1:10,000) was nearly
concurrent with the Zimmerman et al. (1967) data. The 1943 and 1965
aerial photographs were georeferenced to the most current orthophotograph
(1999) using ArcMap software (ESRI, Redlands, CA). Once
georeferenced, we overlaid the aerial photographs to determine areas
of reforestation.
3.3. Analytical methods
Data collected in the Zimmerman et al. (1967) study included
channel dimensions (bed widths and occasional depths) and riparian
vegetation type for approximately five different tributaries to Sleepers
River. Individual width measurements could be determined from
graphs in the Zimmerman et al. (1967) paper for two of the study
streams: a 500-m segment of upper Pope Brook and the entire length
(3 km) of stream W12. Zimmerman et al. (1967) specifically selected
stream segments to avoid confounding disturbances such as backwater
from weirs, trampling from cattle, and other human-caused
alterations. The Zimmerman et al. (1967) study classified the riparian
condition of different sections of W12 as either sod (i.e., grass), thicket,
or forest. Data categorized as "thicket" were excluded from our
analysis because this vegetation type was not well described.
As a precursor to statistical testing of the bed-width data, we
checked the distribution and created a testable subset of the data. We
created a subset of the past and current bed-width measurements by
using the bed-width data points falling within the nine detailed
reaches of this study. This resulted in ~10 data points per reach for a
total of 79 data points for 1967 and 87 data points for 2004. One reach
(NF1) was increased to 140 m in length to include a sufficient number
of data points from the Zimmerman et al. (1967) data. We found the
bed-width data (past and present) to be approximately normal by
assessing normal probability plots, which allowed for parametric
testing (Zar, 1999).
Differences in mean bed width over nearly 40 years of time
(between 1966 and 2004) were tested using a series of two-sample t
tests and assessed using a percent difference. A total of nine t tests
were performed on the seven W12 reaches and the two upper Pope
Brook reaches. Homogeneity of variance was tested in all cases using
Levene's test (SAS, 2004). In one case where homogeneity of variance
Fig. 1. Location of the SRRW showing drainages and reaches of W12 and upper Pope Brook.
447M. McBride et al. / Geomorphology 102 (2008) 445­459
was rejected (reach R4), we used the Satterthwaite version of the twosample
t test (SAS, 2004). Percent differences (PD) between 1966 and
2004 data were determined as
PD  W2004-W1966 =W1966 Â 100; 1
where W2004 is the mean bed width as measured in 2004, and W1966 is
the mean bed width as measured by Zimmerman et al. (1967). We tested
whether the PD of reforested reaches were greater than other reaches
using the nonparametric, one-tailed Mann­Whitney test (Zar, 1999).
Key parameters were identified from the LWD data, cross-sectional
surveys, and riparian vegetation plots. To describe the LWD characteristics
of each reach, we determined total counts and volumes. The
decay classes were combined for the three forested reaches and the
four reforested reaches to assess the frequency distributions. Bankfull
channel width, cross-sectional area, mean depth, and width-to-depth
ratio were determined for each reach. For each riparian plot, we
determined summary statistics for the trees and debris measured,
including counts, mean DBH, and maximum DBH. Forest ages of the
riparian zones of F1, F2, and F3 were estimated by counting the tree
rings present in the increment bores.
Channel dimension data from previously published, paired
forested and nonforested studies were also compiled to produce
hydraulic geometry relationships between bankfull width, bankfull
depth, and drainage area (Davies-Colley, 1997; Hession et al., 2003).
Because the largest drainage area of our study was 2.06 km2
, we
selected data from previous studies for reaches that had drainage
areas smaller than 3 km2
. The drainages from the previous studies had
land cover that was similar to the SRRW with mixed agricultural and
forest land cover (Davies-Colley, 1997; Hession et al., 2003). Climate
data indicates that the SRRW is colder and dryer than the drainages in
the mid-Atlantic states (Hession et al., 2003) or in northern New
Fig. 2. Aerial photographs of W12 in 1965 (A) with the riparian classifications of Zimmerman et al. (1967) and in 1999 (B) with reforested segments highlighted and sample reach
locations identified.
Fig. 3. Photographs of nonforested reaches NF1 (left) and NF2 (right) of W12.
448 M. McBride et al. / Geomorphology 102 (2008) 445­459
Zealand (Davies-Colley, 1997). The Hession et al. (2003) drainages
have a mean annual precipitation of 117 cm; and in nearby Wilmington,
DE, the mean annual temperature is 12.4 °C (NCDC, 2006).
The Davies-Colley (1997) drainages are closest to the city of Hamilton,
which receives 119 cm of annual precipitation and the mean annual
temperature is 13.7 °C (National Institute of Water & Atmospheric
Research (NIWA), 2007). Eighteen reaches from the two studies were
used to create the hydraulic geometry relationship between bankfull
width and drainage area. Only seven reaches from the Hession et al.
(2003) study were similarly used for the bankfull depth relationship,
as channel depths were not collected by Davies-Colley (1997).
4. Results
4.1. Riparian reforestation
Substantial riparian reforestation has taken place adjacent to W12
since the Zimmerman et al. (1967) study (Fig. 2). In 1966, W12 had
~50% of its total stream length bordered by sod or nonforested
vegetation (Zimmerman et al., 1967), while in 1999 only 22% of the
stream remained bordered by nonforested vegetation. Almost 1 km of
W12's riparian zone has reforested within the last 40 years. Only two
sections of W12 remain nonforested (Fig. 3) because of mowing
practices of the landowners, and in these sections nearby trees were at
least 5 m from the stream's edge. Interpretation of the aerial photographs
indicated that riparian forests at reaches F1, F2, and F3 were
established prior to 1943, indicating a forest age of at least 60 years.
The riparian forest at upper Pope Brook (F2 and F3) is likely older
than the riparian forest surrounding reach F1 along W12 based on the
tree-coring results. A yellow birch with a 50-cm DBH in reach F2 was
dated as ~85 years old, and a balsam fir with a 35-cm DBH in reach F3
was ~60 years old. The largest tree near reach F1 was a white pine
(Pinus strobus) with a 50-cm DBH, but the estimated age was only
~50 years.
Visual observations and riparian plot measurements highlight
differences between the forest vegetation at reach locations. All
reforested reaches appeared to have immature forested vegetation
both from our visual observations (Fig. 4) and the plot measurements.
Forest vegetation in reforested reaches appeared more homogeneous
than forested reaches, and this observation was confirmed by the
lower standard deviations of the DBH measurements (Table 1).
Commonly, we observed "wolf" trees in the reforested reaches, providing
evidence for the previously nonforested riparian zones. A
"wolf" tree differentiates itself from other trees by its large size and
horizontal branches, because it once grew in an open area without
competition from other trees (Spirn, 1998). The characteristics of the
reforested reaches were variable and unique to each reach: reach R1's
floodplain was densely populated with moderate-sized trees with
very little woody debris on the floodplain; reaches R2 and R3 had the
smallest trees and woody debris of all reaches; and reach R4 had few
Fig. 4. Photographs of reforested reaches R1 (A), R2 (B), R3 (C), and R4 (D) of W12.
Table 1
Riparian vegetation plot measurements
Reach Stream Stem
count
DBH
(cm)
DBHmax
(cm)
Debris
count
Debris size
(cm)
Debris
sizemax (cm)
NF1 W12 0 na na 0 na na
NF2 W12 0 na na 0 na na
R1 W12 56 11.8 (8.5) 36.0 0 na na
R2 W12 30 9.0 (7.6) 32.6 4 4.4 (3.6) 5.2
R3 W12 28 8.6 (6.1) 27.9 16 4.7 (1.3) 10.5
R4 W12 6 10.7 (6.3) 16.2 8 24.9 (6.9) 42.0
F1 W12 33 11.3 (9.4) 34.5 12 12.2 (3.0) 42.0
F2 Pope 15 17.4 (10.5) 32.5 7 17.8 (10.5) 33.0
F3 Pope 31 12.1 (8.9) 27.0 6 12.1 (12.9) 37.6
DBH: mean diameter at breast height of all tree stems, standard deviation in parentheses;
DBHmax: maximum diameter at breast height; debris size: mean diameter of woody debris
at the midpoint, standard deviation in parentheses; debris sizemax: maximum diameter of
woody debris.
449M. McBride et al. / Geomorphology 102 (2008) 445­459
trees, but large-sized woody debris on the floodplain. We observed
grassy, herbaceous vegetation along portions of the streambanks of
R2, R3, and R4, but mosses were the predominant ground cover in R1.
The three forested reaches (Fig. 5) where the forest vegetation was
N60 years old had trees with greater mean DBH (13.6 cm) than the
more recently reforested reaches (10.0 cm); however, this difference
was not statistically significant.
4.2. Bed width
The reforested reaches of W12 exhibited the greatest increase in
mean bed width between 1966 and 2004; however, all reaches, except
F2, were significantly wider in 2004 than in 1966 (Table 2; Fig. 6).
Among all reaches evaluated, the increase in mean bed width ranged
from 9% to 376%. Three of the four reforested reaches in W12 (reaches
R1, R2, and R4) experienced the greatest change in mean bed width, as
shown by the three highest PD (231%, 263%, and 376%, respectively).
When the reforested reaches were compared against all other reaches,
they had significantly greater PD (p=0.05). Mean bed widths of F2 in
upper Pope Brook exhibited the least change in the last 40 years, and
current mean values (2004) were not significantly different from the
mean values derived from the Zimmerman et al. (1967) estimates. In
general, bed widths were more variable in 2004 than those measured
in 1966; the standard deviation of bed widths in most reaches is
greater for the 2004 data.
4.3. Large woody debris
LWD was more abundant in the forested reaches than the reforested
reaches and was virtually absent from the nonforested
reaches (Table 3). Forested reaches had 10.7 pieces of LWD per 75-m
reach on average, while reforested reaches had only 6.8 pieces. In
contrast, average LWD volume was not significantly different between
forested and reforested reaches, highlighting that LWD pieces in
reforested reaches were larger. Forested and reforested reaches were
also similar in the number of debris dams and the volume of LWD in
debris dams. LWD pieces in forested reaches were classified in the
oldest decay classes, but LWD in reforested reaches were fairly evenly
distributed throughout the six decay classes (Fig. 7). In general,
reforested reaches had LWD that was more recently recruited to the
stream.
4.4. Current bankfull channel dimensions
Bankfull channel dimensions were a function both of drainage area
and riparian vegetation type. Although the drainage areas of the
reaches only ranged from 0.55 km2
to 2.06 km2
, the range in bankfull
widths was more than an order of magnitude (Table 4). Differences in
channel size and undercut banks in nonforested, reforested, and forested
reaches are displayed in Figs. 8, 9, and 10, respectively. Bankfull
widths and bankfull depths were each plotted with hydraulic geometry
relationships developed for forested and nonforested reaches
Fig. 5. Photographs of forested reaches F1 (A) of W12, and F2 (B) and F3 (C) of upper Pope Brook.
Table 2
Current and past bed-width measurements
Reach Stream n Mean bed width (m) Percent
difference
1967 2004 1967 2004
NF1 W12 6 10 0.81 (0.22) 1.19 (0.23) 47%
NF2 W12 6 7 0.67 (0.12) 1.11 (0.31) 66%
R1 W12 9 10 0.70 (0.38) 2.32 (0.33) 231%
R2 W12 10 10 0.65 (0.30) 2.36 (0.42) 263%
R3 W12 9 10 1.24 (0.34) 2.06 (0.68) 66%
R4 W12 8 10 0.51 (0.27) 2.43 (0.74) 376%
F1 W12 11 10 1.23 (0.58) 2.69 (0.70) 119%
F2 Pope 10 10 1.70 (0.77) 1.86 (0.41) 9%
F3 Pope 10 10 1.42 (0.33) 2.09 (0.43) 47%
Bold mean values are significantly different with =0.01. Standard deviations are in
parentheses. Reach name indicates riparian vegetation type.
450 M. McBride et al. / Geomorphology 102 (2008) 445­459
of previous studies (Figs. 11 and 12). Although hydraulic geometry
relationships are generally specific by region, our data corresponded
well with data from Pennsylvania (Hession et al., 2003) and New
Zealand (Davies-Colley, 1997). Forested reaches were substantially
wider than nonforested reaches for a given drainage area, and they
generally align with the hydraulic geometry of forested reaches.
Widths of reforested reaches were narrower than the forested reaches
of this study and smaller than the widths predicted by the hydraulic
geometry relationship for other forested reaches. Reforested reaches
were wider than the nonforested reaches in this study, but they
aligned with the widths expected from nonforested reaches from
other studies (Davies-Colley, 1997; Hession et al., 2003). In general,
widths from our study may be relatively smaller than widths from
Pennsylvania and New Zealand because the northeastern Vermont
climate is dryer and cooler.
The range in mean bankfull depths was less pronounced than the
range in bankfull widths. Mean bankfull depths for forested and
nonforested reaches corresponded well with the hydraulic geometry
relationship derived from previously published data (Fig. 12), where
forested and nonforested sites were combined because Hession et al.
(2003) found no differences in depth. Reforested reaches tended to
have mean bankfull depths that were at least 10 cm deeper than the
mean bankfull depth expected for a given forested or nonforested
reach with the same drainage area. A paired t test of measured and
expected bankfull depths for reforested reaches revealed a significant
difference (p=0.004), but measured and expected bankfull depths for
the five other reaches were not significantly different.
4.5. Hydrology
Although much of the historical hydrologic record from Sleepers
River gages is missing, we were able to compare the annual mean and
annual peak flows of Pope Brook in order to identify potential
differences between historic and current stream flow that might affect
channel morphology. Stream gage records for Pope Brook (USGS gage
01135150) from 1992 to 2005 are available and indicate an annual
mean discharge of 0.17 cm. Annual peak flows for the same time
period range from 2.61 to 7.05 cm, and the mean annual peak flow is
4.47 cm. Zimmerman et al. (1967) provided similar statistics for the
same gaging station for a period of record from 1960 to 1966, where
the mean annual discharge was 0.15 cm and the maximum peak
discharge on the record was 4.47 cm. From this limited comparison of
hydrologic summary statistics, significant differences are not apparent
in the background hydrology over the period of interest.
5. Discussion
5.1. Reforestation and land-use change
Much of the state of Vermont has reforested passively as agricultural
lands were abandoned. In fact, ~30% of late nineteenthcentury
Vermont was forested, while over 70% of Vermont is forested
at present (Albers, 2000). The reforestation of portions of the riparian
zone of W12 was a result of changes in land-use practices.
Nonforested areas, maintained by local landowners as pastures or
fields, reverted back to forest once maintenance declined. Portions of
W12's riparian zone are still used as pasture for cattle, but these areas
were excluded from the analysis because of known impacts to channel
dimensions from cattle grazing (Clifton, 1989; Belsky et al., 1999).
The jumbled history of deforestation and reforestation in Vermont,
and undoubtedly in other locations as well, sets a complex context for
geomorphic inquiries. Within this context the response of a stream or
river to a single anthropogenic or natural change may be difficult to
discern, such as the riparian reforestation impacts. The widespread
deforestation following European settlement changed the hydrologic
regime and sediment loading to streams and rivers; several studies in
Vermont using sediment coring techniques and alluvial fan trenching
determined that upland erosion rates increased following European
settlement (Bierman et al., 1997; Noren et al., 2002; Jennings et al.,
2003). When hillslopes were cleared, they were primed for erosion,
Fig. 6. Box plots of bed width data from 1966 and 2004 showing median values (mid-line), 25th and 75th percentiles (box), and 10th and 90th percentiles (whiskers).
Table 3
Number and volumes of individual LWD and debris dams by reach
Reach Stream LWD
count
LWD volume
(m3
)
Debris dam
count
Debris dam volume
(m3
)
NF1 W12 0 0.00 0 0
NF2 W12 1 0.03 1 0.03
R1 W12 10 0.40 2 0.20
R2 W12 8 0.55 2 0.39
R3 W12 3 0.20 1 0.08
R4 W12 6 0.33 2 0.31
F1 W12 14 0.76 2 0.50
F2 Pope 10 0.19 3 0.11
F3 Pope 8 0.23 1 0.02
451M. McBride et al. / Geomorphology 102 (2008) 445­459
landslides, and mass wasting. Streams and rivers aggraded in response
to the increased sediment load, evidenced by aggradation in floodplain
deposits (Costa, 1975) and in the expansion of the Winooski
River delta in Lake Champlain, VT (Bierman et al., 1997). Streams and
rivers may be working through these legacy sediments. Indeed,
conditions may not remain stable long enough for many streams and
rivers to reach an equilibrium (Knighton, 1998). Although our study
site may still be "recovering" from the historic deforestation centuries
ago and although the site may continue to evolve in response to new
input conditions (e.g., climate change), the decadal change in riparian
vegetation has created a strong and discernable imprint on the
channel morphology.
Similarly, other conditions, including channelization, impoundments,
beaver activity, and grazing can complicate the geomorphic
history. To the best of our knowledge, our study reaches were located
in stream segments with limited historic and current impacts. Historic
maps from the mid-1800 s of the two study streams did not show any
impoundments or channel straightening; however, two homesteads
and a former road crossing were located on W12 between reaches R3
and R4. Beaver are present in the region, and during our field data
collection in 2004 and 2005 they maintained a dam in the headwaters
of W12. We assume that whatever effects the beaver dam may have
had on the sediment or hydrologic regime of W12 that the study
reaches are equivalently influenced by this background condition.
Portions of W12 (between R3 and R4 and between NF1 and R1) are
currently accessible by cattle, but these segments were excluded from
our analysis.
5.2. Riparian vegetation and channel size
We found that forested reaches were significantly wider than
reaches with nonforested riparian vegetation, confirming the findings
of Zimmerman et al. (1967) and other studies (Murgatroyd and
Ternan, 1983; Sweeney, 1992; Peterson, 1993; Davies-Colley, 1997;
Trimble, 1997; Scarsbrook and Halliday, 1999; Hession et al., 2003;
Sweeney et al., 2004; Allmendinger et al., 2005). As Zimmerman et al.
(1967) described, in small streams, riparian vegetation exerts more
influence on channel size than drainage area. For example, reach NF1
had the largest drainage area of all W12 reaches, but was roughly half
as wide as the other forested or reforested reaches. Reach F1 was the
widest of all reaches, even though reaches R1 and R2 had larger
drainage areas than F1.
Our results suggest that reforested reaches may not be as wide as
forested reaches, and thus we infer that reforested reaches will
continue to widen for some unknown period of time. Reach F1 had the
largest mean bed width among all seven reaches in W12 (Table 1), and
this reach has been forested for at least 60 years. Reaches that have
reforested within the last 40 years (reaches R1, R2, R3, and R4) had
mean bed widths that were larger than the nonforested reaches, but
smaller than the mean bed width of reach F1; however, an analysis of
covariance, using drainage area as the covariate, found no significant
difference between the bed widths of forested and reforested reaches.
Although we did not have sufficient data to perform statistical testing
on the bankfull widths, we found that the bankfull widths of the four
reforested reaches aligned more closely with previously published
nonforested hydraulic geometry relationships (Davies-Colley, 1997;
Hession et al., 2003) than the forested ones (Fig. 11).
Another major difference between reforested reaches and the
other reaches was the deeper channel depth. The mean bankfull
depths of the three forested and two nonforested reaches corresponded
well with the hydraulic relationship between mean bankfull
depth and drainage area (Fig. 12). Similarly, other studies have found
no difference in either the hydraulic geometry of depth and discharge
Fig. 7. Histogram of decay classes for LWD in forested and reforested reaches (class 1 had green leaves or needles present, class 2 had twigs present, class 3 had secondary branches
present, class 4 had primary branches present, class 5 had no branches, and class 6 was severely decayed).
Table 4
Current bankfull dimensions at representative riffle feature
Reach Stream Drainage area
(km2
)
Width
(m)
Mean depth
(m)
Cross-sectional
area (m2
)
W:Da
NF1 W12 2.06 1.57 0.22 0.35 7.1
NF2 W12 0.91 0.33 0.28 0.09 1.2
R1 W12 1.98 2.68 0.47 1.25 5.7
R2 W12 1.95 2.12 0.50 1.07 4.2
R3 W12 1.72 2.06 0.45 0.93 4.6
R4 W12 1.02 2.04 0.37 0.76 5.5
F1 W12 1.91 3.84 0.33 1.28 11.6
F2 Pope 0.55 2.84 0.13 0.38 21.1
F3 Pope 1.06 2.27 0.29 0.65 7.9
a
W:D: bankfull width to mean bankfull depth ratio.
452 M. McBride et al. / Geomorphology 102 (2008) 445­459
or depth and drainage area with different riparian vegetation types
(Murgatroyd and Ternan,1983; Andrews,1984; Hey and Thorne,1986;
Trimble, 1997; Hession et al., 2003). The four reforested reaches were
significantly deeper than the other forested or nonforested reaches for
a given drainage area, which suggests that they may be incised at this
stage of transition between a nonforested morphology and a forested
morphology.
Although few studies have monitored channel change with reforestation,
close inspection of previous literature suggests that incision
may be an active process following reforestation. We found
mention of slumping banks in two studies, where presumably an
incised channel made banks increasingly high and steep enough to
cause slumping failures (Murgatroyd and Ternan, 1983; Davies-Colley,
1997). In addition, during a 5-year period, Murgatroyd and Ternan
(1983) observed neither overbank nor bankfull flows in the reaches
with 50-year-old forest vegetation, while upstream in nonforested
reaches, overbank flow occurred on average two to three times per
year. A third study focusing on the water quality of two headwater
streams after nine years of riparian reforestation described the reaches
as incised and found an adverse riparian forest effect on water quality
(Smith, 1992). We suspect that although reforested reaches may widen
and contribute a greater sediment load during a transitional period,
reforested reaches will eventually have slower lateral migration
(Allmendinger et al., 2005) and additional sediment storage associated
with woody debris (Keller and Swanson,1979; Montgomery,1997; Bilby
and Bisson, 1998) that might compensate for a temporary increase in
sediment load.
5.3. Channel widening
Our results suggest that riparian reforestation led to stream widening
in W12 because the greatest difference in bed widths occurred
in reaches that have partially or completely reforested since 1966.
Reforested reaches are likely still adjusting to an equilibrium channel
form. We commonly found features indicating recent channel change
in reforested reaches, such as undercut, eroding banks, and avulsions.
Davies-Colley (1997) speculated that the "streambank recession
phase" following reforestation might continue for decades, while
the channel recreates a forested equilibrium morphology. Parkyn et al.
(2003) found little evidence for channel widening in reaches with
forest vegetation planted 2 to 24 years prior to their study and suggested
that the plantings were too young for widening to have begun.
However, their finding was based on comparisons of nine forested
reaches to upstream or nearby control reaches that were unfenced and
actively grazed (Parkyn et al., 2003), and grazed reaches can be
widened because of streamside trampling from livestock (Clifton,1989).
The time needed for reforested stream reaches to attain equilibrium is
unclear. Our results suggest that this process may take longer than a few
decades and perhaps last as long as a century, as the reforested reaches
appeared to be roughly half-way between the nonforested and older
forested reaches in terms of channel width (Fig. 11). Additionally, the
forested reaches (F2, F3, and especially F1) may still be adjusting to
earlier forest-clearing episodes, as those forest parcels are likely
secondary or tertiary growth.
Although reforested reaches experienced widening that was significantly
greater than the other reach types, both nonforested and
forested reaches also had greater mean bed widths than when measured
in 1966. Some possible explanations for this finding include (i)
measurement error, (ii) long-term channel evolution, and (iii) a recent
disturbance. First, we might not have measured widths at the same
feature as Zimmerman et al. (1967); however, given the small stream
size, measuring widths at different features would unlikely create
threefold and larger differences. Second, results may indicate a longterm
response to the historic land-use change accompanying European
settlement centuries ago. The sediment supplied by the uplands
of these drainages likely peaked during the large-scale land
clearing of colonial times and has likely declined continually since that
time. Throughout the watersheds of this region, streams may be widening
in response to an overall reduction in upland sediment supply and
may be slowly eroding legacy sediments previously trapped in floodplains,
channel margins, or within the channel (Walter and Merritts,
2008). Third, natural or anthropogenic events may have caused larger
Fig. 8. Cross sections of nonforested reaches NF1 (A) and NF2 (B) of W12.
453M. McBride et al. / Geomorphology 102 (2008) 445­459
bed widths overall (e.g., unusually large storm events, beaver dam
breaching, road construction, or other developments). Despite these
complicating factors, the effect of reforestation on channel width outweighs
all other background effects, as the change in channel width was
significantly greater for reforested reaches than the forested and
nonforested reaches. Although all study reaches may be on different
paths of adjustment to watershed, riparian, or climatic changes, channel
widening following riparian reforestation is considerable.
The frequently discussed driving mechanisms for channel widening
(i.e., either bank weakness from canopy shading of grassy vegetation
or scour around LWD) did not appear to be the primary drivers
for channel widening given our results; however, this study was not
intended to specifically test for these mechanisms. First, the absence
or presence of grassy bank vegetation did not appear to modify the
amount of widening. Reach R1 was nearly completely shaded with
mossy bank vegetation, while reaches R2, R3, and R4 had sufficient
canopy opening to allow for ample grassy vegetation on their banks,
yet all four reaches had widened to a similar degree (Figs. 4 and 11).
Second, the amount of LWD did not appear to affect the extent of
widening. Reforested reaches had widely ranging amounts of LWD
(Table 3), but bankfull width did not appear to correspond with that
parameter. Likewise, LWD counts and volumes were not significantly
different between reforested and forested reaches. Furthermore, we
did not observe greater streambank scouring adjacent to individual
Fig. 9. Cross sections of reforested reaches R1 (A), R2 (B), R3 (C), and R4 (D) of W12.
454 M. McBride et al. / Geomorphology 102 (2008) 445­459
Fig. 10. Cross sections of forested reaches F1 (A) of W12, and F2 (B) and F3 (C) of upper Pope Brook.
Fig. 11. Bankfull width hydraulic geometry for forested and nonforested reaches from
Davies-Colley (1997) and Hession et al. (2003) plotted with individual bankfull widths
of nine study reaches.
Fig. 12. Bankfull depth hydraulic geometry for combined forested and nonforested
reaches from Hession et al. (2003) data plotted with bankfull depths of nine study
reaches.
455M. McBride et al. / Geomorphology 102 (2008) 445­459
LWD pieces or debris dams. Bed widths measured near LWD or debris
dams were not significantly wider than bed widths measured
elsewhere in either forested or reforested reaches.
Differences found between the LWD characteristics of reforested
and forested reaches may reveal aspects of the forest recovery process.
In-stream LWD and woody debris on the floodplain were not significantly
more plentiful in forested reaches. Because these are small
streams where LWD is not in transport, the quantity of in-stream LWD
was expected to be a function of the age of the nearby forest. Studies
have found that older riparian forests contribute more LWD than
younger forests (Evans et al., 1993; Ralph et al., 1994; Bilby and Bisson,
1998) and that recovery of LWD levels following a disturbance may
take at least 40 to 60 years (Gregory et al., 1987). Although LWD was
abundant in reforested reaches, LWD was likely younger and more
recently recruited. In reforested reaches, LWD pieces were larger, had
collected in fewer debris dams, and exhibited fewer signs of decay. The
comparatively abundant amount of recently recruited LWD in reforested
reaches, in spite of the age of the forest, may be a result of
channel widening where young trees that grew along the edge of the
once narrow stream have fallen prematurely because of the incision
and widening (Hupp, 1999).
5.4. Conceptual model for channel widening
Based on our findings and previous work, we formulated a conceptual
model to explain channel change over time at Sleepers River
tributaries. Reforestation of riparian buffers appears to cause a multiphase
channel adjustment where the channel first incises and then
widens in response to changes in the local hydraulics and bankresisting
forces. Our conceptual model is similar to other models that
have been proposed for channel evolution following other disturbances
such as channelization and urbanization (Schumm et al., 1984;
Booth, 1990; Hupp and Simon, 1991), but our model does not explain
how either forested or nonforested reaches maintain a dynamic equilibrium
(e.g., Allmendinger et al., 2005). The Schumm et al. (1984)
model described a systematic evolution along the longitudinal profile
Fig.13. Sketch of conceptual model with original channel profile repeated throughout. Stage I -- equilibrium nonforested; stage II -- reforestation begins and channel incises; stage III --
incision slows, channel widens, forest matures; stage IV -- channel reaches forested equilibrium by either (a) aggradation or (b) creating an inset floodplain. Relative levels of in-stream
LWD, near-bank turbulence, and floodplain inundation are graphed adjacently.
456 M. McBride et al. / Geomorphology 102 (2008) 445­459
of incised channels within an impacted drainage. In a downstream
direction from an active nickpoint, reaches are first incised, then
overwidened, and finally the most distant reaches aggrade back to a
quasi-equilibrium state (Schumm et al., 1984). Although we recognize
that reach-scale changes in riparian vegetation may provoke channel
network-scale responses, our conceptual model describes at-a-station
morphologic change. A network-scale response was exemplified by
Clifton (1989) where a formerly grazed meadow reach of Wickiup
Creek aggraded by 1 m, creating a "bulge" in the valley and channel
bed profiles over a 50-year period following the exclusion of livestock.
Another possible network-scale response to riparian vegetation
change is the sediment supply regime. Although we recognize that
sediment supply plays an interactive role in the processes described
below, for lack of data and for simplicity we assume that the sediment
supply is essentially constant through time.
Our conceptual model can be described in four basic steps, recognizing
that this likely oversimplifies some of the complexities of the
process (Fig. 13). The first stage of the model (stage I; Fig. 13) represents
the nonforested condition as observed in reaches NF1 and
NF2. The channel size is relatively small, providing for frequent
floodplain inundation. LWD is largely absent within the channel and
on the floodplain. Given the absence of LWD and the flexibility of the
grassy vegetation, turbulence during overbank flood events is expected
to be relatively low. The transition process begins when forest
species colonize this nonforested floodplain and grow along the
streambanks (stage II; Fig. 13).
In the second stage, the small woody plants have established along
the stream's edge and on the floodplain. At this time, the trees are too
young to shade out grassy vegetation or to provide any LWD to the
stream; therefore, minimal change is expected in either the bank
resistance or the in-stream roughness. The key change at this stage is
the increased roughness on the floodplain that alters the local hydraulics
during overbank flows (McBride et al., 2007). The additional
roughness from the rigid woody stems reduces velocities on the
floodplain and creates a larger velocity differential between the
floodplain and in-stream flows. This velocity differential generates
turbulence at the interface between the floodplain and the channel.
McBride et al. (2007) found that turbulent kinetic energy was doubled
in a narrow zone along the bank's edge when rigid woody stems were
added to a physical model of a small nonforested stream. When
increased turbulence is coupled with high downstream velocities,
erosion is likely amplified (Thompson, 2004). In keeping with our
observations of reforested reaches, we surmise that the channel
responds to the increased turbulent energy by scouring at the base of
the banks. This scouring will promote degradation across the channel
bottom, especially if the grassy cohesive streambanks are more resistant
than the bed material. The amount of vertical and lateral
adjustment to the increased turbulence will be site specific and will
depend upon many factors such as bed substrate and armoring, bank
cohesion, rooting depth, and upstream sediment supply.
The third stage of the model represents the current conditions at
the reforested reaches (R1­R4). In stage III (Fig. 13), the channel is
deeper and wider than stage I or II. Assuming the hydrologic regime is
constant throughout the stages of the model, the floodplain inundation
frequency will have decreased as the channel has incised and
widened. Records of peak stages in reaches NF1 and R1 showed that a
greater proportion of the active channel was filled in reach NF1 than in
reach R1 during several different peak flow events (McBride, 2007).
The near-bank turbulence phenomenon is likely less active because of
the reduced floodplain inundation in this larger channel and because
of the increased roughness within the active channel from recently
recruited LWD. LWD has been recruited to the stream at this stage as
banks collapse and the forest vegetation ages. The forest vegetation
has likely matured at this stage to provide more shade to limit the
growth of grassy vegetation, potentially weakening streambanks, as
speculated in previous studies (Davies-Colley, 1997; Trimble, 1997).
Streambanks may be additionally weakened by the weight of young
trees along the edge of the stream channel (Thorne, 1990; Simon and
Collison, 2002).
In the final stage, the channel reaches a new "forested" equilibrium.
Reaches F1, F2, and F3 are likely still approaching this final
stage of the model. We suspect that the channel will return to a
normal depth either by aggrading back to the original bed level (stage
IVa; Fig. 13) or by creating a new lower floodplain surface (stage IVb;
Fig. 13), depending on the background discharge and sediment
regimes. If the sediment supply is great, we expect that LWD will
trap sediment and that the floodplain will be reactivated, at least
partially. Forested floodplains of small streams may be best described
as a complex patchwork of different elevations that have responded to
in-stream debris dams, which can locally increase the frequency and
extent of overbank flows (Jeffries et al., 2003). Ultimately, the new
"forested" channel reaches an equilibrium size such that the channel
has adequate capacity for the discharge and sediment regimes, given
the added roughness of both in-stream LWD and the forested
floodplain. Near-bank turbulence is greater than in stage I but lower
than in stages II or III because of the increased channel size and the
increased in-stream complexity. Compared to the original stage I
channel, the channel is wider with lower stream velocities (Sweeney
et al., 2004), higher in-stream roughness (Sweeney et al., 2004),
slower migration rates (Allmendinger et al., 2005), and greater benthic
habitat (Sweeney et al., 2004).
In summary, the conceptual model may provide guidance for
future research efforts. Ideally, a next step would be to monitor stream
reaches that have recently reforested or that have been planted as a
restoration effort over a long time span and document change over
time. A more plausible option might be to conduct a field investigation
with a broader geographic scope where stream reaches typical of the
stages described in the model could be monitored over a shorter time
span. Either of these options might shed light on whether our
conceptual model of incision-widening-recovery is widely applicable
or site specific.
6. Conclusions
Reforested reaches were shown to have widened by a comparison
of measurements collected in 1966 and in 2004­2005. Several
characteristics of the reforested reaches indicated that these reaches
were not in equilibrium and will likely continue to widen and adjust to
the change in riparian vegetation. Similar to many other studies of
small streams, we found that nonforested reaches were considerably
narrower than forested reaches. Our observations did not provide
strong evidence that previously hypothesized mechanisms were primarily
responsible for differences in channel width, namely that
widening is a result of either scour around LWD or bank weakness
from the suppression of grassy bank vegetation. Alternatively, we
present a conceptual model describing a process of incision, widening,
and recovery primarily instigated by a change in stream hydraulics.
Although we have documented channel widening associated with
reforestation, this study was not intended to investigate the effect of
riparian vegetation on bank stability. Additionally, our small sample
size and limited geographic extent restrict generalized conclusions;
however, we hope results from this study will provoke future
investigations into the timing of channel widening associated with
reforestation and the effects of widening on sediment delivery,
sediment transport, inundation frequency, and stream habitat. Our
results provide valuable information for stream restoration efforts
that involve the conversion of nonforested riparian vegetation to
forests. Replanting forested buffers on small streams may invoke
unanticipated channel widening with yet unknown consequences for
sediment delivery to downstream water bodies. With an awareness of
the channel widening response, perhaps stream restoration efforts
could be modified to accommodate or mitigate for possible undesirable
457M. McBride et al. / Geomorphology 102 (2008) 445­459
short-term outcomes. Undoubtedly, further investigations are needed to
determine the effects of riparian reforestation to sediment supply,
habitat complexity, and ecosystem structure and function.
Acknowledgements
Research supported in part by the National Science Foundation
Graduate Fellowship Program and a graduate research assistantship
from the Vermont Experimental Program to Stimulate Competitive
Research (VT EPSCoR, EPS0236976). We thank T. Owen, A. Pearce, J.
Clark, and T. Clason for their helpful field assistance. We thank J. Shanley
of the U.S. Geological Survey for his support of our research. We are
especially grateful to K. Dwyer, R. Boyle, and S. Boyle for allowing us
access to research sites on their property. This paper improved significantly
thanks to the insightful reviews of D. Thompson, P. Bierman,
three anonymous reviewers, and the editor.
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