channelization effects on floodplain functions in western ...€¦ · river basin management 19 1...

Channelization effects on floodplain functions in western Tennessee

S.B. ~ranklin', J.A. hpfer2, S.R. ~ezeshki', N. van ~ e s t e l ' & R.W. end I Department of Biology, University of Memphis, USA 2 Department of Geography h Regional Development, University of Arizona, USA r round Water Institute, University of Memphis, USA

Abstract

We examined six river reaches in western Tennessee over a two-year period to determine how channel alteration affected floodplain hydrology and nutrient pools. Four sites, two depression and two non-depression, were established on the floodplains of each river, and data on vegetation, water table depth, redox potential, and soil and leaf nutrient pools were collected. Chamelized streams had higher mean water tables and lower soil redox potentials than non-channelized or channelized and leveed streams. Leveed systems appeared to have mostly oxidized soil conditions, similar to uplands. Leaf and soil nutrient pools were generally higher in non-depression sites, especially for channelized streams. A drought between the first and second years of sampling rendered very different results between the two sampling occasions. Following the drying of the floodplain, nutrient pools were not significantly different between depression and nondepression sites. These results underscore the need for a better understanding of the relationships among channel modifications, floodplain hydrology, vegetation, and nutrient cycling.

1 Introduction

River channels in many areas of the world have been straightened, deepened, widened and leveed to accelerate storm water drainage, increase overbank flood stage, lower the water table in bottomland areas, and protect agricultural land fiom flooding. Channelized rivers may sometimes incise further as a result of steeper channel gradients, creating drier conditions [l, 21, or aggrade with the formation of valley plugs where channelized tributaries enter a main stem river, increasing the

Transactions on Ecology and the Environment vol 50, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541

190 River Basin Management

depth, area, and duration of inundation [3, 41. Channelization may thus alter hydrologic regimes in complex manners and may in turn alter other floodplain ecosystem functions, such as nutrient cycling [5,6,7].

Flooding generally serves as a nutrient input to floodplains due to the organic matter [S] and nutrients in deposited sedirnents [9, 101. However, flooding is also a stress to terrestrial ecosystems, and productivity may be diminished due to changes in soil oxygen and nutrient availability 110, 111. Changes in flood duration and periodicity have been shown to affect floodplain vegetation composition [12], nutrient retention [13], and soil redox potential [14]. However, research examining the overall effect of channelization on floodplain functions is rare so the present study was developed to determine how human alterations to low-gradient rivers in western Tennessee have affected floodplain hydrology and nutrient pools.

To do so, we selected six rivers for study, including two non-channelized reaches, two channelized and leveed reaches, and two channelized but non-leveed reaches. We hypothesized that channelization has created a flashier hydrology, greater flood depths and shorter flood periods. However, in leveed systems, we anticipated that the erection of levees has cut off river inputs and restricted these sites to rain-fed systems. Leveed floodplains would thus be drier than either of the floodplains still connected to river hydrology. Further, we hypothesized that levels of some nutrients would be lowest on leveed sites because levees would remove the inputs associated with occasional flood events. However, two important factors may condition the hypothesized responses. First, several studies have documented the importance of floodplain microtopography on a range of processes (e.g., [15]). We hypothesized that depression sites would have slower decomposition and lower nutrient pools than nondepression sites. Second, forest age may affect nutrient cycling and retention. Thus, while we did not include age in our analyses, we did sample forests of various ages fiom 30-100 years old within all of the treatments.

2 Methods

2.1 Study sites and study design

Six low-gradient (< 0.02 % slope) riverine systems were selected representing three treatment conditions: 1) unchannelized, 2) channelized, and 3) channelized and leveed (see Table 1 for river names and background information). Most of the stream alterations in the modified streams occurred prior to 1970. The unchannelized streams (reference systems, senw [16]) included an upper section of the Wolf River and the Hatchie River, a nationally scenic river. Although the main channels of these streams have not been altered, their tributaries often have been channelized. Within each river reach, two plots were randomly located in depressions (areas of concave surface microtopography) and two plots were located in nondepressions (straight or convex microtopography). The study design is a two- factor split-plot, with topography as subplots of rivers.

Western Tennessee is part of the Mississippi Embayrnent, an extension of unconsolidated coastal plain materials. The soils for all six rivers are mostly of the


River Basin Management 19 1

Table 1. Characteristics of the six rivers in western Tennessee used to examine the effects of channel alteration on floodplain forest nutrient pools. Data represent the section of the drainage upstream fiom the specific study area and are from the Groundwater Institute archives, university of ~ e m ~ h i s .

Site River Length Drainage % Basin % Basin Study (km) Basin (ha) Wetlands Agriculture Section

Lensth (km) Tigrett North Fork; 64.7 120,525 12 87 5 W.M.A. Forked Deer Lucius (Lower) 86.6 192,257 25 30 3 Burch N.A. Wolf Milan Rutherford 36.8 31,384 29 68 5 Arsenal Fork; Obion Stokes Stokes Creek 12.9 3,112 2 98 4 Wolf (Upper) Wolf 50.1 79,303 50 49 8 Hatchie Hatchie 60.2 227,798 42 32 5

Waverly-Swamp association, which consists of poorly drained, level, silty soils on low, broad first bottoms [17, 181. These soils formed in medium acid and strongly acid loess washed £tom uplands. Swamp soils are under water most or all of the year year. The yearly average temperature is 16" C. July is the warmest month averaging 26.6" C, and January is the coldest month averaging 4' C. Average yearly precipitation is 132 cm with a majority of precipitation occurring during the winter and spring months. The average growing season is 230 days.

2.2 Floodplain vegetation

Because soil and leaf nutrient pools are strongly influenced by local vegetation, we first tested for differences in forest composition among treatments and between depression and non-depression sites. Sites were sampled using 10 m X 20 m rectangular plots aligned parallel to the watercourse. At each plot, all individuals > 3 cm diameter at breast height (dbh) were measured to the nearest 0.1 cm and identified by species. To test for compositional differences between the treatments and the microtopographic classes, we performed multi-response permutation procedures (MRPP; [19, 201). This analysis calculates the similarity within groups (selected a priori) to the similarity between groups for all possible randornly- assigned groupings (permutations), and compares the random results to the results from the analysis of the original groups. The permutation allows a significance test of the differences found between groups.

2.3 Floodplain hydrology and nutrient pools

In the spring of 1998, surface wells were installed at each plot to examine water table fluctuations. Data were collected at least once a month during the growing season. In the summer of 1999, redox potential electrodes were installed at each of the 24 sites at three depths (15, 30, and 60 cm). Redox potential measurements were taken every two weeks from June 1 - October 30. Redox potential measurements from the three depths were pooled for analyses. Soils were collected during a two- week window in the springs of 1999 and 2000. Soils were air dried, sifted through a



2 X 2 mm sieve and sent to A & L Agricultural Laboratories for analysis of pH, total extractable K, Mg, Ca, P, and nitrate-N (all in ppm), organic matter (%), estimated nitrogen reserve, and cation exchange capacity.

We also established 1 m2 leaf catch nets at each site. These nets were set 1 m off the ground but had floats and were engineered to slide up poles during high water, inhibiting any leaching or additions due to floods. Catches were established by October 1 and taken down early in January, providing a three-month record that corresponded to the period of maximum leaf fall in 1998 and 1999. Leaf samples were collected at least once a month. Samples were dried at 60' C for 48 hours, weighed (oven dry weight, O.D.W.), and then combined for chemical analysis at A & L Labs. Biomass data were summed for all collections of a particular site. Total biomass at each site was multiplied by the value from chemical analysis (ppm or %) to generate leaf nutrient values as g O.D.W. m-2.

A split-plot ANOVA was used to examine the effect of channelization treatments (TRT), topography (DND), and their interaction (TRTeDND); topography was a subplot of rivers (RIVERS). Due to limitations on space, we do not discuss the differences found between rivers in this paper. All statistics were run in SAS.

3 Results

3.1 Hydrology

The piezometer data collected at each of the sample plots elucidated the differences in hydrologic regimes for the study areas and for depression and non-depression microsites. Channelized sites tended to have higher water tables than the leveed and unchannelized sites. Depression sites had significantly higher water tables in channelized and unchannelized streams, but leveed sites had only weakly significant topographic differences (Fig. 1) (TRT*DND, F = 3.10, p = 0.08). There was also a significant location effect (RIVERS, F = 20.64, p 0.0001).

Soil redox potential was greatest in the channelized and leveed sites, as might be expected due to their disconnection from the river hydrology. Both the channelized sites and unchannelized sites, where the river and floodplain hydrology are still connected, had significantly lower redox potentials in depressions compared to non-depressions (TRT*DND, F = 8.82, p = 0.0002; Fig. 2). While soils remained in the oxidation zone (Eh > 350 mV) throughout the sampling season for channelized and leveed sites, the channelized sites and one unchannelized site began the growing season with reducing soils (Fig. 3). Soil redox potential was similar between channelized streams and between channelized and leveed streams, but a great disparity was evident between the two unchannelized streams.

3.2 Floodplain vegetation

A significant difference was found between depression and non-depression communities (MRPP p I delta = 0.075). Taxodium distichum was a dominant


River Basin Management 193

C&L C U Treatment

Figure 1. Mean water table depth taken over a two-year period for six rivers in western Tennessee. Positive values represent above-surface water tables. C&L = channelized & leveed; C = channelized; U = unchannelized; Topographic settings: D = depression, ND = non-depression.

Figure 2. Soil redox potential measurements 6om six rivers in western Tennessee under three types of alterations: U = unchannelized; C =channelized; C&L = channelized and leveed.

June July August

Month (1 999)

Figure 3. Soil redox potential measurements from six rivers in western Tennessee under three types of alterations: U = unchannelized; C = channelized; C&L = channelized and leveed. Rivers: H =

Hafchie, W = Wolf, S =Stokes Creek, M = Obion, LB = lower Wolf, and T = Forked Deer.



Table 2. Mean basal area for overstory (> 3 cm dbh) vegetation from six rivers in western Tennessee. Data are presented by treatment (channelized (C), channelized & leveed (CL), and unchannelized (U)) and topography (depression (D) and nondepression (ND)); n = 4. Species U, ND CL, ND C, ND C, D CL, D U, D

Taxodium distichum 9.25 8.01 7.78 Planera aquatica 11.3 Salix nigra 13.94 5.51 3.70 12.36 Platanus occidentalis 9.09 2.96 2.76 3.67 Fraxinus pennsylvanica 2.8 1 6.23 1.24 Quercus lyrata 15.85 Acer rubrum 1.24 6.87 9.25 20.28 1.21 Acer saccharinum 4.04 Betula nigra 1.15 Quercus nigra 10.17 Liquidambar styraciflua 14.66 9.34 3.54 1.01 Quercus michauxii 3.10 Populus deltoides 2.91 Quercus palustris 12.47 4.02 Ulmus americana 1 .08 1.33 5.81 2.1 1

component of depressions, along with Salix nigra and Acer rubrum (Table 2) . The latter are more typical of younger forest on western Tennessee floodplains that often accompany human alterations. Acer rubrum, for example, was much more dominant on channelized and channelized & leveed sites. Unchannelized sites had greater amounts of Liquidambar styraciflua, Platanus occidentalis and Acer saccharinum. However, there was not a significant difference in community composition among treatments (MRPP p I delta = 0.79).

3.3 Nutrient pools

A significantly greater amount of leaf biomass fell on the non-depression sites than the depressions of channelized streams in 1998 (Table 3, Fig. 4a), resulting in increased leaf nutrient pools as well (Fig. 4). No significant differences were found in leaf biomass or nutrient pools between depressions and non-depressions in either the leveed sites or the unchannelized sites. Although there were no differences by treatment or microtopography in 1999, there were significant differences in leaf biomass by river. Overall, more leaf biomass was recorded during the 1999 collection than during the 1998 collection.

The trend in biomass was for channelized and leveed sites to have the greatest biomass and unchannelized sites the least (Fig. 4). Phosphorus, magnesium and calcium followed the biomass trend while nitrate-N had the opposite trend (greatest in unchannelized sites) (differences are statistically insignificant). The prior elements are all structurally bound in leaf tissues. There was at least a 10-fold increase in the amount of nitrate-N found between 1998 and 1999 (Fig. 4a, b). All nutrients were significantly different among rivers in 1999.



Table 3. Split-plot ANOVA results for leaf and soil nutrient pools of six rivers in western Tennessee. Treatments (TRT) include channelized, channelized & leveed, and unchannelized sheams. Topographic effects (DND) are depression and non- depression floodplain sites. Bold indicates significance (a = 0.1).

Variable TRT TRT DND DND TRT* TRT* River f iver F p > F F p > F DND DND (TRT) (TRT)

F p > F F p > F Biomass 98 6.05 0.09 5.66 0.03 7.88 0.005 0.63 0.61 Biomass 99

LeafN 98 Leaf N 99 Leaf P 98 Leaf P 99 Leaf K 98 Leaf K 99 Leaf Mg 98 Leaf Mg 99 Leaf Ca 98 Leaf Ca 99

Soil N 99 Soil N 00 Soil P 99 Soil P 00 Soil K 99 Soil K 00 Soil Mg 99 Soil Mg 00 Soil Ca 99 Soil Ca 00 2.37 0.24 4.61 0.05 0.60 0.56 2.90 0.08

Soil nutrient pools also varied greatly from 1999 to 2000. Nitrate was significantly affected by the interaction of topography and treatment in 1999 (Table 3, Fig. 5a). On channelized and unchannelized streams, nitrate pools were greatest in depression sites. There was a 10-fold decrease in soil nitrate-N fiom 1999 to 2000 (Fig. 5a, b).

Both magnesium and calcium soil pools were higher in non-depression sites of channelized and unchannelized streams than depressional sites in both 1999 and 2000 (Fig. Sg, h, i, j). Magnesium increased on all sites fiom 1999 to 2000, while calcium decreased. Phosphorus pools tended to be highest in non-depression sites (Fig. Sc, d), but the results were not significant. Potassium was significantly greater in non-depressions of channelized sites (Fig. Se, f), but no differences were apparent in leveed and unchannelized streams. Both potassium and phosphorus tended to be highest in channelized sites.

4 Discussion

While other research has shown channelization to affect vegetation [7], we did not find a significant difference in overstory vegetation between the channelized, channelized and leveed, and unchannelized streams. Common species, such as



600

% xn, W - m

.E g zoo

J loo

18

Fall 1998

a) I

Fall 1999

b)

CBL C U C&L C U Treatment

Figure 4. Leaf biomass and nutrient pools from litter catch nets along six rivers in western Tennessee, 1998 and 1999. C&L = channelized and leveed, C = channelized, U = unchannelized.



Spring 1999 14

I 14

12 1 a) , 12

Spring 2000

J 0.1-

C8.L C U Treatment

Figure 5. Soil nutrient pools from six rivers in western Tennessee, 1999 and 2000. C&L = channelized and leveed, C = channelized, U = unchannelized.


l98 River Basin Management

Liquidambar styraczjl'ua, were found in all three treatments. In addition, we targeted various ages of forest during sampling on all sites, and the successional status of altered streams is a maior com~onent of the differences found between altered and unaltered streams [7]. In addition, most of these forests were already in place prior to chmelization (mainly during the 1960s). We, thus, believe that differences found in nutrient pools due to treatment effects or interactions of treatment and topography are directly related to alterations of stream hydrology by channel modifications.

We found few differences in the leaf and soil nutrient pools due to treatment effects alone. We hypothesized that the channelized and leveed sites would have fewer soil nutrients than the streams where the floodplains are connected to the rivers and flooding events could replenish nutrient budgets. Instead, few relationships were evident from the data. Two possible explanations include the subsidy-stress effects on floodplains and the differences between rivers. The subsidy-stress model [21] suggests that periodically flooded systems will be recharged with soil nutrients d h g flood events, but will also be stressed during these events leading to a decreased ability of biota to use the nutrients. These two factors may actually cancel each other out as was shown in the swamps of South Carolina and Louisiana [10].

A second reason for lack of significance may be the confounding river factor. Significant differences in nutrient pools were found between the six rivers. This may be best explained using the soil redox potential data given in Fig. 3. The re~licate rivers £tom the channelized and channelized and leveed sites showed similar patterns in redox potential, while the unchannelized replicates were vastly different. The variation between these two reference systems may be due to the large variations in hydrologic regime typically found on unaltered floodplains. With a small sample size the differences in rivers may have masked differences due to treatments.

Vegetation may have been an important factor contributing to differences in soil and leaf nutrient pools between depression and nondepression sites. The compositional difference between these microtopographic positions was significant. ~ re i i ous research has elucidated the effects of microtopography on floodplain vegetation, suggesting a few centimeters in elevation can yield vastly different vegetation assemblages [22]. We found soil redox potentials to be significantly lower in depressional sites representing moderately reducing soil conditions and mean water tables significantly higher as compared to nondepressional sites. In addition, several studies have shown that different species vary in their tolerance to flooding and nutrient uptake [23, 24, 251, both of which result in different foliar nutrient pools [24, 25, 261. The feedback links between vegetation composition, flooding regimes, and nutrient cycling are confounded in this study, but in general, depression sites fimction differently than non-depression sites in western Tennessee floodplains.

Soil nutrient pools in depressions tended to be lower than non-depression sites. Leaf nutrient pools were also generally lower in depressions, although only for the channelized streams. While continuous flooding of depressions, resulting in anoxic conditions (reducing soil conditions), would increase the mobilization of some



minerals for plant uptake, it would decrease decomposition rates [27] and nutrient uptake capacity of roots [28]. In addition, periodic flooding is important in the release of nutrients fiom litter. Thus, the depressions had fewer soil nutrients than nondepression sites supporting our hypothesis.

There were stark differences in the results fiom the data collected in 1998 and 1999 for leaf and soil nutrients, respectively. The summer of 1999 was a severe drought for the area, resulting in a 21 cm decrease in precipitation during July- September from 1998 to 1999. Leaf fall tended to be higher following the summer drought as trees shed their leaves. In addition, differences in leaf nutrients between microtopographic positions found in 1998 disappeared in 1999, likely due to the fact that all sites dried for extended periods of time during the 1999 summer (see Fig. 3). Thus, it is likely that the 1998 leaf fall data are more typical of wet year nutrient pools, while the 1999 leaf fall data represents dry year nutrient pools.

Certain temporal changes seem to fit expected results. For example, leaf nitrogen increased by 10-fold between sampling periods while soil nitrogen decreased by 10-fold, suggesting a tight cycling of nitrogen. Mitsch et al. [9] also found a tight cycling of phosphorus in floodplains of southern Illinois. Other nutrients did not show this pattern.

During the wetter year prior to the drought, 1998 leaf and 1999 soil nutrient pools, depression sites of unchannelzed streams had higher leaf nutrient pools and lower soil nutrient pools than the altered streams. Data from non-depression sites suggest channelized and leveed sites have nutrient pools more similar to unchannelized streams than to channelized steams. The channelized streams in this study were in the latter stages of recovery following channelization, including aggradation in the channel [7]. The resultant hydrology tends to be more frequent flooding and of longer duration than recently channelized streams. Thus, the channelized streams are linked to the river and function much differently than channelized and leveed systems. The nutrient subsidy from flood events on channelized streams is likely greater than for the unchannelized streams due to nonpoint source fertilizer movement from the surrounding watershed. Channelized streams averaged 78% agricultural land in their watersheds while unchannelized streams averaged only 40% agricultural land. Thus, nutrient inputs and productivity are expected to be higher in the channelized streams than unchannelized streams, as was generally found in our study.

Our results suggest that human alterations of streams in western Tennessee have a significant impact on the functions of adjacent floodplains. However, further work is necessary to examine the interactions of nutrients, vegetation, and hydrology. Current efforts are underway to examine the productivity of these systems and their decomposition rates.

Acknowledgements

This research was supported in part by contracts with the Tennessee Department of Environment and Conservation and the U.S. Army Corps of Engineers. We especially wish to thank Ellen Williams (TDEC) and Dan Smith (USACE) for their help. We also wish to thank the many people who assisted in the data collection and



analysis, especially T. Scheff, R. Hansen, S. Anderson, K. Gage, K. Brown, M. Lee, M. Elcan and J. Farmer. The facilities provided by the Edward J. Meeman Biological Field Station were essential for the project's completion.

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