WETLANDS, Vol. 29, No. 2, June 2009, pp. 451–464
’ 2009, The Society of Wetland Scientists
COMPLEX EFFECTS OF CHANNELIZATION AND LEVEE CONSTRUCTION ON
WESTERN TENNESSEE FLOODPLAIN FOREST FUNCTION
Scott B. Franklin1,2, John A. Kupfer2,3, S. Reza Pezeshki4, Randy Gentry5, and R. Daniel Smith6
1
School of Biological Sciences, University of Northern Colorado
Greeley, Colorado, USA 80639
E-mail: [email protected]
2
Edward J. Meeman Biological Field Station
Millington, Tennessee, USA 38053
3
Department of Geography & University of South Carolina
Columbia, South Carolina, USA 29208
4
Department of Biology, University of Memphis
Memphis, Tennessee, USA 38152
5
Southeastern Water Resources Institute, The University of Tennessee
Knoxville, Tennessee, USA 37996
6
US Army Corps of Engineers Waterways Experiment Station
Vicksburg, Mississippi, USA 39180
Abstract: Data on vegetation composition and structure, soil and leaf nutrient pools, soil redox
potential, and surface water hydrologic connectivity were collected from floodplains along six river
reaches in western Tennessee to examine the effects of channel modifications on associated riparian
systems. Comparisons among channelization treatments (non-channelized reaches, channelized and
leveed reaches, channelized but non-leveed reaches) and floodplain geomorphology (depression and
nondepressional sites) showed that hydrologic connectivity was affected by channelization treatments,
particularly leveeing. The disconnected floodplains were drier, maintained higher nutrient pools, and had
greater herbaceous biomass than floodplains still connected to channel hydrology. Runoff onto
floodplains from the agriculturally dominated landscape of channelized and leveed tributaries, and
flooding stress in the form of scour on floodplains along streams without levees may explain the observed
pattern. Channel and floodplain hydrologic processes were most strongly connected for unchannelized
streams. Unchannelized streams were varied in soil redox potential, water table, and nutrient pools.
Vegetation composition reflects both historical hydrologic regimes and disturbances, and thus complex
relationships to channel modifications. Results suggest both the subsidy (i.e., nutrient inputs) and the
stress of flood events have been altered by anthropogenic activities, but these alterations were greatest in
channelized systems compared to unchannelized systems.
Key Words: ecology, hydrology, nutrient pools, productivity, soil redox
INTRODUCTION
water flow and alter the flood regime. With greater
pressure on the utilization of floodplain systems,
there exists a global effort to understand floodplain
functions so land stewards may assess effects of
anthropogenic manipulations (Van Looy et al. 2003,
Oswalt and King 2005) and success of restoration
efforts (Palmer et al. 2005, Orr et al. 2007).
River channels in many areas of the world have
been straightened, deepened, widened, and leveed to
accelerate storm water drainage, decrease overbank
Floodplains have the ability to maintain or
improve water quality (e.g., Tabacchi et al. 2000),
but this function relies on the preservation of intact
riparian ecosystems that in turn limits the conversion of potentially productive floodplains to agriculture. In the southeastern U.S., changes in land
use and land cover in riparian zones have been made
possible by the implementation of stream management practices designed to expedite or regulate
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WETLANDS, Volume 29, No. 2, 2009
flood events, lower water tables of bottomland
forests, and protect agricultural land from flooding
(Brookes 1989). Channelization alters stream power
and sediment transport capacity of a river, which in
turn triggers changes in erosion and deposition that
result in morphological adjustments of the river
channel (Landwehr and Rhoads 2003). Channelization may thus alter hydrologic regimes and fluvial
processes in complex manners that are manifested at
a range of spatial scales and are related not only to
local conditions but also to landscape-level patterns
and processes (e.g., modifications to upstream river
systems).
Levee construction is another ubiquitous alteration, often accompanying channelization in a
further effort to constrain floodwaters within the
active channel. Levees are barriers to flood pulses
(Junk et al. 1989, Bayley 1995), eliminating the
connection between channels and floodplains important for fish spawning (Bayley 1995), macroinvertebrate dispersal (Jenkins and Boulton 2003),
nutrient cycling (Valett et al. 2005), and other
landscape-level functional processes (Ward et al.
2002). Alteration of flood regimes has repercussions
on floodplain ecosystem structure as well, albeit not
well understood, simply due to the decreased
heterogeneity of disturbance and environmental
conditions throughout the floodplain leading to
lower overall diversity (Simon and Hupp 1992,
Sparks 1995, Shankman 1996).
Effects of channel modifications are likely complicated by a range of confounding factors, including microtopography (e.g., depressions vs. nondepressions), differences in geomorphic responses of
the streams (e.g., incision vs. aggradation), nonequilibrium conditions and responses of the floodplain
ecosystem (e.g., the ‘‘storage effect’’: Chesson 1990),
and the extent and effectiveness of the channel
modifications themselves (which may change with
time). Such inherent variability among modified
streams has hampered generalizations of their
modification effects (Steiger et al. 2005).
The overall objective of this study was to clarify
the effects of channelization and levee construction
on floodplain forest composition, diversity, structure, productivity, surface hydrologic connectivity,
and leaf and soil nutrient pools. Hydrologically, we
expected that channelization could result in either: 1)
lowered water table levels, as water is moved more
quickly out of channelized reaches, or 2) higher
water table levels, in areas where upstream channelization is exporting water at a faster rate than can be
accommodated due to nearby impoundments such
as valley plugs, beaver dams, or other channel
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452
Figure 1. Location of river floodplain study areas and
selected USGS gage stations in western Tennessee, USA.
Unchannelized reference reaches: Hatchie and Upper
Wolf Rivers; Channelized stream reaches: Stokes Creek
and Obion Rivers; Channelized and leveed reaches: Lower
Wolf and North Fork-Forked Deer Rivers. Channelized
reaches are shown in grey; unchannelized reaches are
shown in black.
blockages. In leveed systems, we anticipated that
levees had cut off river inputs and converted these
sites to rain-fed systems (although sub-surface
hydrology may still be connected), making leveed
floodplains drier than non-leveed systems still
connected to river hydrologic processes. This
conversion could also lead to lower soil nutrient
levels because levees would remove inputs associated
with periodic flood events, or increased nutrients
deposited by sheet flow from neighboring agricultural fields. Finally, we expected that changes in
floodplain hydrologic connectivity and nutrient
availability would result in structural and compositional differences of the floodplain forest communities, but that such differences would not be
consistent among rivers with comparable modifications due to stream-to-stream variability.
METHODS
Study Areas
We selected six river sections ranging in length
from 3–8 km for this study, two from unchannelized
reaches (the upper Wolf River and the Hatchie River
in the Hatchie National Wildlife Refuge), two from
Cust # 08-59
Franklin et al., EFFECTS OF CHANNELIZATION AND LEVEE CONSTRUCTION
channelized reaches (Stokes Creek, a tributary to the
North Fork of the Forked Deer River, and the
Rutherford Fork of the Obion River in the Milan
Arsenal), and two from channelized and leveed
reaches (the North Fork of the Forked Deer River
in Tigrett Wildlife Management Area and the lower
Wolf River in the Lucius Burch Natural Area)
(Figure 1). Although the main channels of the
unchannelized stream reaches have not been altered,
many of their tributaries have been channelized,
affecting their hydrologic processes and geomorphology. Our reference rivers are thus not truly
natural, but they represent the best remaining
examples of large, free flowing unchannelized rivers
in the region and should be classified as the least
disturbed condition based on Stoddard et al. (2006).
Further, they have been used to develop reference
standards for the hydrogeomorphic classification
system for low gradient riverine systems in western
Tennessee (Franklin et al. 2009).
Vegetation in the reference systems is typical of
the northern portion of the southern floodplain
forest (Sharitz and Mitsch 1993, Hodges 1998), with
prominent species including Taxodium distichum (L.)
Rich., Salix nigra Marsh., and Quercus nigra L. in
depressions and Liquidambar styraciflua L., Platanus
occidentalis L., and Acer saccharinum L. on floodplain nondepressions (Franklin et al. 2001a,b).
However, timber cutting has been pervasive
throughout West Tennessee and has likely affected
forest structure and composition in all of the reaches
to some degree. Regionally, clearing for agriculture
began in the early 1800s as cotton farming
developed. Additional waves of forest loss and
conversion coincided with railroad expansion during
the turn of the 20th century and agricultural
expansion in the first third of the century, so that
all areas of this study were likely logged at least once
(Table 1). More recently, bottomland hardwood
area in the southeastern United States, and western
Tennessee specifically, decreased again by 25%
between 1950 and 1971 (Steed et al. 2002).
Concurrent with increases in agriculture, modifications to many rivers in western Tennessee,
including snag removal, dredging, widening,
straightening, and leveeing, began in the early
1900’s, and most major stream channels in western
Tennessee had been dredged and straightened by
1926 (Simon and Hupp 1992). Subsequent periods
of debris accumulation and channel filling from
sediment deposition necessitated further stream
clearing, channel snagging, and occasional widening
and straightening of stream reaches (Robbins and
Simon 1983, Johnson 2007). Even though these early
attempts at channelization failed, a re–invigorated
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453
effort was begun in 1961 and led by the Corps of
Engineers as part of the West Tennessee Tributaries
Project. These additional efforts also failed and were
stopped by landowners who petitioned not to have
their land channelized (Johnson 2007). Of the
reaches examined in this study, none have experienced channel modifications (excluding clearing and
snagging) since the mid-1970’s. ‘‘Natural’’ channel
widening in the modified areas, an indicator of
channel–bed degradation and ongoing stream adjustment, was occurring at rates in the late-1980’s
that were comparable to those due to widening from
bank caving on the unchannelized reaches (Simon
and Hupp 1992).
As might be expected, the channelized systems
examined in this study had less forest cover in the
surrounding watersheds (Table 1) and higher sediment loads. For example, shortly after the most
recent channel adjustments, sediment output in the
Forked Deer River averaged 9178 kg ha21 versus
only 2608 kg ha21 in the Hatchie (Johnson 2007).
Beyond the effects of overbank flow events,
bottomland forests can be affected by burial from
sediments eroding from uplands and tributary
channels and increased inundation due to levees
that impeded drainage of floodplains and valley
plugs.
Climatically, the annual average temperature in
the region is 16uC, with mean January and July
temperatures of 4uC and 26.6uC, respectively.
Average growing season is 230 days, and average
annual precipitation is 132 cm with a majority
occurring during the winter and spring months
(USDA 1978). Soils along all six reaches are of the
Falaya-Waverly-Swamp associations (coarse–silty,
mixed, active, acid, thermic Fluvaquentic Endoaquepts and thermic Aeric Fluvaquents), which
consist of level, poorly drained silty soils on low,
broad first bottoms (USDA 1965, 1978). In the
absence of channel modifications, these areas would
have been flooded in most years during the winter
and spring and occasionally during the summer,
with periods of inundation ranging from weeks to
months. These soils formed in acidic loess washed
from uplands and are thus highly susceptible to
erosion.
Data Collection
We established four to nine transects in each of
the study reaches, with river study sections ranging
from 3 to 8 km. Transects were aligned perpendicular to the stream channel, positioned at least 100 m
apart on forested first bottoms, and spatially
stratified along the entire river reach. To avoid
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Cust # 08-59
Unchannelized
Unchannelized
Upper Wolf
River
Hatchie
River
Channelized
Stokes
Creek
Channelized
& Leveed
Lucius
Burch NA
Channelized
Channelized
& Leveed
Tigrett
WMA
Milan
Arsenal
Treatment
River
Loess, and
Claiborne &
Wilcox
Formations
Loess, and
Claiborne &
Wilcox
Formations
Loess and
Holocene
Alluvium
Loess, and
Claiborne &
Wilcox
Formations
Loess and
Holocene
Alluvium
Loess and
Holocene
Alluvium
Geology
3,112
79,303
259,204
12.9
50.1
60.2
31,384
192,257
86.6
36.8
120,525
64.7
Length Drainage
(km) Area (ha)
42
50
2
29
25
12
% Basin
Wetlands
32
49
98
68
30
87
% Basin
Ag
5
8
4
5
3
5
Study
Section
Length (km)
Logged in
early
1900s,
ponding
death
None in
recent
history
Logging
History
Logged in
early
1900s,
ponded
death,
planted
cypress
Selectively
Middle of
thinned
study area
1947–
in main
1948,
channel
ponded
death
Privately
Above and
owned
below study
patchy
area in main
cutting,
channel
ponded
death
Above study 1960s, but
not on
area in
study sites
tributaries
Tributaries
Not seen,
but
floodplain
leveed for
hunting
Not seen
Channel
Sediment
Plugs
1964
Enlarging
and
Straightening
none
none
None
1970s
None
Clearing
and snagging
1976–1978
Clearing
and
Snagging,
below study
area
1977
Enlarging
and
Straightening
Recent
Date
Modifications Completed
Table 1. Modification history of the six West Tennessee river reaches used to examine the effects of channel alteration on floodplain forests. Nearly all original
channelization projects were begun in the early 1900s. Geology was from Simon and Hupp (1992). Channel sediment plugs were from personal observation and
Diehl (2000). Dates for recent modifications are from Hupp (1992) and Johnson (2007). Recent modifications on the Upper Wolf and Hatchie Rivers are only on
tributaries and not given. Logging history was developed from Johnson 2007 and interviews with local residents.
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Franklin et al., EFFECTS OF CHANNELIZATION AND LEVEE CONSTRUCTION
autocorrelation of samples, sample sites (1 to 4) were
located at least 50 m apart along each transect but
within 1 km of the stream channel and stratified
evenly according to their locations in depressions
(areas of concave surface microtopography) or
nondepressions (areas of straight or convex microtopography). Thus, half of the sites were in
depressions for each sample area, and depressions
and nondepressions alternated along transects.
Data on water table level and variability were
collected from January 1999–June 2000 using
surface wells installed at each sampling site in
1998. A total of 588 measurements were recorded,
ranging from 29 to 146 samples per site, spaced over
three to eight time periods. While such limited data
collection does not permit a refined analysis of
floodplain hydrologic processes, it does allow for a
limited comparison of mean soil water table levels
and variability among research areas. In the summer
of 1999, we installed redox potential electrodes at 24
sites, with two each randomly selected from
depressions and nondepressions at each study reach.
Redox potential measurements at three depths (15,
30, 60 cm) were taken every two weeks from June 1
through October 30, 1999. Historical data from
USGS river gages were collected and compared to
floodplain soil water levels to examine the strength
of connection between channel and floodplain
hydrologic processes.
Data on forest structure and composition were
recorded in summer 1998 using 20 3 10 m quadrats
established at 89 sites, with 36 in unchannelized
reaches, 28 in channelized reaches, and 25 in
channelized and leveed reaches. Diameter and
species of each live tree $ 10 cm in diameter at
breast height (dbh; 1.37 m) were recorded to provide
information on overstory trees while individuals 0.1–
10 cm dbh were identified by species and counted to
determine density in the midcanopy (3.0–
9.9 cm dbh) and sapling (0.1–2.9 cm dbh) strata.
At the 24 sites where redox potential was
measured, we established nine 1 m2 herbaceous
biomass subplots in an area of uniform herbaceous
flora composition. In June 1999, we removed and
separately bagged all above ground herbaceous
biomass and dead litter biomass from three randomly selected plots. All bagged material was dried
at 60uC for 48 hours and weighed to the nearest
0.01 g, and averages of the three plots were
determined. We also placed 1 m2 litter fall traps to
collect leaf fall on these same 24 sites from October
1–December 31 in 1998 and 1999. Nets were emptied
every other week, and the downed material was
oven–dried and weighed. Ten surface soil samples
were collected from the same 24 sites in spring 1999
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and 2000 using a 7.62 cm diameter soil corer to a
depth of 20 cm. Soil samples from each site were
mixed thoroughly, dried, and sifted through a 2 3
2 mm sieve. Both leaf litter tissue and soil samples
were sent to A & L Agricultural Laboratories
(Memphis, TN) for nutrient analyses.
Data Analysis
We examined the effects of channelization and
leveeing on a wide range of biotic and abiotic
characteristics, including:
Site hydrologic processes: mean water table depth
(cm) below ground surface, water table variability
(reach: among plots on a given date; local: within
surface well during year 2000), and soil redox
potential.
Forest structure: overstory basal area (m2 ha21),
overstory density (# ha21), mid–canopy density
(# ha21), and sapling density (# ha21);
Forest composition and diversity: species composition, species richness (# of species per plot), and
species evenness (based on equitability of Shannon-Wiener Index values; McCune and Mefford
1997), in both the overstory and mid–canopy;
Nutrient pools: leaf litter nutrient pools: Ca (ppm),
K (ppm), N (ppm), P (ppm), and C/N (carbon/
nitrate–N ratio); soil nutrients and characteristics:
N (ppm), P (ppm), organic matter (%), cation
exchange capacity (meq/100 ml), and C/N (carbon/nitrate–N ratio);
In most cases, a nested ANOVA (reaches were
nested in TRT, SAS PROC: GLM) was used to
examine the effect of channelization treatments
(TRT; unchannelized vs. channelized vs. channelized
and leveed), topography (DND; depressions vs.
nondepressions), and their interaction. Treatment
was also included as a subplot of reach to test for
differences among stream sections. Upon a significant REACH effect, factors were subsequently
blocked by reach to test for significant TRT,
DND, and interaction effects. Significant treatment
effects were examined with a Tukey post hoc
analysis. All samples were considered replicates
because of their spacing and alternation of depression and nondepression sites, decreasing autocorrelation.
Floristic differences among the sites were investigated using multi–response permutation procedures
(MRPP; McCune and Mefford 1997), a technique
that tests for differences among groups defined a
priori based on the ratio of dissimilarity among
groups compared to within groups in Euclidean
space. Hydrologic relationships were explored with
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WETLANDS, Volume 29, No. 2, 2009
correlation analyses of channel discharge and
floodplain water table readings from a particular
day. To examine the concept that water table
fluctuations yield greater productivity (Robertson
et al. 2001), leaf and soil nutrients were correlated
with mean water table and water table variability
(i.e., standard deviation). All inferential analyses
were performed in SAS (2003).
RESULTS
Floodplain Hydrologic Processes
Daily water table readings were correlated with
stream discharge for both unchannelized reaches
and one channelized reach. The strongest correlation
was for the Hatchie River (r 5 0.48), while the
Upper Wolf and Rutherford Fork of the Obion
could only be modeled about half as well (r 5 0.22
and 0.20, respectively). Both the Upper Wolf and
Hatchie scattergrams had a consistent relationship
between floodplain water–table level and stream
gage discharge, while channelized and channelized
and leveed stream reaches show the entire range of
floodplain water–table levels under low flow conditions, suggesting a disconnect between channel and
floodplain hydrologic processes at low flows.
Mean depth to water table differed as a function
of topographic setting (F 5 7.3; p.F 5 0.02) and
reach (F 5 20.6; p.F 5 0.0001). Sites along stream
reaches without levees had much shallower water
tables that differed substantially between depressions and nondepressions (Figure 2a). Those along
leveed reaches, on the other hand, had a greater
depth to water table and little difference between
topographic settings. The floodplain hydrologic
regime also differed among reaches (Figure 2b),
with sites at Lucius Burch (channelized and leveed)
being the driest and those along the Upper Wolf
(unchannelized) the wettest. Water table variability
(standard deviation of individual surface wells) had
a treatment by topography interaction (F 5 4.9;
p.F 5 0.01), with differences between depressions
and nondepressions in channelized and unchannelized reaches but not in channelized and leveed
reaches. Variability was higher in depressions for
unchannelized reaches and higher in nondepressions
for channelized reaches (Figure 3a). Water table
variability also differed among reaches (F 5 7.8;
p.F 5 0.0002), with less variability in channelized
and leveed reaches than channelized or unchannelized
reaches (Figure 3b). River water table reach variability (variance in water tables across wells for a day in
March) was extremely low at the Lucius Burch reach,
and thus differed from all other reaches (Figure 3c).
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456
Figure 2. Mean depth to water table and soil redox
potential in floodplain sites along six river reaches in
western Tennessee: (a) and (c) treatments include topography, depression versus nondepression sites, and river
alterations, including channelized (C), channelized &
leveed (C&L), and unchannelized (U); (b) and (d) rivers
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Franklin et al., EFFECTS OF CHANNELIZATION AND LEVEE CONSTRUCTION
457
Patterns of soil redox potential mirrored but were
even stronger than those for water table depth; a
significant treatment by topography interaction (F
5 8.8; p.F 5 0.001) and reach effect (F 5 24.8;
p.F 5 0.0001; Figure 2c). Redox potential was
greatest in channelized and leveed reaches (irrespective of topographic position), with soils remaining in
the oxidation zone (Eh . 350 mV) throughout the
sampling season. Sites along both channelized and
unchannelized reaches, where the river and floodplain hydrologic processes were still connected, had
lower redox potentials in depressions compared to
non–depressions (Figure 2c), and the channelized
reaches and one unchannelized reach (Upper Wolf
River) began the growing season with reducing soils
(Figure 2d). The reach effect occurred because while
soil redox potential was similar between channelized
reaches and between channelized and leveed reaches,
there was a greater disparity between the two
unchannelized locations.
Forest Structure and Composition
Overstory basal area did not differ among
treatments. There was a reach effect on mid–canopy
density (F 5 4.8; p.F 5 0.04), indicating variability
among river systems unrelated to their treatment.
Otherwise, forest structure showed little response to
treatment, topography or location.
Overstory and mid–canopy composition, however, both showed an interaction between treatment
and topography (overstory: MRPP delta 5 0.86, p
, 0.0001; mid–canopy: MRPP delta 5 0.82; p ,
0.00001). Of particular note was the absence or
reduced prominence of characteristic depressional
species Taxodium and Nyssa in the modified stream
reaches, the variability of specific early successional
species among samples (e.g., Salix nigra, Betula
nigra, Quercus michauxii, Liquidambar styraciflua),
and the prominence of Acer rubrum in the modified
stream system overstories and in nearly all mid–
canopies (Table 2). Also notable is the strong
r
include two channelized and leveed stream sections, the
North Fork of the Forked Deer Tigrett Wildlife
Management Area (NFFD) and Lucius Burch (LB, lower
Wolf River), two channelized stream sections, the
Rutherford Fork of the Obion River in the Milan Arsenal
(RF Obion) and Stokes Creek (Stokes), and two
unchannelized stream sections, the Hatchie River Wildlife
Refuge (Hatchie) and the Upper Wolf River near Moscow
(Up Wolf). Bars are standard error. Different letters
represent significant differences among treatments.
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457
Cust # 08-59
Figure 3. Mean of standard deviations for individual
surface wells from six river reaches in western Tennessee:
a) Treatments include topography, depressions versus
nondepressions, and river alterations, including channelized, channelized & leveed, and unchannelized; b) rivers
include two channelized and leveed stream reaches, the
North Fork of the Forked Deer in Tigrett Wildlife
Management Area (NFFD) and Lucius Burch (LB, lower
Wolf River), two channelized stream reaches, the Rutherford Fork of the Obion River in the Milan Arsenal (RF
Obion) and Stokes Creek (Stokes), and two unchannelized
stream reaches, the Hatchie River Wildlife Refuge
(Hatchie) and the Upper Wolf River (Up Wolf); c)
standard deviation of surface wells for one day in March
2000 for each stream section. Bars for a and b are
standard error. Different letters represent significant
differences among treatments.
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WETLANDS, Volume 29, No. 2, 2009
Table 2. Mean density (stems ha21) for midcanopy (3–10 cm dbh) from floodplain sites along six West Tennessee river
reaches. Data are presented by treatment (channelized, C; channelized & leveed, CL; unchannelized, U), and topography
(depressions, D; nondepressions, N).
Species
n5
CL, N
CL, D
15
Taxodium distichum
0.0
Nyssa aquatica
0.0
Planera aquatica
0.0
Salix nigra
0.0
Platanus occidentalis
3 6 13
Carya spp.1
20 6 75
Quercus spp.2
7 6 18
Fraxinus pennsylvanica
123 6 230
Ilex decidua var. decidua
3 6 13
Walt.
Acer rubrum
253 6 437
Carpinus caroliniana Walt.
60 6 112
Celtis laevigata Willd.
17 6 36
Acer saccharinum
3 6 13
Acer negundo
57 6 111
Betula nigra
0.0
Liquidambar styraciflua
0.0
Ulmus americana
30 6 72
Other species
57 6 123
Total Density
683 6 410
Richness
2.5 6 1.8
Evenness
0.49 6 0.4
C, N
C, D
13
14
19
16
3 6 13
27 6 107
23 6 70
0.0
20 6 47
53 6 173
67 6 267
7 6 27
29 6 81
0.0
0.0
79 6 216
5 6 23
29 6 96
5 6 17
11 6 37
61 6 147
0.0
7 6 17
0.0
71 6 161
7 6 17
18 6 75
0.0
10
20 6 53
0.0
10 6 32
40 6 68
30 6 63
0.0
15 6 41
90 6 211
90
265
60
30
40
15
5
85
75
970
2.4
0.66
6 198
6 610
6 87
6 79
0.0
6 67
6 48
6 16
6 171
6 180
6 853
6 1.3
6 0.5
8 6 36
0.0
238 6 400
8 6 40
8 6 32
0.0
8 6 33
92 6 157
88
588
19
12
12
77
27
15
50
1400
3.9
0.65
6 121
6 1335
6 38
6 42
6 42
6 177
6 48
0.0
6 42
6 100
6 813
6 2.4
6 0.3
73
120
17
17
7
10
90
37
770
2.9
0.70
0.0
6 280
6 211
6 53
6 53
6 26
0.0
6 27
6 281
6 120
6 703
6 1.2
6 0.3
U, N
0.0
529 6 1141
50 6 111
3 6 12
0.0
8 6 26
58 6 151
8 6 34
42 6 98
24 6 77
1129 6 1430
4.7 6 1.8
0.84 6 0.1
U, D
14
325
68
21
7
43
7
96
57
68
1171
2.9
0.52
6
6
6
6
6
6
6
6
6
6
6
6
6
39
770
151
75
17
126
17
341
141
153
1910
2.1
0.4
1
Carya glabra (Mill.) Sweet, C. illinoensis (Wangenh.) K. Koch, C. laciniosa (Michx.) Loudon, C. ovata var. ovata (Mill.) K. Koch, C.
tomentosa (Poir.) Nutt.
Quercus lyrata, Q. michauxii, Q. nigra, Q. texana Buckl., Q. phellos L., Q. pagoda Raf.
2
difference in mid-canopy density of A. rubrum and F.
pennsylvanica between nondepressions and depressions on channelized and unchannelized sites, but not
on channelized and leveed sites (Table 2). Despite the
mid-canopy differences, sapling composition was not
different based on either treatment or topography.
Overstory richness and evenness were comparable
among treatments and between topographic settings, but an interaction effect for richness (F 5 6.2;
p.F 5 0.003) suggested the importance of stream
modifications on floodplain forest diversity. While
nondepressions in the unchannelized reaches had the
highest richness values (R 5 4.9), depressions in the
same reaches had the lowest values (R 5 3.1). In
contrast, richness values were intermediate in the
modified stream reaches, and depressions were
either comparable to or richer in species than
nondepressions, suggesting a treatment effect but
one that is altered by topographic setting. These
patterns were reinforced by comparisons of mid–
canopy diversity, with treatment–level differences
again obscured or altered by topographic setting
(nondepression sites had higher richness; F 5 7.0;
p.F 5 0.01) or a treatment by topography interaction (same evenness relationship as overstory; F 5
Wetlands wetl-29-02-04.3d 7/4/09 11:14:15
458
4.9; p.F 5 0.01). Unchannelized reaches had mid–
canopy diversity values similar to those for the
overstory, but richness was distinctly lower (and
relatively equal between depressions and nondepressions) in the channelized and leveed reaches (Table 2).
Maximum litter biomass was different based on
treatment (F 5 92.34; p.F 5 0.0004), with sites
along channelized and leveed reaches having higher
biomass than those along channelized or unchannelized reaches. No treatment effects were found for
live shoot or root biomass.
Soil Characteristics and Leaf Nutrients
Analyses of soil characteristics showed almost no
direct treatment effects but were complicated by
interaction effects, year-to-year variation, and reachto-reach variability (Figure 4). A treatment by
topography interaction was found for all measures
in at least one year, indicating that channel
modifications may influence floodplain nutrient
dynamics but that any changes are likely dependent
on the topographic setting (Table 3). For example,
C:N ratios were greater in depressions compared to
nondepressions in channelized and unchannelized
Cust # 08-59
Franklin et al., EFFECTS OF CHANNELIZATION AND LEVEE CONSTRUCTION
459
Figure 4. Percent organic matter, cation exchange capacity, and carbon/nitrate-nitrogen ratios from floodplain soils
along six western Tennessee river reaches during two spring sampling periods (1998 and 1999). Treatments include
topography, depression versus nondepression sites, and river alterations, including channelized (C), channelized & leveed
(C&L), and unchannelized (U). Bars are standard error. Different letters represent significant differences
among treatments.
reaches in 2000 (Figure 4f). On the other hand, in
1999, C:N was higher only in nondepression sites of
channelized and leveed reaches. In 1999, organic
matter was greatest in depressions (Figure 4a),
resulting in lower cation exchange capacity as well
(Figure 4c), but only for sites in channelized reaches.
Further, a reach effect was noted for all measures
except those involving nitrogen (which was most
strongly affected by topographic setting), meaning
that the different stream reaches had different soil
nutrient pools and structure.
A topographic effect (F 5 7.2; p.F 5 0.01) for
leaf calcium in 1998 was the only significant
treatment effect; depression sites had lower leaf
calcium pools than nondepression sites. While
relationships between leaf and soil nutrients might
be expected, soil and leaf nutrient data were
Wetlands wetl-29-02-04.3d 7/4/09 11:14:16
459
Cust # 08-59
uncorrelated (p . 0.10) for all treatments, and there
were no relationships between mean water table
level and either soil or leaf nitrogen and phosphorus
(largest r 5 0.27; lowest p 5 0.26). Negative
correlations were found, however, between the
variability of the water table at a particular well
(i.e., standard deviation) and leaf nitrogen (r 5
20.58; p 5 0.01) and soil phosphorus (r 5 20.44; p
5 0.06).
DISCUSSION
We hypothesized that channelization and leveeing
of western Tennessee rivers would directly affect
floodplain hydrologic processes and thereby alter
floodplain forest structure, composition and function. In particular, we thought that levees should
460
WETLANDS, Volume 29, No. 2, 2009
Table 3. Nested ANOVA results for comparisons of soil characteristics from floodplains along six westernTennessee
river reaches. TRT 5 treatment (channelized, channelized & leveed, and unchannelized); DND 5 topography (depression
or nondepression); RIVER 5 the individual stream. Bold indicates significance ( 5 0.05< 0.2).
Variable
dferror
Soil Organic Matter
1999
2000
15
Cation Exchange Capacity
1999
2000
15
Carbon/Nitrate–N ratio
1999
2000
15
Nitrogen
1999
2000
15
Phosphorus
1999
2000
15
Potassium
1999
2000
15
Calcium
1999
2000
15
TRT
DND
TRT*DND
River (TRT)
df 5 2
df 5 1
df 5 2
df 5 3
F
p.F
F
p.F
F
p.F
F
1.85
0.35
0.30
0.71
0.93
0.03
0.34
0.88
3.00
3.78
0.05
0.05
7.83
4.2
0.0003
0.03
0.66
0.16
0.58
0.86
0.12
0.35
0.73
0.57
5.57
0.00
0.007
0.99
8.69
7.37
0.0001
0.004
1.38
1.82
0.38
0.30
3.80
1.32
0.06
0.27
3.14
5.47
0.05
0.02
1.79
1.05
0.17
0.41
3.71
1.48
0.16
0.35
14.10
3.13
0.001
0.10
6.30
3.12
0.004
0.08
1.36
0.78
0.27
0.53
0.99
7.29
0.47
0.07
5.87
0.39
0.02
0.55
3.77
2.17
0.03
0.16
2.50
0.85
0.07
0.49
0.82
0.25
0.52
0.79
1.68
0.12
0.20
0.73
7.10
0.47
0.002
0.63
1.16
0.54
0.42
0.63
0.35
1.38
0.56
0.26
6.35
0.49
0.004
0.62
have the greatest impact, and indeed, the clearest
distinction between altered and unaltered channels
occurred when levees were constructed. Levees
detached the floodplain from the channel, decreased
overbank flood events and rendered lower water
tables and higher soil redox potential compared to
non–leveed stream systems (Gergel et al. 2002, Giller
2005, Kang and Stanley 2005).
While treatment effects were evident for some
aspects of floodplain form and function, the effects
of stream channel modifications were often manifested through interactions of river modifications
with other factors influencing site hydrologic processes, including topographic setting and reach–
specific conditions. Direct treatment effects were
thus absent from many comparisons of forest
structure, composition, and soil nutrient pools, even
though stream modifications (as altered by interaction effects) were clearly crucial. The interactions
underscore the complexity of floodplain ecosystems
and the importance of topographic heterogeneity
(Richter and Richter 2000, Ward et al. 2002, Oswalt
and King 2005, Steiger et al. 2005).
Wetlands wetl-29-02-04.3d 7/4/09 11:14:21
460
18.0
27.4
4.95
5.45
p.F
,0.001
,0.001
0.005
0.01
If a stream is channelized and leveed and the
levees are not breeched, then the floodplain is
completely cut off from the channel with no surficial
exchanges of nutrients, moisture, or organic matter,
except potentially during extreme flood events. An
incising (i.e., degrading) channel, caused by an
increased gradient and stream power, will result in
a drier floodplain with greater soil aeration (Hupp
1992). The lower Wolf River reach (Lucius Burch) is
the best example from this study. If the levees are
breeched or no levees were created, then flooding
remains a potentially important function of channelized reaches, albeit to a varying degree. A
primary goal of channelization is to export water
from the modified area more quickly and thereby
prevent flooding and lower the water table. High
water tables have nonetheless been shown to occur
in channelized stream reaches as a result of upstream
channelization, a flashier basin hydrology (with land
use conversion to agriculture), and the development
of stream blockages (e.g., Shankman and Pugh
1992, Diehl 2000) that locally maintain higher water
tables during low flow periods (Piégay 1997, Oswalt
Cust # 08-59
Franklin et al., EFFECTS OF CHANNELIZATION AND LEVEE CONSTRUCTION
and King 2005). Such blockages occurred on both
unchannelized and channelized reaches in our study
area and contributed to the high water table levels
observed in the latter. We found only one significant
correlation between channel stage and floodplain
water table of the four channelized stream reaches,
while both unchannelized reaches had a significant
correlation. This suggests that while channelized
streams may still exhibit hydrological variability,
floodplain water table was not as strongly connected
to channel flow as unchannelized streams, especially
during low flow periods (Valett et al. 2005).
Aggradation of river channels may be due to
several factors, including bank collapse, headcutting, and sediment inputs from surrounding agricultural land (Brown 1988, Shankman and Samson
1991, Nakamura et al. 1997), so morphological
stream adjustments and the amount of agriculture in
the watershed are directly related to stream sediment
input (Coleman and Kupfer 1996, Craft and Casey
2000). Uplands in western Tennessee are capped
with highly erodible loess soils, with annual erosion
estimates averaging 15 MT ha21 (Troy Taylor,
NRCS, pers. comm., September 2001). Deposition
in litter bags from a decomposition study at our sites
(Molavi unpubl. thesis) showed high values for both
the channelized (688–748 g m22 yr21) and unchannelized (757–1628 g m22 yr21) sites, while channelized and leveed streams averaged an annual
deposition rate of only 22.7 g m22 yr21. Hupp and
Bazemore (1993) also suggested unchannelized
streams would have high sedimentation rates.
The aggradation or scouring of sediment on
floodplains has repercussions on other functions.
The nutrient subsidy (Odum 1979) from flood events
in channelized systems is likely greater than for
unchannelized systems due to nonpoint source
fertilizer movement from the surrounding watershed. Channelized reaches averaged 78% agricultural land in their watersheds while unchannelized
reaches averaged only 40% agricultural land. Thus,
nitrogen, phosphorus, and potassium load were
expected to be higher in channelized reaches than
in unchannelized reaches, as was evidenced with soil
nutrient pools (this study, Franklin et al. 2001b).
Grubaugh and Maier (unpublished data) found
higher water nutrient concentrations in Stokes
Creek, surrounded by agriculture, compared to the
Upper Wolf River surrounded by mainly forest,
corroborating our results. Obviously, this is an
additional subsidy, but these streams are also
receiving an additional stress due to increased
sediment loads. We found negative relationships
between water table variability, and leaf nitrogen
and soil phosphorus. While periodic flooding
Wetlands wetl-29-02-04.3d 7/4/09 11:14:22
461
Cust # 08-59
461
generally results in higher nutrient availability
(Mitsch et al. 1979, Baldwin and Mitchell 2000,
Brunet and Astin 2000), higher productivity (Robertson et al. 2001), and increased decomposition
(Brinson 1981, Lockaby et al. 1996), it is not the case
when flows carry heavy sediment loads (Brookes
1986); sedimentation stress effects outweigh subsidy
effects.
The importance of floodplain microtopography in
shaping patterns of hydrologic connectivity, species
composition, and some soil nutrients was not
surprising. Previous studies suggest that continuous
flooding of depressions, resulting in anoxic soil
conditions, increases the mobilization of some
minerals for plant uptake (e.g., phosphorus) while
decreasing nutrient uptake capacity of roots (Pezeshki 1994) and decomposition rates (Mitsch and
Gosselink 1993, Brunet and Astin 2000). As
expected for sites along unchannelized and channelized reaches, mean depth to water table was less in
depressions than nondepressions, and soil redox
potential was lower, representing moderately reducing soil conditions. Contrarily, floodplain water
table and soil redox potential of sites along
channelized and leveed sites did not differ between
depressions and nondepressions. Thus, while we
hypothesized that depressions would have lower soil
and leaf nutrient pools because more of the nutrients
would be tied up in slowly decaying biomass,
microtopography and nutrient levels differed only
when considered within the context of channelization effects. By decoupling the floodplain and
stream systems, levees minimize the differences
between depressions and nondepressions and potentially reduce the amount of floodplain environmental heterogeneity (Hughes and Cass 1997, Ward et
al. 2002).
The two key factors determining the responses of
the floodplain ecosystems to stream modifications
are thus the effects of the modifications on site
hydrologic connectivity itself and the manner in
which the energy regime and sediment transport
capacity of rivers is altered, particularly as it affects
the movement and accumulation of sediment, the
same two factors suggested by Steiger et al. (2005).
Consequently, flood regimes and site hydrologic
processes are considered the key factors constraining
many aspects of floodplain forest structure, composition, and function due to the varying tolerances of
species to water table dynamics (Hosner 1960, Hook
and Brown 1973, Sharitz and Mitsch 1993, Pezeshki
and Anderson 1997). For example, both A. rubrum
and F. pennsylvanica had greater density on nondepressions than depressions of channelized and
unchannelized sites, but no difference on channel-
462
WETLANDS, Volume 29, No. 2, 2009
ized and leveed stream reaches, again suggesting loss
of topographic heterogeneity following levee construction.
While we found differences in composition, the
cause was confounded by disturbances that occurred
on all rivers (e.g., logging and ponding) and the
storage effect of long-lived forest species (Warner
and Chesson 1985). For example, floodplains
completely disconnected from the channel (Tigrett
WMA and Lucius Burch NA) supported some of
the best examples of older Quercus forest but were
typified by greatly impoverished understory communitites. The older forest is a remnant of the
hydrologic processes that existed prior to channel
modifications and survives now only due to the long
life span of the trees. The unchannelized systems,
while still affected by ponding and cutting, did not
have as much of their floodplain area affected by
these disturbances and also had natural degradation
and aggradation occurring along their meandering
channel, leading to structural and functional heterogeneity (Kupfer and Malanson 1993). Thus, they
tended to be more diverse overall when considering
overstory and understory (this study) and floodplain
(beta) diversity (Franklin et al. 2009). Oswalt and
King (2005) similarly concluded that channelization
impacts on floodplain forests along the Middle Fork
of the Forked Deer River (TN) were more
temporally and spatially complex than previously
thought. Due to such confounding factors, direct
effects between vegetation composition and/or
structure could not be established, and were less
helpful in understanding channelization effects.
Another factor leading to greater interaction
effects than direct effects was the amount of
variability among the six reaches. Basin characteristics, site hydrologic connectivity and subsequently
stream modification effects can vary greatly even
within a relatively small region such as western
Tennessee. Indeed, Ward and Tockner (2001)
suggest hydrologic diversity is the unifying theme
for river ecology. Our soil water table and soil redox
potential data supported the obvious: channelized
and leveed sites were less variable. The unchannelized river reaches displayed the greatest amount of
variation in several characteristics, likely due to the
large variations in hydrologic regime typically found
on unaltered floodplains (Robertson et al. 1978,
Richter and Richter 2000, Steiger et al. 2005). This
variation was evident, for example, in considering
soil redox potential, where the two unchannelized
reaches had the highest and lowest values of all six
rivers in the July sample, while soil redox potentials
were quite similar within (but not between) the
channelized and the channelized and leveed stream
Wetlands wetl-29-02-04.3d 7/4/09 11:14:23
462
replicates. One important indirect effect of channelization (leveed or not) may thus be a decrease in
floodplain variability, both topographical and hydrological (Ward et al. 2002, Valett et al. 2005).
In conclusion, levees had the greatest impact on
floodplain forest functions, eliminating exchanges of
nutrients and organic matter and rendering a drier
floodplain environment, but many responses to
stream modifications were predicated on topographic setting. Channel hydrologic regime was tightly
linked to floodplain hydrologic regime for unchannelized reaches, while channel and floodplain
hydrologic regimes were decoupled in channelized
stream reaches during low flow events. Thus,
channelization (accompanied by levee construction
or not) led to decreased hydrologic variability and
subsequently decreased floodplain habitat heterogeneity. Both channelized and unchannelized stream
systems suffer from sediment movement and accumulation, decreasing decomposition rates, nutrient
cycling, and productivity.
ACKNOWLEDGMENTS
This research was supported by the Tennessee
Department of Environment and Conservation and
the U.S. Army Corps of Engineers. We especially
wish to thank Ellen Williams (TDEC) for her help
and Steve Stephenson for allowing data collection
from the Milan Arsenal. We also thank the many
people who assisted in the data collection and
analysis, especially Natasja van Gestel, Tanya
Scheff, Ryan Hanson, Stacy Anderson, Karla Gage,
Kit Brown, Melissa Lee, Mitch Elcan, and Jason
Farmer. Facilities provided by the Edward J.
Meeman Biological Field Station were essential for
the project’s completion.
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