MISSISSIPPI RIVER FRESHWATER DIVERSIONS IN 

SOUTHERN LOUISIANA:

EFFECTS ON WETLAND VEGETATION, 

SOILS, AND ELEVATION

A Position Paper by the Technical Panel from the 

Workshop on Response of 

Louisiana Marsh Soils and Vegetation to Diversions

Coastal marsh near Leeville, Louisiana. Photo Credit: Dennis Demcheck (USGS)

Final Report to the 

State of Louisiana and

U.S. Army Corps of Engineers through the

Louisiana Coastal Area Science and Technology Program

Coordinated by the

National Oceanographic and Atmospheric Administration

December 5, 2012
This report should be cited as:

Teal, J.M., R. Best, J. Caffrey, C.S. Hopkinson, K.L. McKee, J.T. Morris, S. Newman and B. 

Orem. 2012. Mississippi River Freshwater Diversions in Southern Louisiana: Effects on Wetland 

Vegetation, Soils, and Elevation. Edited by A.J. Lewitus, M. Croom, T. Davison, D.M. Kidwell, 

B.A. Kleiss, J.W. Pahl, and C.M. Swarzenski. Final Report to the State of Louisiana and the U.S. 

Army Corps of Engineers through the Louisiana Coastal Area Science & Technology Program; 

coordinated by the National Oceanic and Atmospheric Administration. 49 pages.
Authors (Workshop Technical Panel)

John M. Teal, Scientist Emeritus, Woods Hole Oceanographic Institution Partner, Teal Partners, 

Rochester, MA; jteal@whoi.edu

Ronnie Best, U. S. Geological Survey, Greater Everglades Priority Ecosystems Science, c/o 

University of Florida/IFAS, Fort Lauderdale Research and Education Center, 3205 College 

Avenue, Fort Lauderdale, FL 33314; ronnie_best@usgs.gov

Jane Caffrey, Center for Environmental Diagnostics and Bioremediation, University of West 

Florida, 11000 University Parkway, Pensacola, FL 32514; jcaffrey@uwf.edu

Charles S. Hopkinson, 229 Marine Sciences Bldg. Department of Marine Sciences, University of 

Georgia, Athens, GA 30602; chopkins@uga.edu

Karen L. McKee, U. S. Geological Survey, National Wetlands Research Center, 700 Cajundome 

Blvd., Lafayette, LA 70506; email: mckeek@usgs.gov

James T. Morris, Belle W Baruch Institute for Marine & Coastal Sciences, University of South 

Carolina, Columbia, SC 29208; jtmorris@mailbox.sc.edu

Susan Newman, South Florida Water Management District, Everglades Systems Assessment 

Section, 8894 Belvedere Road, Bldg 374, West Palm Beach, FL 33411; snewman@sfwmd.gov

Bill Orem, U. S. Geological Survey, 12201 Sunrise Valley Drive, Reston, VA 20192; 

borem@usgs.gov

i
Editors (Workshop Steering Subcommittee)

Alan J. Lewitus, National Oceanic and Atmospheric Administration, National Ocean Service, 

National Centers for Coastal Ocean Science, 1305 East West Highway, Silver Spring, MD 

20910; alan.lewitus@noaa.gov

Miles Croom, National Oceanic and Atmospheric Administration, National Marine Fisheries 

Service, Southeast Regional Office, 263 13th Avenue South, St. Petersburg, FL 33701; 

miles.croom@noaa.gov

Todd Davison, National Oceanic and Atmospheric Administration, National Ocean Service, 

Coastal Services Center, Gulf Coast Services Center, 4 Falcon Dr., Peachtree City, GA 30290: 

todd.davison@noaa.gov

David M. Kidwell, National Oceanic and Atmospheric Administration, National Ocean Service, 

National Centers for Coastal Ocean Science, 1305 East West Highway, Silver Spring, MD 

20910; david.kidwell@noaa.gov

Barbara A. Kleiss, U.S. Army Corps of Engineers, Mississippi Valley Division, 1400 Walnut 

Street, Vicksburg, MS 39181-0080: Barbara.A.Kleiss@usace.army.mil

James W. Pahl, Louisiana Coastal Protection and Restoration Authority, Planning and Research 

Division, Studies and Environmental Section, PO Box 44027, Capitol Station, Baton Rouge, LA 

70804-4027; James.Pahl@la.gov

Christopher M. Swarzenski, U. S. Geological Survey, Louisiana Water Science Center, Baton 

Rouge, LA 70808: cswarzen@usgs.gov

ii
Executive Summary

Coastal Louisiana lost 4,877 km2 of land area between 1932 and 2010 (Couvillion et al. 2011). 

Freshwater Diversions are a technique to address one cause of wetland loss: failure to maintain 

elevations sufficient to support emergent vegetation forming the wetland habitat. This report 

examines the effects of Freshwater Diversions from an ecological perspective, and focuses on 

wetland plant community productivity and composition, wetland elevation, and wetland soil 

strength. Each section assesses available information, information gaps, and identifies data

needed to ensure successful marsh stabilization and restoration.

Diversions of river water into adjacent coastal wetlands are a part of all plans to mitigate for the 

extensive loss of Louisiana’s coastal wetlands. Diversions can be broadly characterized as 

Sediment Diversions, designed for significant land-building in areas that currently are open 

water, and Freshwater Diversions, designed to flow into existing, but degrading marsh systems to 

reverse or slow the rates of degradation. This paper explores the effects on marsh properties of 

diversions in the lower Mississippi River designed for freshwater delivery (i.e. Freshwater 

Diversions). Evaluation of the effects of Sediment Diversions is not emphasized in this 

investigation.

Freshwater Diversions may affect wetland plants and their responses to changes in salinity, 

nutrients, and herbivory. Freshwater Diversions represent a major shift in water-quality 

conditions, with reduced salinity and increased nutrient availability that affect plant communities 

directly by influencing their distribution (salt tolerance) and their growth characteristics 

(root:shoot ratios, absolute productivity). Salinity and nutrient conditions also have indirect 

effects on herbivory, interactively influencing plant communities and marsh stability. Lower 

salinities lead to shifts in plant species to those more typical of freshwater communities. 

Nutrient increases brought by Freshwater Diversions may also change aboveground and 

belowground production and ratios. Better forage quality from nutrient enrichment also 

stimulates herbivory (e.g. nutria), indicating the importance of including the quantitative impact 

of herbivory in predictive models on vegetation responses to Freshwater Diversions. A general

conclusion on the expected short-term and long-term responses of marsh belowground 

production to Freshwater Diversions in Louisiana could not be drawn from the available 

evidence. This uncertainty stems from several research limitations: data collection was highly 

variable from one diversion project to the next; design review was lacking, especially when 

considering ecosystem level outcomes; monitoring was highly variable from one site to the next; 

pre-diversion data for sites were not collected, making it difficult to understand results of 

diversions; and conditions varied considerably from site to site so data from one site were not 

necessarily useful at another. 

Freshwater Diversions may alter wetland elevation trajectories by modifying the processes 

controlling wetland elevations, which may be physical, biological, and/or hydrological, and 

operate in response to drivers such as sea level change, river flows, storms, climate change, 

nutrient loading, and human modifications. One of the many causes of deterioration of 

Mississippi River Delta wetlands is the separation of the wetlands from the river and its sediment 

due to control structures and channel stabilization. Diversions may deliver mineral sediment 

directly to marsh surfaces, increasing rates of accretion and/or freshwater and nutrients to 

iii
stimulate plant production (biomass, stem density), which in turn promote sediment deposition 

and trapping (aboveground) and/or increase organic matter accumulation (primarily 

belowground where decomposition is slow). Existing data showing diversion effects on marsh 

accretion or elevation are sparse and based on limited sample sizes and/or inadequate sampling 

designs. Major information gaps exist with respect to how much diversions may alter marsh 

elevation trajectories (especially in relation to balancing relative sea level rise), which specific 

mechanisms influencing soil volume (organic vs. inorganic) might be most affected by 

diversions, and what other factors might modify the impact of diversions on marsh elevation 

dynamics. With regard to Freshwater Diversions, data are particularly needed on how changes in 

water chemistry or plant community composition may influence plant production-decomposition 

processes and resultant effects on soil volume and elevation change.

Freshwater Diversions may also affect the shear strength of soils, which is determined by soil 

composition (e.g. mineralogy, grain size, particle shape, ionic forces, organic matter content), 

void ratio, water content, and pore water chemistry (e.g. salinity, pH), the soil structure (e.g. 

particle arrangement, fissures, cementation), and loading conditions (e.g. magnitude of shear 

stress, rate, history). It is also controlled by plant roots and rhizomes: it increases proportionally 

to the cross-sectional area of roots crossing the shear plane. Storm and hurricane surge modeling 

and predictions of water level, short wave height, pressure field variation and frictional energy 

dissipation due to bay bottom and wetland vegetation interactions were not included in most 

storm surge models of the Louisiana coast prior to Hurricane Katrina in 2005. The wave energy 

lost in passing over marsh vegetation and the ability of the marsh/root/soil complex to hold up 

against wave shear stress are the reasons wetlands provide a buffer for inland property. Soil

strength is expected to increase with depth as a result of compaction by overburden loading. 

Surface soil drainage and desiccation enhances compaction, which can increase soil strength near 

the surface. Wetland ponding decreases soil strength and increases wetland vulnerability to 

storm scour and loss. Plant roots change the depth profile of soil strength, with increased soil 

strength where roots are present. Abrupt changes in soil strength slope vs. depth, especially from 

positive to negative slope, can result in failure planes - regions above which entire blocks of 

wetland soil can be scoured. Diversions can increase scour vulnerability if they result in ponding 

or decreases in rooting depth or root tensile strength.

All diversion projects would benefit from formal adaptive management. These programs 

function when goals are agreed to among the actors, knowledge of pre-diversion conditions exist, 

monitoring is done to measure progress toward goals, and there is a process in place to adjust 

functions to improve the likelihood that goals can be met. At present, considerable investment 

has been made in designing, engineering and building diversions, and an emphasis of Coastal

Protection and Restoration Authority of Louisiana’s 2012 Coastal Master Plan is development of 

a comprehensive Adaptive Management Framework, to be completed in 2013, that will apply to 

the initial stages of master plan implementation. Elements of the Framework will include 

expansion and refinement of existing monitoring and modeling efforts, and development of new 

projects to fill information and management gaps. 

A well-designed monitoring program is essential to assess whether diversions are promoting 

marsh sustainability and to support adaptive management. To answer questions about the spatial 

extent of diversion influence, it is important to establish multiple monitoring stations at suitable 

intervals (distances from discharge and in relation to discharge flows) and sufficiently replicated 

iv
to detect significant differences. Preferably, pre-diversion measurements should be made over a 

suitable duration to provide baseline trends, including seasonal variation. Monitoring of pore

water chemistry (salinity, nutrients, sulfide) would assist in interpreting the influence of 

diversion discharges on environmental conditions affecting plant growth. Measurements of soil 

texture (particle size distribution), soil strength, dry bulk density, and organic content conducted

at each sampling station would allow more detailed monitoring of changes in soil texture and soil 

integrity. End of season standing live biomass (above- and belowground) would provide 

information about the health of the plant community and relationship to elevation changes. 

Herbivory is a confounding issue because nutria remove both above- and belowground biomass, 

leading to reductions in soil organic matter, further reducing the ability of the marsh to maintain 

itself as sea level rises. 

The Panel concluded that little evidence was available that any Freshwater Diversion in the 

Louisiana deltaic plain has significantly reversed the rate of marsh degradation and land loss. 

While there is evidence that the minor sediment load of Freshwater Diversions has enhanced 

accretion in localized areas, lack of uniform sheet flow across marshes, coupled with prolonged 

flooding and ponding of wetland vegetation, and rapid shifts in salinity have compromised the 

combined processes of macrophyte biomass accumulation, peat accumulation, building of soil 

strength and building wetland elevation and extent. These processes are necessary for reversing 

the high rates of wetland loss through most of the Louisiana deltaic plain. It is unlikely that any 

currently planned Freshwater Diversion without substantial sediment load will reverse wetland 

loss in Louisiana. Restoration of Louisiana wetlands may only be possible through significant 

inputs of sediment.

A number of “Sediment Diversions” are planned along lower portions of the Mississippi and 

Atchafalaya Rivers, but the science is still evolving to inform model predictions on the amount 

of sediment needed for adequate wetland rebuilding (not addressed here) and the role of high 

nutrient river water in contributing to or detracting from marsh restoration (addressed here). 

Effective adaptive management, therefore, will be paramount to ensure that diversions have their 

intended outcome. To this end, an Adaptive Management Framework is being developed by the 

Coastal Protection and Restoration Authority of Louisiana (CPRA) as a critical element of the 

2012 Coastal Master Plan so that diversion effects can be monitored, future effects more 

accurately predicted, and management strategies adjusted as needed. The Framework offers an 

excellent opportunity to develop a comprehensive research design that adequately tests the 

alternative hypotheses on wetland response to nutrient-rich river water characterized here, as a 

critical step to ensuring intended outcomes of restoring and sustaining Louisiana wetlands. 

v
Table of Contents

Executive Summary..........................................................................................iii

Introduction .....................................................................................................................................1

Effects of Diversions on Wetland Plants – Response to Changes in Salinity, Nutrients, and 

Herbivory ........................................................................................................................................ 5

 Background.................................................................................................................................5

 Environmental factors that alter wetland plant communities .....................................................6

 Effects of salinity on plant productivity.................................................................................6

 Effects of salinity on plant nutrition ......................................................................................8

 Effects of increased nutrient loads on plant productivity ......................................................9

 Effects of herbivory on plant communities..........................................................................10

 Observed effects of diversions on vegetation communities .....................................................11

 Information needed ...................................................................................................................12

 Conclusions...............................................................................................................................14

 Suggestions for Moving Forward .............................................................................................15

Effects of Diversions on Wetland Elevation................................................................................. 15

 Background...............................................................................................................................15

 Environmental factors that alter marsh elevation .....................................................................18

 Observed effects of diversions on marsh elevation ..................................................................19

 Information needed ...................................................................................................................20

 Conclusions...............................................................................................................................21

 Suggestions for Moving Forward .............................................................................................22

Effects of Diversions on Wetland Soil Strength........................................................................... 22

 Background...............................................................................................................................22

 Environmental factors that alter soil strength ...........................................................................23

 Observed effects of diversions on soil strength ........................................................................25

 Information needed ...................................................................................................................25

 Conclusions...............................................................................................................................27

 Suggestions for Moving Forward .............................................................................................28

Monitoring and Adaptive Management........................................................................................ 28

Summary........................................................................................................................................32

References......................................................................................................................................34

vi
Introduction

John M. Teal

A stable and growing Mississippi delta has many valued features, not only for the citizens of 

Louisiana, but also for the nation as a whole. The delta protects and preserves the City of New 

Orleans, it supports the oil and gas industries, protects navigation channels, mitigates storm 

damage, provides recreational and social benefit to human populations, and preserves and 

enhances fisheries and wildlife. 

Delta formation in coastal Louisiana over the last 6,000 years was marked by shifting 

geomorphology, with wetland gains and losses through active deltaic lobes. A net increase in 

wetland area occurred up until the 20th century, after which an estimated 25% loss (4,800 km2) of 

coastal wetlands has occurred, due in part to human-related activities that reduced sediment 

delivery down the Mississippi River (e.g. dams, ship channels), altered marsh flooding and 

drying cycles (e.g. weirs and pipeline canals), increased subsidence rates (e.g. sediment 

consolidate and mineral or petroleum extraction), and reduced river input to the deltaic plain 

(flood control measures such as levees and distributary closures), and sea level rise (Britsch and 

Dunbar 1993, Barras et al. 1994, Day et al. 2007, 2009a). An estimated 4,877 km2 of coastal 

Louisiana wetland area was lost from 1932 to 2010 (Couvillion et al. 2011).

Controlled diversions of river water into adjacent coastal wetlands are a significant part of all 

plans to mitigate for the extensive loss of Louisiana’s coastal wetlands. Diversions can be 

broadly characterized as Sediment Diversions, designed for significant land-building in areas that 

currently are open water, and Freshwater Diversions, designed to flow into existing, but 

degrading marsh systems to reverse or slow the rates of degradation. Sediment Diversions 

typically have large discharge volumes and transport substantial quantities of mineral sediment. 

Freshwater Diversions generally have small discharge volumes and carry little mineral sediment. 

Their main effects are in changing water quality, namely by reducing salinity and adding 

nutrients. The boundaries between the two types are somewhat blurred. Some diversions 

primarily intended for freshwater delivery can nonetheless transport substantial mineral 

sediments (e.g. the Caernarvon Freshwater Diversion transported enough sediment to result in 

infilling of the Big Mar receiving basin). By the same token, a Sediment Diversion may carry 

less mineral sediments than some Freshwater Diversions if located in a mineral sediment poor 

area.

Sixteen constructed diversions currently exist in the lower Mississippi River (defined as starting 

at the Old River Control Structure), and up to 12 more are proposed through the 2012 Coastal 

Master Plan (Coastal Protection and Restoration Authority of Louisiana 2012); Figure 1, Table 1. 

The maximum releases of existing diversions range from 250 cfs (cubic feet per second) to over 

740,000 cfs (Table 1). Of the existing diversions in the Lower Mississippi River, 14 may be 

considered Freshwater Diversions and 4 Sediment Diversions, which were relatively recently 

constructed (West Bay, Delta Management at Fort St. Phillip, and Channel Armor Gap) or 

developed naturally (Mardi Gras Pass).

1
Figure 1. Map of constructed (closed symbols) and proposed (open symbols) Freshwater (blue 

circles) and Sediment (yellow squares) Diversions. Numbers correspond to diversions listed in 

Table 1 below.

2
Table 1. Constructed and proposed diversions from the Mississippi and Atchafalaya Rivers shown in 

Figure 1 include natural crevasses and structures capable of diverting freshwater as “Freshwater 

Constructed,” including emergency and navigation structures as well as restoration projects. Projects that 

share identifiers indicate where existing projects overlap with proposed projects, or where multiple 

proposed projects are being studied. Except for the Bayou Lafourche Diversion, discharges listed for the

planned diversions in the State of Louisiana’s 2012 Comprehensive Master Plan are indicative of scale 

(small diversions were modeled at 5,000 cfs; medium diversions were modeled at 50,000 cfs, and large 

diversions were modeled at 250,000 cfs). The exact discharges for those projects will be determined 

during future investigations. 

Identifier Diversion Type Status Constructed

Discharge

Capacity

(cfs)

Proposed

Discharge

Capacity

(cfs)

1 Old River Control Structure Freshwater Constructed 740,000

2 Morganza Floodway * Freshwater Constructed 600,000

3 Walter Lehmann Pump Station Freshwater Constructed 300

Master Plan Bayou Lafourche Diversion Freshwater Proposed 1,000

4 LCA Small Diversion at Convent / Blind River Freshwater Proposed 5,000

Master Plan West Maurepas Diversion Sediment Proposed 5,000

5 Bonnet Carré Spillway * Freshwater Constructed 250,000

6 Davis Pond Freshwater Diversion Freshwater Constructed 10,650

7 Violet Siphon Freshwater Constructed 300

Master Plan Central Wetlands Diversion Sediment Proposed 5,000

8 Caernarvon Freshwater Diversion Freshwater Constructed 8,800

Master Plan Upper Breton Diversion Sediment Proposed 250,000

9 White Ditch Siphon Freshwater Constructed 250

LCA Medium Diversion at White Ditch Sediment Proposed 35,000

10 Naomi Siphon Freshwater Constructed 2,100 2,100

11 LCA Medium Diversion at Myrtle Grove with Dedicated Dredging Sediment Proposed 75,000

Master Plan Mid-Barataria Diversion Sediment Proposed 250,000

12 Master Plan Mid-Breton Diversion Sediment Proposed 5,000

13 West Point a la Hache Siphon Freshwater Constructed 2,100

14 Master Plan Lower Breton Diversion Sediment Proposed 50,000

15 Mardi Gras Pass Sediment Constructed 2,500

16 Bayou Lamoque Floodgate Removal Sediment Proposed 2,500

17 Master Plan Lower Barataria Diversion Sediment Proposed 50,000

18 Empire Lock Freshwater Constructed N/A

19 Ostrica Lock Freshwater Constructed 11,000

20 Delta Management at Fort St. Phillip Sediment Constructed 5,400

21 Channel Armor Gap Sediment Constructed 2,500

22 West Bay Sediment Diversion Sediment Constructed 20,000

23 Master Plan Increase Atchafalaya Flow to Eastern Terrebonne Sediment Proposed 20,000

24 Master Plan Atchafalaya River Diversion Sediment Proposed 150,000

25 Wax Lake Delta Sediment Constructed 440,000

1,655,900 735,600

440,000 170,000

Diversions Off the Mississippi River

Diversions Off the Atchafalaya River

Total Potential for Diversion from Mississippi River

Total Potential for Diversion from Atchafalaya River

* The Morganza and Bonnet Carré Spillways are emergency floodways, with discharge restricted to extreme river 

flood years. Present project authorizations do not allow for more frequent, non-emergency use.

3
This paper explores the effects on marsh properties of diversions in the lower Mississippi 

River designed for freshwater delivery (i.e. Freshwater Diversions). Evaluation of the 

effects of Sediment Diversions is not emphasized in this investigation. We do not address 

effects on swamp forests (flood plain hardwoods, cypress stands, etc.) but do mention the 

increasing importance of black mangroves in the southern delta marshes.

The use of Freshwater Diversions has become a controversial approach for restoring and 

recreating Louisiana coastal habitats. There remains considerable uncertainty regarding when, 

where, and how diversions can be used to replenish Louisiana coastal habitats, and the responses

of marsh soils and vegetation is a key question in understanding the efficacy of Freshwater 

Diversions as a tool to restore degrading marshes. The science is still evolving, and our 

understanding remains incomplete, so that the siting, sizing, timing, and duration of freshwater 

inflows are not yet refined to the point where we can always be confident that diversions will 

have the intended effect. 

The incomplete knowledge base makes it difficult to know when and where to use diversions, 

and though we have considerable knowledge, we still don’t know enough to predict in any detail 

the outcomes caused by a particular diversion. Research done in one area or under some salinity 

regime will not necessarily be applicable in another area or under other conditions. However, 

because diversions are known to work in some situations, there is pressure to increase the use of 

diversions to reverse the substantial and continuing loss of coastal wetlands. Clearly, there is a 

need for additional research to develop the information needed by resource managers to improve 

our predictive and management capability, and there is an equally pressing challenge to deliver 

the results of research to decision-makers and the general public to understand the difficulties 

faced in determining when the use of diversions is the appropriate approach to improve the 

quality and increase the quantity of coastal habitats.

On 23-24 February 2011, the Louisiana Coastal Area Program (LCA) Science and Technology 

Office (established by the State of Louisiana and the U.S. Army Corps of Engineers) and 

National Oceanic and Atmospheric Administration (NOAA) convened the “Workshop on 

Response of Louisiana Marsh Soils and Vegetation to Diversions” 

(http://www.mvd.usace.army.mil/lcast/pdfs/DiversionWorkshop.pdf) at the University of 

Louisiana, Lafayette to evaluate the current scientific understanding of the effects of Freshwater 

Diversions on Louisiana marshes, develop a consensus to inform coastal managers of expected 

soil and vegetation responses, and identify gaps in understanding to inform future research 

priorities and management strategies. The Workshop was the third in a series of meetings to 

inform the LCA Science and Technology Board, decision makers, and the public on technical 

issues regarding existing and planned diversions. Support for the Workshop was provided by the 

NOAA Fisheries Southeast Region, NOAA National Centers for Coastal Ocean Science, NOAA 

Coastal Services Center, the State of Louisiana, the U.S. Army Corps of Engineers, the United 

States Geological Survey (USGS) Louisiana Water Science Center, the University of Louisiana, 

Lafayette, the National Audubon Society, the Coalition to Restore Coastal Louisiana, and The 

Nature Conservancy.

The Workshop brought together a panel of 10 technical experts, 18 presentations, and an 

audience of about 250 people. Over the course of the Workshop, a broad range of topics was 

4
discussed that examined how introductions of freshwater and nutrients into coastal marshes 

affect marsh soils and vegetation. The Technical Panel of subject matter experts was selected by 

the Workshop’s Steering Committee. This Position Paper is a consensus assessment of the 

Technical Panel, based on Workshop proceedings and an extensive literature review, on the state 

of knowledge and information gaps on expected soil and vegetation responses to Freshwater 

Diversions in coastal Louisiana. The Position Paper will be presented to the USACE and Coastal 

Protection and Restoration Authority of Louisiana (CPRA) to inform planning, monitoring, and 

adaptive management of diversions, and to the Gulf Coast Ecosystem Restoration Task Force to 

inform incorporation of diversions as a restoration option.

The following sections present theoretical arguments and empirical observations used to examine 

the potential application of Freshwater Diversions to marsh restoration. They address the 

influence of Freshwater Diversions on wetland plant community productivity and composition, 

wetland elevation, and soil strength. 

Effects of Freshwater Diversions on Wetland Plants – Response to Changes in 

Salinity, Nutrients, and Herbivory

Susan Newman, Ronnie Best, Jim Morris, and Jane Caffrey

Background

The vegetation communities of coastal Louisiana marshes are defined by their location along a 

salinity gradient. They range from the low salinity freshwater and intermediate marshes, to 

brackish and saltwater marshes. Freshwater marshes represent over one-third of the coastal 

wetland area, the largest spatial extent of marshland in the region (Sasser et al. 2008). With 

salinities less than 0.5 ppt, they are dominated by the freshwater plants, Sagittaria lancifolia and 

Panicum hemitomon. A unique community observed in this environment is the floating marsh. 

Dominated by emergent vegetation, primarily P. hemitomon, healthy floating marshes have thick 

highly organic buoyant mats of intertwined roots and rhizomes (Sasser et al. 1995). More 

recently, though, “thin” mats, dominated by Eleocharis baldwinii, have become more prevalent 

(Visser et al. 1999). Intermediate, low salinity (0.5-5 ppt) marshes encompass 26% of the 

coastal marsh and are dominated by species characteristic of both freshwater and brackish 

environments, S. lancifolia, Schoenoplectus americanus, and Spartina patens. S. patens is also 

one of the dominant species in brackish marshes, along with the more salt tolerant Spartina 

alterniflora. Brackish marsh represents 21% of the coastal marsh area, while the remaining 17% 

is true saltwater marsh, dominated by S. alterniflora. However, over the last 20 years, Avicennia 

germinans (black mangrove) has expanded into these areas (Henry and Twilley 2011). While 

there is no good estimate of the aerial expanse of A. germinans, a 5-fold increase in abundance 

from 2002 to 2009 (Michot et al. 2010) suggests the mangrove community is expanding rapidly.

Although nutrient availability is a controlling factor for the productivity of wetland plants, with 

different species having different nutrient requirements, there is no grey literature or peer 

reviewed literature that indicates coastal Louisiana marshes are suffering from a lack of 

nutrients. Nutrients in the Mississippi and Atchafalaya Rivers can be high due to high nutrient 

inputs in the watersheds of the Mississippi River (Turner and Rabalais 1991). Total nitrogen 

5
concentrations in the river are between 137 and 140 μM (1.9 to 2.0 mg N/L), with 60 to 90% of 

the nitrogen in the form of nitrate (Turner and Rabalais 1991, Lane et al 1999, 2004). Total 

phosphorus in the river ranges from 5.1 to 7.4 μM (0.16 to 0.23 mg P/L) (Turner and Rabalais 

1991, Lane et al. 2004). Thus, freshwater from river flooding or through diversions represents a 

significant nutrient input to marsh communities which can rapidly reduce these concentrations 

(Lane et al. 2004). Nitrogen concentrations within the estuaries and marshes not receiving direct 

riverine inputs are much lower, with nitrate concentrations less than 20 μM (0.28 mg N/L) and 

ammonium concentrations about 10 μM (0.14 mg N/L) (Caffrey and Day 1986, Madden et al. 

1988). Nutrient inputs to coastal marshes from the Louisiana continental shelf and Gulf of 

Mexico are low because concentrations are usually less than 5 μM (0.07 mg N/L) and 1 μM 

(0.03 mg P/L) of nitrate and dissolved inorganic phosphate, respectively (Lohrenz et al. 1999). 

Herbivory by large mammals such as nutria and muskrats can be significant in Louisiana 

wetlands (Shaffer et al 1992, Evers et al. 1998). The effects of large mammal herbivory have 

been more thoroughly studied than herbivory by waterfowl or invertebrates such as grasshoppers 

or snails. While some damage to marshes by muskrats has been documented, grazing by nutria 

is concentrated in fresh and brackish marshes, particularly since the decline of the fur industry in 

the late 1980s (Scarborough and Mouton 2007). Nutria, native to South America, were released 

in coastal Louisiana in the late 1930s and have few natural predators except for adult alligators 

(Keddy et al. 2009). 

Environmental factors that alter wetland plant communities

To put Freshwater Diversions into context, it is important to understand how environmental 

factors independently, and interactively, influence wetland plant communities. Freshwater 

Diversions represent a major shift in water-quality conditions, with reduced salinity and 

increased nutrient and herbicide availability (Swarzenski et al 2008). Agricultural runoff from 

the midcontinent spring flush characterizes water quality of the inflowing water during the spring 

flush now, as opposed to before the 1950s. Changes in salinity and nutrients affect plant 

communities directly by influencing their distribution (salt tolerance) and their growth 

characteristics (root:shoot ratios, absolute productivity), and these responses can be manifested 

in altered marsh plant production, composition, and diversity. Salinity and nutrient conditions 

may also have indirect effects on herbivory, interactively influencing plant communities and 

marsh stability. 

Effects of salinity on plant productivity

Diversions of freshwater from the Mississippi River into adjacent wetlands and bayous reduce 

the salinity of receiving wetlands and water bodies (e.g. Sasser et al. 1986). Extensive canal 

dredging has increased the number of tidal channels across the Delta, resulting in localized 

saltwater intrusion and the conversion of large areas of low-salinity marsh to open water (Sasser 

et al. 1986). Consequently, diversions that reduce salinity are a prerequisite for restoration of 

forested and fresh marsh communities that have been damaged by salt intrusion. 

Generally, increased salinity is associated with reduced wetland plant species numbers and 

biomass (Gough and Grace 1998). In general, all higher plants have problems dealing with salt. 

6
Even halophytes (salt tolerant plants) such as S. alterniflora grow better at lower than higher 

salinity (Nestler 1977). Plants are directly affected by the salinity of the pore water (the water in 

the pores between grains of sediment), but only indirectly affected by the surface water or flood 

water salinity effect. Pore water salinity is usually lowest near sources of fresh water and often 

increases at elevations where neap flood tides are not high enough to consistently inundate the 

site. Evapotranspiration can concentrate the salts in sediment and raise the salinity to levels that 

exceed the salinity of tidal flood water. High salinity in pore water affects plant gas exchange 

and stunts growth. A reduced rate of gross photosynthesis, reduced transpiration, and a higher 

aboveground respiration rate are common responses to elevated salinity (Hwang and Morris 

1984, Pezeshki et al. 1987, Hester et al. 2001).

The pore water salinity of brackish and salt marsh is rarely constant, fluctuating with storms, 

flood frequency, evapotranspiration, and rainfall (Morris 1995). The variability in salinity has an 

effect that is somewhat independent of the actual concentration. That is, plants can be 

acclimated to a higher concentration of salt that may be lethal if experienced as a pulse. 

Sensitivity to rapid changes in salinity is probably greater for oligohaline marsh macrophytes 

than for salt marsh species, and even among the oligohaline species there are differences in 

sensitivity. For example, growth of Scirpus americanus was actually stimulated by exposure to 6 

g/L of salt, and was able to recover even under the most extreme conditions of exposure to 12 

g/L salinity for 3 months, while the ability of Eleocharis palustris, P. hemitomon, and 

S.lancifolia to recover from salt exposure decreased with increased salinity and increased 

duration of exposure (Howard and Mendelssohn 1999). Recovery was also suppressed in these 

species by a rapid salinity influx rate compared to a slow influx rate. The conclusion is that 

short-term exposures to salt can change plant community composition and species dominance, 

depending on the pattern of exposure (Howard and Mendelssohn 1999), and the pattern of 

exposure in wetlands receiving Freshwater Diversions will depend on the operation of the pumps 

and siphons.

There also are effects of salt exposure on the allocation of biomass between growth above- and 

belowground. Loss of soil strength, which would increase the erosion of sediment, has been 

postulated to be one of the side-effects of nutrient-rich, Freshwater Diversions from the 

Mississippi River (Turner 2011b). Soil strength is affected by the production of roots and 

rhizomes, and any factor, such as salinity, that modifies the absolute growth of roots and 

rhizomes is relevant. Osmond (1980) posited that the root:shoot ratio of halophytes decreases 

with increased salinity, while Munns and Termaat (1986) suggested that the root:shoot ratio of 

glycophytes (non-halophytes) increases with increased salinity due to a dramatic reduction in 

shoot biomass. Parrondo et al. (1978) reported that S. alterniflora and Spartina cynosuroides, 

grown in a controlled environment, had similar responses to salinity in that increasing the 

salinity of the root medium reduced the shoot growth more than root growth, raising the 

root:shoot ratio. Likewise, Hester et al. (2001) found increased root:shoot ratios in S. alterniflora 

with elevated salinity. These studies indicate a direct relationship between salinity and 

root:shoot ratio in the tested halophytes – the implications to short- and long-term responses of 

plant production, soil strength, and wetland elevation in brackish or saline marshes receiving low 

salinity water remains open. 

Experimental responses in environmental regulation of root:shoot ratios in isolation may be 

7
misleading, and should be considered in conjunction with responses in absolute production of 

roots and rhizomes, which can be more relevant to soil strength and sediment accretion. 

Although Parrondo et al.’s (1978) study of several salt marsh species showed an increased 

root:shoot ratio with increased salinity of the root medium, both shoot and root growth 

decreased. Results from a European study were consistent. Five of six marsh species tested had 

reduced root growth when plants were grown under saline conditions (Cooper 1982). Similarly, 

Pezeshki and Delaune (1993) reported reduced root growth in S. patens when grown at 25 ppt 

compared to growth at 0-15 ppt. In general, there appears to be a negative growth response of 

roots to salt stress and the response is greatest among glycophytes, but there are exceptions. 

There are differences among species; roots and shoots of the true halophytes such as Salicornia

spp. grow best at moderate salinity. Other differences may be related to environmental 

conditions like nutrient availability, waterlogging, and the level of salinity, all of which 

interactively control growth.

Effects of salinity on plant nutrition

There are important interactions between salinity and nutrients at a physiological level that can 

affect plant nutrition and the nutrient balance of marshes. A variety of organic solutes such as 

sorbitol, proline, and quaternary ammonium compounds accumulate in the cytoplasm where they 

serve as nontoxic osmotica, i.e. these compounds reduce the water potential of the cells and serve 

the essential function of maintaining turgor pressure when the plant is exposed to salt-stress or 

drought (Jefferies et al. 1979). The nitrogen-based compounds proline and glycinebetaine are 

used for this purpose by S. alterniflora (Cavalieri 1983). These compounds accumulate more or 

less depending on the soil salinity and availability of nitrogen (Cavalieri and Huang 1979, 1981, 

Hester et al. 2001). Their cellular concentrations increase when plants are fertilized with 

nitrogen, which increases their tolerance of high salinity and their productivity (Cavalieri and 

Huang 1981).

Ironically, nitrogen uptake is depressed at high salinity due to competition among ammonium 

ions and seawater cations for carriers on the root membrane. Increasing the salinity of aerobic 

nutrient solutions from 3 to 32 ppt decreased Vmax (the maximum root-weight specific rate of 

uptake) by 30% in S. alterniflora and S. patens; while under anoxic conditions this same increase 

in salinity decreased Vmax in S. patens by 57% but had no effect on S. alterniflora (Morris 1984). 

Similarly, Brown et al. (2006) found significant decreases in uptake by S. alterniflora of all 

nutrients in response to increases in salinity. These same interactions between salinity and 

nutrient uptake should be common in all plants, because these are physiological principles that 

apply to all. 

Changing salinity levels also effect changes in sediment chemistry and thus nutrient cycling. 

The ammonium exchange capacity of sediments is greatly affected by salinity. There is 

competition among cations for exchange sites on silt and clay particles, and, owing to the super 

abundance of sea water cations that greatly exceed cations such as NH4

+

, the exchange sites will 

be completely occupied by seawater cations Na+ and K+

. Consequently, ambient exchangeable 

ammonium concentrations in freshwater sediments are generally higher than those reported for 

marine sediments (Seitzinger et al. 1991). Sundareshwar and Morris (1999) reported that the 

phosphate sorption capacity of sediments from a freshwater marsh was higher than the sorption 

8
capacity of sediments from brackish and saline marshes, due perhaps to differences in Fe and Al 

availability, surface area of sediment particles, and anion exchange capacity. 

Another salt-related factor that appears to control nitrogen release from sediment is inhibition of 

nitrification by sulfide, which is typically abundant in anoxic marine sediments. In estuarine 

sediments amended with 60 and 100 μM hydrogen sulfide, the rate of nitrification was inhibited 

by 50 and 100%, respectively (Joye and Hollibaugh 1995). A freshening of sediment, which 

would lower the sulfate and sulfide concentrations, should lead to greater rates of nitrification 

and, consequently, denitrification. In addition, the sulfate concentration of waters is an 

extremely important variable controlling phosphorus release from sediments, presumably due to 

competition between phosphate and sulfide for iron (Caraco et al. 1989). Thus, for a variety of 

physical and biochemical reasons, a freshening of sediment should lead to greater nutrient 

availability to wetlands. Consequently, to the extent that Freshwater Diversions reduce salinity, 

the efficiency of nitrogen uptake by the plant community should be increased, while the 

requirement for intracellular osmotica should decrease.

Effects of increased nutrient loads on plant productivity

There is a significant interaction between nutrients, salinity, and vegetation responses. In 

salinities < 0.8 ppt, salinity stress of S. patens can be reduced by the addition of nutrients (Foret 

2001, DeLaune et al. 2005, Merino et al. 2010). However, freshwater species, such as P.

hemitomon and S. lancifolia, are particularly sensitive to salinity increases, with mortality 

occurring at 4 and 6 ppt, respectively (Spalding and Hester 2007). Unlike more salt tolerant 

species, increased nutrients did not alleviate the effects of even minor increases in salinity in P.

hemitomon (Hester and Fisher 2011). 

In theory, the elimination of nutrient limitation will increase marsh productivity. However, there 

is variable experimental evidence for this occurring in Louisiana marsh communities. 

Significant increases in P. hemitomon shoot, root, and rhizome biomass were observed in 

experimental floating mats exposed to nitrogen loads of 50 g/m2

/y relative to 25 g/m2/y 

(Mayence and Hester 2010). Graham and Mendelssohn (2011) observed that 11 years of nutrient 

enrichment of a S. lancifolia dominated marsh with nitrogen (20-120 g m2

/y), phosphorus (6.6-

39.6 g m

2

/y) and potassium (6.6-39.6 g m

2

/y) did not significantly affect belowground 

production. Similarly, while increased nitrogen loads (55-900 g/m2/y) to field plots of S.

alterniflora resulted in 18-138% greater live aboveground biomass, there was no effect on live 

belowground biomass (Darby and Turner 2008a). By contrast, live belowground biomass 

decreased 40-60% when phosphorus or iron was added, as fewer roots were required to acquire 

limiting nutrients. Comparing the effects of nutrient enrichment on root accumulation in two 

brackish and one salt water marsh, McKee (2011b) found that there was no consistent pattern 

either within a site or over time. What is apparent from these studies is that belowground 

responses to increased nutrient loads are inconsistent and as a result, it is difficult to draw 

general conclusions. 

Nutrient enrichment has frequently been shown to provide a competitive advantage to species 

capable of rapid uptake and growth (Grace and Tilman 1990). So while biomass of P.

hemitomon may increase upon nutrient addition, nutrient additions may alter community 

9
composition and reduce species richness of these freshwater mats (Visser et al. 1999, Hester and 

Fisher 2011). Increased nutrients were one of the factors suggested to contribute to the 37% 

decrease in aerial cover of P. hemitomon in coastal wetlands from 1968-1992. This vegetation 

was replaced by Eleocharis spp. dominated marshes, whose aerial extent increased from 3-41% 

during the same time period. The influence of nutrient enrichment on plant community

compositional changes was confirmed by Hester and Fisher (2011). Nitrogen additions at 35 

g/m2/y decreased the relative abundance of P. hemitomon and increased abundance of Leersia

hexandra, while a combination of nitrogen + phosphorus (35 and 15 g/m2

/y, respectively) 

resulted in a significant increase in the relative abundance of Eleocharis spp. and Hydrocotyl

spp.

It is difficult to make direct comparisons between studies testing the effects of elevated nutrients 

associated with diversions because of differences in the form of nutrients used, particularly 

whether nitrogen was added as NH4 vs. NO3, study conditions (e.g. greenhouse vs. field), study

duration, and the magnitude of added loads. The majority of the studies were short-term (< 1

year), and while many studies used nutrient loads similar to Caernarvon peak mean annual 

nitrogen and phosphorus outflow loads of 50 g N/m2/y and 10 g P/m2/y, respectively (Lane et al.

1999), they were considerably higher than recently reported nitrogen and phosphorus loads 

within the upper basin of 4.8 to 8 and 0.56 to 1.2 g/m2/y, respectively (Hyfield et al. 2008). 

Therefore, changes in biomass and production associated with extremely high loads used in those 

studies are unlikely to be observed under field conditions. 

Effects of herbivory on plant communities

Research on herbivores in coastal Louisiana has shown that large vertebrate herbivores, both 

mammals and waterfowl, can reduce aboveground biomass by 50% (Shaffer et al. 1992, Evers et 

al. 1998, Holm et al. 2011). Specifically, herbivory by nutria, particularly eat outs where large 

areas are heavily grazed, is principally confined to fresh and brackish marshes, with few 

examples of eat outs in salt marshes. The classic paradigm that disturbance associated with 

herbivory leads to changes in plant succession that then alter herbivore populations may not 

apply to wetlands with nutria because populations remain continually high. In experimental 

exclosures in the Pearl River Basin, plant biomass and cover of palatable species like S.

americanus were higher than in plots without exclosures (Ford and Grace 1998). Herbivory 

associated with nutria may amplify the effects of other disturbances such as fire, which may 

enhance plant nutritional quality (McFalls et al. 2010). Thus, regrowth of vegetation in burned 

areas may be preferentially targeted by nutria. In a coastal marsh, S. lancifolia and Panicum 

virgatum were enhanced by fire and exclusion of herbivores at the expense of S. patens, while 

the dominance of S. patens was unchanged in plots that were burned and not fenced (Ford and 

Grace 1998). 

While nutrients may increase plant growth, under field conditions primary production may be 

reduced by secondary consumers. Salt marshes with higher nitrogen levels generally have higher 

muskrat populations (Visser et al. 2006). Visser and Sasser (2011) observed a significant linear 

increase in the number of muskrat houses as water column total Kjeldahl nitrogen concentrations 

increased. Herbivore grazing by waterfowl and nutria in the Atchafalaya and Wax Lake deltas 

significantly reduced end of season aboveground biomass between 25 and 50% (Evers et al. 

10
1998). Shaffer (2010) described how nutria became a nuisance in a waste water treatment 

wetland where plants developed elevated nitrogen concentrations in their tissues. Note that

nutrients are more concentrated in waste water than Mississippi River water, thus their impacts 

may differ. Nonetheless, the cumulative impacts of nutrients and salinity on the flora and fauna 

need to be studied. 

Observed effects of diversions on vegetation communities

The Caernarvon diversion provided the most documented effects of Freshwater Diversions on 

vegetation communities. The Caernarvon project consisted of a structure built in the levee that 

periodically diverts fresh water and its accompanying nutrients and sediments from the 

Mississippi River into coastal bays and marshes in Breton Sound for fish and wildlife 

enhancement. The introduction of fresh water from the Mississippi River was intended to reduce 

the local degradation of wetlands and enhance marine resources in Breton Sound. 

The diversion significantly changed both salinity and nutrients within the Breton Estuary (Lane 

et al. 2007). Water was fresh in the upper estuary and increased to 14 and 30 ppt in the lower 

estuary; however during spring pulses, the entire estuary was fresh for short time periods (< 1

month). From 1999 through 2004, Hyfield et al. (2008) calculated that precipitation provided 48 

to 57% of freshwater input to the estuary, while the diversion provided 33 to 48% and accounted 

for 60 to 71% of nitrogen and 43 to 62% of phosphorus loads. Sites closest to the discharge 

tended to have higher nutrient concentrations than further downstream, though nutrient 

regeneration was suggested to occur throughout the basin (Day et al. 2009b). 

Aerial mapping of vegetation types within the Caernarvon project area suggests that diversions 

caused a significant increase in freshwater marsh area and an associated decrease in brackish 

area (GEC 2011). However, this conclusion assumes no change in marsh aerial coverage from 

1988 until the diversions started in 1991. On-the-ground sampling confirmed that the diversion 

caused significant shifts in vegetation with increased coverage of submersed aquatic vegetation 

(Rozas et al. 2005). At sites influenced by the diversion, SAV coverage averaged 66%, almost 

four-fold higher than the 18% average observed at sites unaffected by the diversion. Similarly, 

while end-of-season peak live aboveground biomass of the emergent marsh was highly variable, 

the greatest biomass was found at sites within 20 km of the discharge (Day et al. 2009b). 

Unfortunately in 2005, Hurricane Katrina confounded future field assessments of the sites used 

by Day and his colleagues. Hurricane Katrina created a lateral fold which, along with an 

adjacent spoil bank, caused hydrologic isolation of one of the paired transects. However, it also 

provided opportunities for direct comparisons of a system with and without the influence of river 

water, albeit with few sites and no replication, as well as an examination of recovery following 

hurricane disturbance. Using this diminished number of sites, Moerschbaecher (2008) observed 

that aboveground biomass did not show any trend with distance from inflow. By contrast, 

belowground responses appeared to show a significant positive response to river water inputs. 

All sites exhibited increased belowground biomass during the year-long study, showing rapid 

recovery from hurricane disturbance.

Pore water salinity was significantly lower at sites nearest the Caernarvon diversion in response 

11
to freshwater input and highest when diversion discharge was lowest. Sulfide levels were 

highest at the sites that had greater salinity and when redox levels were lowest, suggesting that 

the anoxic soil conditions were a controlling factor in sulfate reduction in more saline areas. 

Total soluble sulfide levels ≥ 1 mM inhibit biomass production of S. alterniflora (Koch and 

Mendelsohn 1989). However, while a couple of sites had sulfide values > 1 mM in March 2006, 

most sites were well below 1 mM (Moerschbaecher 2008), suggesting that these sulfide levels 

were not likely to adversely impact biomass production.

Direct, large scale studies of the effects of herbivores on plant communities at diversion sites 

such are Caernarvon are lacking. The effects of herbivory may be altered by diversions in 

several ways. First, increased nitrogen loading associated with diversions may enhance eat outs 

compared to areas without diversions. Nutria (and other herbivores) prefer nitrogen-rich plants, 

so that experimental fertilization plots are sometimes preferentially targeted for grazing 

compared to unfertilized areas (McFalls et al. 2010). The scale of field studies is an important 

issue. Targeted enrichment at small sites can attract herbivores to a location compared to largescale enrichment which would lead to an increase in herbivore populations. While examples of 

large-scale nutrient enrichment leading to increased herbivory are less common, some areas 

receiving treated sewage have experienced increased herbivory by nutria (Lundberg 2008, 

Shaffer et al. 2009). Also, increased freshwater associated with diversions will likely increase 

flooding frequency and reduce salinity levels leading to changes in plant community 

composition. Any enhancement of more palatable species would enhance herbivory leading to 

reduced aboveground biomass during the growing season and reduced belowground biomass in 

the winter when nutria target rhizomes. Finally, nutria abundance in the marsh will be lower 

when the diversion is active and water levels are high, so that grazing effects will be 

concentrated during periods when water levels are low. 

Information needed

Greenhouse studies, as well as results from the Caernarvon diversion, suggest that freshwater 

inflows will reduce salt water intrusion and thus benefit freshwater plant communities. 

However, there is significant uncertainty regarding the effects of additional nutrient loads that 

occur as part of river water diversions, particularly the potential to increase belowground 

decomposition of both roots and organic matter (Swarzenski et al. 2008; also covered in further 

detail in “Effects of Diversions on Soil Strength” section). In addition, despite the differential 

response of belowground production compared to aboveground production observed in 

greenhouse studies, limited field measurements of belowground production have been made. 

Historically, belowground measurements are limited because they are labor intensive. While 

recent advances such as computed tomography (Davey et al. 2011) may be used for greater 

assessment of root responses and thus also capturing the deterioration of the organic matter, 

these approaches are still limited.

Ideally we could predict the net nutrient exchange between the water column and the sediments 

of wetlands receiving diversions of river water. Nutrient exchange will vary with time as well as 

with salinity. We can predict qualitatively that as salt desorbs and diffuses from sediments, that 

ammonium will gradually sorb onto the sediments. However, over time the sorption of 

ammonium should diminish as exchange sites become fully saturated. This type of information 

12
could be helpful for forecasting changes in water quality.

Because soil salinity fundamentally impacts sediment biogeochemical processes, and the 

diversion of water and sediment from the Mississippi River will impact salinity as well as 

nutrient flux, information is needed on how these variables interact. First and foremost, more 

information is needed about the impact of a change in salinity, particularly a lowering of salinity, 

on the nutrient balance of the wetland soils. As discussed above, the cation exchange properties 

of sediments change dramatically with salinity. Sediments will sorb nutrients as salinity is 

reduced, but the capacity will likely depend on the degree of freshening. Additionally, 

anticipating the pulses of salinity brought in by storms, it would be useful to know how the 

sediment nutrient balance changes with aperiodic salt intrusions. 

Studies to evaluate the best pulsed approach to maximize sediment transport to the downstream 

marsh, while also providing freshwater flow when the probability of salt water intrusion to the 

freshwater marshes is greatest, would provide much needed information.

Increased wave action and high diversion flow rates were both expected to have negative effects 

by increasing the potential to uproot vegetation. However, limited documentation of such effects 

exist, particularly flow rates. Floating marshes were particularly sensitive to hydrologic 

disturbance, but also returned quickly. Thus, greater information on the effects of flow and wave 

action would be beneficial in understanding plant community responses. 

The effects of freshwater pulses on floating mats appear dependent on the stage of mat 

development, thus a greater understanding of mat successional trajectories would improve 

predictability of responses to diversions.

Information is needed on the degree to which invasive species (e.g. Phragmites, Typha) will 

expand following a freshening of salt and brackish marshes. The effect of freshening on invasive 

species will have direct as well as indirect effects. Because many studies were short-term and 

laboratory-based, there is limited information on plant community composition changes as a 

result of increased nutrients. Invasive species like Phragmites may expand as a consequence of 

increased nutrients as well as reduced salinity. Recent evidence confirms that Phragmites 

australis communities along the Louisiana coast are comprised of both native and non-native 

geonotypes (Howard et al. 2008). The non-native genotype spread to cover about 82% of the 

plots to which it was introduced. By contrast, the native genotype only covered 18%. Thus 

further information on the spatial extent of non-native Phragmites should be obtained so that true 

estimations of invasion potential under changing salinity and nutrients can be obtained. Also, 

invasion of Typha species with softer belowground roots/tubers is possible, and could be 

detrimental overall. Typha occurrence is frequently correlated with ponding water in coastal 

Louisiana freshwater systems (Penfound and Hathaway 1938).

While the consensus among many Louisiana wetland scientists at the NOAA Diversion 

Workshop held in Lafayette, Louisiana, in February 2010 

(http://www.mvd.usace.army.mil/lcast/pdfs/DiversionWorkshop.pdf) seems to be that eat outs

do not lead to fundamental changes in a wetland, confirming this view appears to be the big 

unknown, especially in the context of diversions and changes in plant communities.

13
Managers have the following specific information needs:

1. What is the potential threat of salt water intrusion and how could diversions be operated 

to mitigate for this poorly understood possibility?

2. What is the effect of diverted river flow on the partitioning into below- and aboveground 

production of plant communities? Will this partitioning affect marsh sustainability?

3. Do diversions result in plant communities that are less or more resilient to scour from 

hurricane storm surge and other heightened hydrodynamic flows?

4. In what ways does the marsh vegetation response depend on the operation of the 

diversion (duration, discharge rates, seasonality, etc.)? 

5. What is the effect of diverted river flow (primarily nutrients, but other components as 

well) on herbivory of wetland vegetation? Do diversions result in herbivory levels that 

would outweigh potential benefits to other ecological resources?

6. Would it be possible to eliminate nutria (it was done in England on a small scale)? Would 

the enhancement of large predators (e.g. large alligators) reduce nutria populations?

Conclusions

• In assessing the responses of marsh vegetation to Freshwater Diversions, it is important 

to consider the effects of low salinity, high nutrient water on both absolute plant 

production and the relative production of aboveground and belowground portions, the 

latter an important factor in maintaining soil strength critical for long-term marsh 

sustenance. 

• A general conclusion on the expected short-term and long-term responses of marsh 

belowground production to Freshwater Diversions in Louisiana could not be drawn from 

the available evidence:

o Salinity responses: Freshening of sediments should lead to greater efficiency of 

nutrient uptake by wetland plants, based on physiological considerations, and 

some studies have shown an inverse relationship between salinity and root 

(belowground) growth in Louisiana marsh plants; however, a direct relationship 

between salinity and root:shoot ratio (belowground to aboveground production) 

was found in experiments with tested halophytes. Freshening of sediments should 

lead to greater rates of denitrification and alter the partitioning of phosphate in 

sediment. 

o Nutrient responses: Studies on the response of marsh production to nutrient 

increases also drew variable conclusions, but in general, aboveground biomass 

increased and belowground production was either not affected, increased, or 

decreased in response to nutrient enrichment. 

• Studies on the Caernarvon Freshwater Diversion found that freshwater marsh area was 

increased by diversions, with highest aboveground biomass near the discharge site. 

Following Hurricane Katrina, a relative increase in belowground production was 

observed at Caernarvon sites receiving river water, indicating a stimulatory effect of 

diversions on belowground production at least in the short-term (i.e. rapid recovery from 

hurricane disturbance).

• Evidence suggests that nutrient enrichment from Freshwater Diversions could cause plant

14
species compositional changes (e.g. experimental findings that nitrogen additions 

decreased the relative abundance of P. hemitomon), but a precise understanding of 

expected compositional responses is limited because many studies were short-term and 

laboratory-based. 

• Evidence supports an effect of secondary consumers (e.g. herbivores such as nutria) in 

responding to nutrient-enriched plants (i.e. better forage quality) and reducing 

aboveground biomass; the quantitative impact of herbivory on vegetation responses to 

Freshwater Diversions needs to remain a focus in predictive models. 

Suggestions for moving forward

• Measurement of aboveground biomass alone is insufficient to assess the impacts of a 

Freshwater (or Sediment) diversion on plant production. 

• Existing Freshwater Diversions offer an experimental setting to examine some aspects of 

their effects on plant communities but only with rigorous statistical designs, sufficient 

replication, and appropriate reference marshes. Key studies include:

o partitioning between above- and belowground biomass

o plant community composition

o herbivory

Effects of Diversions on Wetland Elevation 

Karen L. McKee

Background

Controlled diversions to build land and promote wetland sustainability in coastal Louisiana were 

reported beginning in the early 1970s (Gagliano et al. 1973, Roberts et al. 1984, Soileau 1984, 

van Heerden 1994, USACOE and LADWF 1998, Council 2006, NRC 2006). There is some 

confusion in the literature over the stated purpose(s) of diversions with respect to “reducing 

marsh loss”. It is important to know whether a diversion was designed to reduce marsh loss by 

directly altering surface accretion of mineral sediment or by modifying plant growth through 

changes in salinity and/or nutrient regimes. Some diversions were initially designed primarily to 

reduce “saltwater intrusion”. For example, the Caernarvon Freshwater Diversion was created to 

address “the alarming rate of loss of these wetlands....caused by many factors, among them 

saltwater intrusion.” (USACE and LADWF 1998). However, a later report (LADNR 2003)

stated that one of the objectives was to reintroduce sediments, along with freshwater into 

marshes of Breton Sound estuary. This 2003 report, based in part on several studies conducted 

by scientists at Louisiana State University and University of Louisiana-Lafayette, reported 

effects of the diversion on salinity, wildlife, and vegetation (mainly shifts in marsh vegetation 

type related to salinity changes). However, this 2003 report was mainly focused on salinity and 

consequent effects on wildlife and vegetation composition, and did not include an examination of 

elevation changes (although accretion rates were measured by Delaune et al. 2003). 

The underlying assumptions (relative to wetland vegetation) apparent in these reports are that: 

(1) saltwater intrusion was the primary cause of wetland loss, because saltwater intolerant 

15
species were killed faster than tolerant species could invade, and (2) amelioration of salinity 

stress would promote vegetative growth overall (perhaps aided by addition of nutrients and 

sediment). Several early reports suggested that introduction of freshwater and nutrients, in 

addition to sediments, might offset factors causing deterioration of brackish and freshwater 

marshes (Gagliano et al. 1973, Roberts et al. 1984). These assumptions can be traced, in part, to 

Gagliano et al. (1973), who pointed out that, in addition to sediment from controlled diversions 

to build land, “relatively small amounts of fresh water introduced into the upper ends of 

interdistributary estuary systems could be used to offset salinity intrusion and introduce badly 

needed nutrients, both of which would offset conditions leading to rapid deterioration of 

brackish and fresh marshes.” Nowhere in this early report, however, are there data supporting 

the contention that the rapid deterioration of wetlands was caused by saltwater intrusion or lack 

of nutrients. Clearly, these early workers were proposing that freshwater introduction would 

lower salinity and deliver nutrients, but would be conducted in conjunction with sediment 

loading sufficient to build land. A later Louisiana Department of Natural Resources (LADNR)

report (Roberts et al. 1984) developed specific recommendations for a Freshwater Diversion into 

Barataria Basin (Davis Pond). This report also emphasized modification of salinity regimes and 

restoration of freshwater marsh, but predicted that nutrients and sediments would increase marsh 

productivity and that “..increased organic production along with increased suspended sediment 

input will help retard subsidence-induced marsh loss in the basin.”

The key point is that since the early 1970s scientists have recognized the need for interventions 

that directly address the cause of wetland loss: failure to maintain elevations sufficient to support 

emergent vegetation forming the wetland habitat. For sustainable wetland restoration, marsh soil 

and vegetation responses to low salinity/high nutrient water influx should be considered in light 

of the long-term maintenance of elevation. 

Wetlands must maintain their soil surfaces relative to prevailing water levels to persist. This 

balance is achieved when the rate of land building equals the rate of submergence (Cahoon et al. 

2006). Despite this simple relationship, the processes involved are complex and interconnected 

by feedback linkages (Figure 2). The processes controlling wetland elevations may be physical, 

biological, and/or hydrological, and operate in response to drivers such as sea level change, river 

flows, storms, climate change, nutrient loading, and human modifications. If the rate of 

submergence increases, soil accretion rates must also increase. If not, increased depth and 

duration of flooding at a lower soil elevation will stress the plants (Mendelssohn and Morris 

2000), eventually exceed their tolerance limits, and cause plant death and wetland loss (Kirwan 

et al. 2010). The processes occurring at or below the soil surface that influence vertical 

movement of a wetland include deposition/erosion of organic or mineral sediment on the soil 

surface, physical compaction of deposited material, soil shrinkage/expansion due to water flux, 

and organic matter production/decomposition (Cahoon et al. 2006, Cahoon et al. 2011, McKee 

2011a,b). These processes do not occur in isolation, but interact to drive elevation change.

16
Figure 2. Diagram showing processes and feedback linkages influencing marsh elevation.

Many wetlands, including those in the Mississippi River Delta, depend on both mineral and 

organic inputs to soil volume for elevation maintenance (Turner et al. 2006, Neubauer 2008, 

McKee 2011b), although the relative contribution may vary with wetland type and location; e.g.

freshwater versus saline, inland versus coastal. Consequently, any alteration in conditions 

influencing accumulation of organic or inorganic materials may alter elevation dynamics and 

marsh stability. Wetlands in the Mississippi River Delta portion of coastal Louisiana are 

deteriorating because levees and control structures along the Mississippi River starve the 

wetlands of sediment and prevent the river from switching course to form a new delta (Blum and 

Roberts 2009). Activities such as canal dredging, fluid withdrawal, spoil bank construction, and 

impoundment construction have altered hydrology, salinity regimes, and subsidence rates 

(Boesch et al. 1994). 

River diversions may change conditions such that either organic or inorganic contributions to 

soil volume increase. A diversion might contribute to elevation gain by delivering: (1) material 

(mineral sediment) directly to marsh surfaces, increasing rates of accretion and/or (2) freshwater 

and nutrients to stimulate plant production (biomass, stem density), which in turn promote 

sediment deposition and trapping (aboveground) and/or increase organic matter accumulation 

(primarily belowground where decomposition is slow). Plant production in coastal wetlands 

may be modified by changes in factors such as salinity (Broome et al. 1995, Merino et al. 2010), 

nutrients (Darby and Turner 2008b), and phytotoxins (sulfide) (Koch and Mendelssohn 1989), as 

discussed above, or flooding depth/duration (Broome et al. 1995, Howard and Mendelssohn 

17
1995). 

Environmental factors that alter marsh elevation

Mississippi River diversions deliver freshwater and constituents (e.g. nutrients, sulfate, and other 

dissolved ions), usually in pulses, that change water chemistry in the receiving basin and alter 

marsh productivity and organic accretion. The Caernarvon diversion greatly modified salinity in 

the Breton Sound estuary, generating a pattern from fresh in the upper estuary to 14 to 30 ppt in 

the lower estuary; spring pulses, however, propagated freshwater conditions throughout the 

estuary for short periods (< 1 month) (Lane et al. 2007). A lowering of salinity may benefit 

freshwater plant communities, which are vulnerable to saltwater intrusion (Flynn et al. 1995), 

and enhance plant growth in intermediate or brackish marshes (Broome et al. 1995, DeLaune et 

al. 2005). However, changes in salinity may also drive shifts in plant community composition 

(Sasser et al. 1986, DeLaune et al. 1987, Baldwin et al. 1996) with concomitant changes in 

production-decomposition processes influencing organic matter accumulation in soils.

Nutrient inputs may also stimulate plant production, either through a direct fertilization effect 

(DeLaune et al. 2005, Izdepski et al. 2009, Merino et al. 2010) or indirectly by introducing iron, 

which may precipitate phytotoxic sulfides (DeLaune et al. 2003). Conversely, addition of 

nutrients may reduce belowground production of biomass, since fewer roots would be required to 

forage the soil for limiting nutrients (Darby and Turner 2008a, Ket et al. 2011, see previous 

section, "Effects of Freshwater Diversions on Wetland Plants"), or may increase organic matter 

decomposition (Laursen 2004, Turner 2011a,b). However, no studies have been conducted that 

experimentally examined the direct effects of river diversions on organic matter decomposition,

although one study correlated Mississippi River water influx with more decomposed root mats in 

a P. hemitomon freshwater marsh in the Penchant basin (Swarzenski et al. 2008). The marsh 

with riverine input had more reducing soils, higher sulfide (a phytotoxin), and higher alkalinity 

in addition to faster decomposition and loss of soil strength, compared to a marsh without river 

influence. Greenhouse and field studies are needed to establish a cause and effect relationship 

between river input and marsh response (in different marsh types) and to better predict how 

diversions that alter nutrient regimes may influence marsh-building processes.

Some river diversions may deliver sediment to the receiving basin and contribute to accretion 

directly, although the amounts vary with the diversion structure, magnitude of flow, and river 

stage (Snedden et al. 2007). The Caernarvon diversion, for example, delivered 1.64 x 103 t/d in 

sediment discharge when the Mississippi River was falling and 3.29 x 103 t/d when it was rising 

(Snedden et al. 2007). These inputs, however, were orders of magnitude lower than historical 

discharges (via crevasses or uncontrolled diversions) and deemed insufficient to offset current 

sea level rise. How much of the discharged sediment ends up on the marsh surface depends upon 

a suite of interacting forces, including water velocity, tides, winds, water levels, and suspended 

sediment load (Day et al. 2009a). Measured deposition can be as high as 2.3 cm/y in the shortterm (sediment traps), but lower (0.26 to 0.30 cm/y) over longer time periods (210Pb or 137Cs 

geochronology) (DeLaune et al. 2003, Wheelock 2003). A key question is where deposition 

measurements were made in the marsh landscape: in the interior marsh or near waterways?

Wheelock (2003) found that deposition of mineral sediment (short-term sediment traps) from 

Caernarvon varied with distance from the diversion, proximity to a major waterway, and with 

18
season. Sediment deposition decreased with distance from the diversion at exterior (marsh edge) 

positions, but was low and did not vary with distance at interior marsh sites. This finding has 

important implications for monitoring and predicting movement of sediments delivered by 

diversions into areas with existing (but deteriorating) marshes. 

Observed effects of diversions on marsh elevation

To our knowledge, there are few published data reporting effects of diversions on marsh 

accretion (DeLaune et al. 2003, Wheelock 2003, Lane et al. 2006, Moerschbaecher 2008, Day et 

al. 2009a) and only one study that measured elevation change in relation to diversion structures 

(Lane et al. 2006). This lack of data is surprising considering the importance of such 

information. A study by Delaune et al. (2003) used both feldspar markers and 137Cs dating to 

assess short-term and long-term accretion rates in the Breton Sound estuary marshes at different 

distances from the Caernarvon diversion (total of 20 sites). Sites were grouped into those in the 

upper basin (closest to the diversion) and those in the lower basin. Highest rates of accretion 

were measured closest to the diversion discharge (within ~12 km: 1.72 cm/y) compared to 

farther away (~12 to 30 km: 1.34 cm/y). Long-term rates measured by 137Cs dating also were 

higher closer to the diversion. In addition to accretion of mineral matter, organic matter 

accumulation (above feldspar) was higher closest to the diversion (432 vs. 166 g/m/6 mo). 

Wheelock (2003) also found higher short-term and long-term deposition closer to the diversion, 

although this pattern only held for sites closest to waterways and not in interior marsh; 

sedimentation was additionally influenced by seasonal variation in winds and other factors. 

These results must be viewed with caution, since there were no reference sites (outside the 

diversion basin) or pre-diversion measurements of accretion, which would be required to 

conclusively demonstrate that the observed differences between upper and lower basins were due 

to effects of the diversion (rather than natural variation across the basin). Also, accretion does 

not equal elevation change, which determines whether a marsh is keeping up with the rate of 

submergence. To fully assess how a diversion may be affecting marsh capacity to keep up with 

relative rates of sea level rise, it is necessary to determine rates of surface elevation change in 

relation to a stable benchmark. The technology to make high-resolution measurements of marsh 

elevation change, accretion, and shallow subsidence has existed for at least ten years (Cahoon et 

al. 2000, Cahoon et al. 2002a, Cahoon et al. 2002b), and is in use at most Coastwide Reference 

Monitoring System (CRMS)-Wetlands monitoring stations 

(http://www.lacoast.gov/crms2/Home.aspx).

Studies by Day and coworkers are the primary sources of information on how diversions may 

affect marsh elevations. Based on a series of field investigations, they concluded that the 

marshes near diversions were generally healthy, accreting, and keeping pace with relative sea

level rise (Lane et al. 2006, Moerschbaecher 2008, Day et al. 2009a). One field study 

investigated the effects of three diversions (Caernarvon, West Pointe a la Hache (WPH), and 

Violet) on accretion, subsidence, and elevation change rates in the receiving basin marshes (Lane 

et al. 2006). At each diversion, three sampling distances were used, each with duplicate stations 

where measurements were made from summer 1999 to spring 2000. This sampling design 

specifically allowed testing of whether the measured rates varied with increasing distance from 

the diversion structures. All sampling stations at Caernarvon and WPH showed sediment 

19
accretion and elevation gains over the study period, whereas substantial elevation losses occurred 

at all Violet sites (Table 2, Figure 5 in Lane et al. 2006). The results, however, did not show a 

consistent pattern with distance from the diversion structures, which would be expected if the 

diversions were influencing elevation dynamics. At Caernarvon and WPH, there were no 

significant differences in elevation change rates with distance from diversions, and elevation 

losses were significantly greater at stations closest to the Violet diversion (Figure 5 in Lane et al. 

2006). Accretion rates at WPH were significantly higher closest to the diversion, but at 

Caernarvon, the highest accretion occurred farthest from the diversion; there were no differences 

in accretion with distance at Violet (Figure 5 in Lane et al. 2006). A companion study of the 

Caernarvon diversion examined effects on plant biomass production in Breton Sound estuary 

from March 2006 to October 2007 in duplicate plots located at 3.4, 12.3, and 18.1 km from the 

diversion structure (Moerschbaecher 2008). This study found high above- and belowground 

biomass in the Breton Sound study area, comparable to or exceeding reference marshes. 

Biomass production did not vary consistently with distance from the diversion structure, but this 

evaluation was confounded by the spatial variation in plant community composition across the 

sampling stations (freshwater species closer to the diversion, saltwater species farther away). 

Failure to detect a significant difference with distance might be attributable to insufficient 

replication or a sampling design unable to account for other environmental or biological (e.g.

species composition) gradients. Even though the studies by Day and coworkers were unable to 

detect a clear effect of diversions on marsh elevations, the data indicated that the marsh areas 

sampled were for the most part productive and keeping pace with relative sea level rise. 

However, these results cannot say whether these sites were already productive and accreting or if 

these values reflect an effect of the diversion. Also, it is unlikely that these data are 

representative of the entire receiving basins. Considering the areal extent (Caernarvon: 1100 

km2

, Violet: 50 km2, West Point a la Hache: 68 km2) and range of marsh types present 

(freshwater, brackish, and saline), six sampling stations are inadequate to characterize the 

marshes or determine health and stability throughout the areas of interest.

In summary, existing data showing diversion effects on marsh accretion or elevation are sparse 

and based on limited sample sizes and/or inadequate sampling designs. These points are raised 

to emphasize data gaps and key considerations in designing future monitoring plans. The 

existing data are nonetheless useful because they provide some insight into accretion and 

elevation dynamics in diversion basin marshes. Published work on salinity and nutrient effects 

on marsh production-decomposition processes also provide hints as to how changes in water 

chemistry might alter organic contributions to soil volume. However, many questions remain 

with regard to how changes in water chemistry may influence elevation dynamics. 

Information needed

Data on how diversions influence marsh elevations and dynamics are needed, but have not been 

acquired in any systematic or comprehensive manner. Specifically, the following questions 

should be addressed:

1. Do diversions promote accretion (mineral or organic) and elevation gains that balance 

rates of submergence in receiving basin marshes? The answer requires knowledge of 

20
pre-diversion rates and/or rates in reference marshes unaffected by diversions.

2. How far does the influence of diversions on marsh elevation extend from the discharge 

structure, and does the zone of influence coincide with measured changes in sediment 

accretion, environmental conditions, and/or changes in plant community composition or 

productivity?

3. Which processes controlling marsh elevations (e.g. accretion/erosion, subsidence, organic 

matter accumulation) are most influenced by diversions, and what environmental changes 

(sediment, nutrients, salinity) are most influential in this regard?

4. What are the tradeoffs among factors controlling marsh elevations that are affected by 

diversions (e.g. nutrient effects on biomass production vs. decomposition)?

5. Are elevation dynamics in all marsh types similarly affected by diversions (or are some 

more sensitive to certain changes in environmental conditions caused by diversions)?

6. What existing features might modify the impact of a diversion on elevation dynamics 

(e.g. barriers to water flow)?

Conclusions

• For wetlands to persist, they must maintain their soil surfaces relative to prevailing water 

levels. 

• For river diversions to succeed in the goal of reversing marsh loss, they must contribute 

to vertical land development at a rate that balances relative sea level rise (change in ocean 

surface height plus local land movement). 

• Diversions may promote vertical land development by (1) adding mineral sediment 

directly to marsh surfaces increasing rates of vertical accretion or (2) by stimulating plant 

production that promotes sediment deposition and trapping (aboveground) or by 

increasing organic matter accumulation (belowground) and soil volume.

• Delivery of nutrients or lowering of salinity may stimulate plant production but may also 

increase rates of organic matter decomposition, the latter potentially leading to loss of 

soil volume. However, empirical data showing a cause and effect relationship between 

river-induced changes in water chemistry and marsh response are lacking.

• A limited number of studies have examined sediment accretion rates in relation to 

diversions and the results show variable deposition patterns depending on distance from 

the diversion, proximity to major waterways, existence of barriers to flows, and with 

season.

• Only one study has directly examined the effect of diversions on marsh elevations and

did not conclusively show that diversions increased net elevation gain.

• Overall, there have been few studies to examine diversion effects on marsh accretion and 

elevation and, although providing some insights, were based on limited sample sizes 

and/or inadequate sampling designs (necessary to rigorously test the effects of diversions 

on either accretion or elevation change). 

• Major information gaps exist with respect to how much diversions may alter marsh 

elevation trajectories (especially in relation to balancing relative sea level rise), which 

specific mechanisms influencing soil volume (organic vs. inorganic) might be most 

affected by diversions, and what other factors might modify the impact of diversions on 

marsh elevation dynamics.

21
Suggestions for moving forward

• Measurement of accretion (i.e. deposition of organic or inorganic sediment on the soil 

surface) alone is insufficient to assess the impacts of a freshwater (or sediment) diversion 

on the long-term stability of a wetland. Information about subsurface movement, along 

with regional sea level trends, is necessary to evaluate the capacity of a wetland system to 

avoid submergence.

• Existing freshwater diversions offer an experimental setting to examine some aspects of 

their effects on elevation dynamics, but only with rigorous statistical designs, sufficient 

replication, and appropriate reference marshes.

• Existing permanent monitoring stations (e.g. the Coastwide Reference Monitoring 

System) provide additional means to quantify spatial and temporal variation in marshbuilding rates and correlations among processes, which will aid in interpreting observed 

responses at existing diversions.

• Emerging data from wetland elevation studies conducted both within and outside the 

region can contribute to adaptive management of existing freshwater diversions by 

suggesting possible outcomes of specific modifications and by contributing to a more 

comprehensive database to guide decisions.

Effects of Diversions on Wetland Soil Strength 

Charles Hopkinson and William Orem

Background

Soil strength describes the geotechnical characteristics of soils and is typically used in designing 

earthworks and foundations. Geotechnical investigations include surface and subsurface 

exploration of a site and in situ and laboratory tests that measure soil properties. Shear strength 

defines the ability of soils to resist displacement or deformation when subjected to shear stresses. 

However, there are no American Society for Testing and Materials (ASTM) standard test 

methods for determining the shear strength of coastal wetland soils, which are typically finegrained cohesive sediments (silts and clays) with high organic matter content, oftentimes 

extensive root mats, and flooded or at least saturated with water during high tide. Shear strength 

is thought to be an indicator of wetlands resistance to shear stresses (e.g. vegetative cover loss) 

from storm-induced waves, currents, and pressure fluctuations, for example during a hurricane. 

 

The shear strength of soils is affected by numerous factors including soil composition (e.g. 

mineralogy, grain size, particle shape, ionic forces, organic matter content), void ratio, water 

content and pore water chemistry (e.g. salinity, pH) , the soil structure (e.g. particle arrangement, 

fissures, cementation), and loading conditions (e.g. magnitude of shear stress, rate, history). 

Shear strength of a soil is also controlled by plant roots and rhizomes: it increases proportionally 

to the cross sectional area of roots crossing the shear plane. Roots add strength because of the 

tensile force required to break individual roots and rhizomes (Howes et al. 2010). Roots also 

contribute strength through their frictional contact with soils. 

Two instruments have been employed in coastal Louisiana wetlands to gauge shear stress: the 

22
Dutch cone penetrometer (Are et al. 2002, Mullins and Fraser 2006) and field vanes (Howes et 

al. 2010, Holm et al. 2011). The penetrometer is widely used in geotechnical analyses of soils. 

It provides measures of tip resistance and shear resistance with depth in soil and can be used to 

calculate other important parameters of soft clay wetland soils, including soil cohesion and stress 

strain modulus. The field-vane torque meter as used in Louisiana wetland soils measures the 

combined resistance along the length of vanes. It must be used with extracted cores or with 

extensions to examine how strength varies with depth. It must be used carefully to examine the 

degree to which the top root mat is tied or bound to lower depths. Additional geotechnical 

analyses provide insight into strength discontinuities in the soil profile, for example bulk density, 

organic matter content, rooting depth, and root density/strength. 

Environmental factors that alter soil strength

Information on wetland soil strength helps us better understand the processes and factors that 

contribute to the sustainability of Louisiana coastal wetlands. Theoretically, if shear stress (e.g.

during storms) exceeds soil strength, wetland soils will be lost or swept away. There are distinct 

differences in soil strength in wetlands undergoing rapid degradation and loss versus other 

wetlands that show little degradation over the past couple of decades (Day et al. 2011, Holm et 

al. 2011). Discontinuities in the soil profile of soil strength can increase the vulnerability of 

overlying wetlands regardless of degradation condition (Howes et al. 2010). We need to know 

how soil strength changes when subjected to Freshwater Diversions and associated decreases in 

salinity, increases in nutrients, and alterations in flooding and wetting regimes. 

Storm and hurricane surge modeling and predictions of water level, short wave height, pressure 

field variation, and frictional energy dissipation due to bay bottom and wetland vegetation 

interactions were not included in most storm surge models of the Louisiana coast prior to 

Hurricane Katrina (NRC 2006). The wave energy lost in passing over marsh vegetation and the 

ability of the marsh/root/soil complex to hold up against wave shear stress are the reasons 

wetlands provide a buffer for inland property. These factors are critical to the sustainability of 

wetlands. Wave shear stress is related to the wave friction factor, which is influenced by stem 

density, canopy height, and leaf geometry (Suhayda and Jacobsen 2008, Howes et al. 2010). 

Examples abound in the Louisiana coast that hurricanes can rearrange the deltaic landscape and 

contribute directly to wetland loss through storm surge and shear physical force (Barras et al. 

2008). Such loss has been documented both in newly emergent marshes formed in the vicinity 

of the mouth of the Atchafalaya River and in geologically older marshes that maintain elevations 

above sea level primarily through organic accretion (Howes et al. 2010). Failure of both types of 

wetlands, across strong gradients in marsh type and mineral and organic content, demonstrates 

that multiple factors contribute to wetland soil strength and influence how marshes respond to 

excessive wave shear strength. 

In 2005, hurricanes Katrina and Rita caused marsh loss throughout coastal Louisiana, including 

Breton Sound marshes in the immediate outfall area of the Caernarvon freshwater diversion 

(Barras 2006, 2007). Raynie (2011) attributed the loss of wetlands in this location to natural 

processes, and excess hydraulic energy in this area due to the funnel configuration of the 

elevated levees. Howes et al. (2010) attributed the extensive loss in the Breton Sound marshes in 

23
part to geomorphic amplification of wave shear stress as well as soil strength weaknesses due to 

shallow rooting depths and weak layers in the soil profile. A lens of inorganic material deposited 

by a deliberate levee breach of the 1927 flood was thought responsible for the weakness because 

it lacked the abundant roots and rhizomes present above. Kearney et al. (2011) documented 

patterns of marsh loss in the same area and hypothesized that operation of the Freshwater 

Diversion contributed disproportionately to the loss of marsh in this area, in part by the 

weakening of organic soils through excess nutrient influx. Considerable uncertainty remains 

therefore whether Freshwater Diversions serve to exacerbate or ameliorate the effects of natural 

physical forces, such as storm surge, on the marsh landscape.

Soil strength increases linearly with increasing live belowground material, when comparing 

across a variety of wetland species (Holm et al. 2011). This relationship is very strong for the 

freshwater plant P. hemitomon and the salt marsh plant S. alterniflora. The relationship is 

weaker for the fresh marsh plant, S. lancifolia, and the black mangrove, A. germinans. Of the 

marsh sites studied to date, Panicum demonstrated the highest live root mass and the highest 

shear strength while S. lancifolia showed the lowest live root mass and the lowest shear strength. 

Thus type of marsh per se (i.e. salt, brackish, or fresh) seems to bear little relation to soil 

strength. It is the species present and its root biomass in various marshes that is most important 

(Holm et al. 2011). 

Howes et al. (2010) may have over-generalized when they concluded that fresh marsh was more 

susceptible to hurricanes than salt marsh. Their conclusion may reflect site specific properties 

that could be expected to differ greatly from site to site within Louisiana. At their study sites, S. 

patens growing in tussock form with low and shallow root mass in an organic rich fresh soil 

contributed less to soil strength than S. alterniflora growing with high and deep root mass in a 

higher mineral content salty soil. If their study had compared P. hemitomon and S. alterniflora

marshes, the fresh vs. salt marsh conclusion may have been reversed.

Compaction brought about by desiccation, drainage, or overloading also contributes to soil 

strength even within the same type of marsh (e.g. salt marsh). Day et al. (2011) showed that 

Bayou Chitigue marshes, which seldom drain, generally lose surficial mineral deposits while 

Oyster Bayou marshes, which do drain and dry out, retain surficial sediments. This is especially 

important when surficial deposits are comprised of fine grain mineral matter and organic matter 

(Paola et al. 2011). Shear strength of Oyster Bayou surface marsh sediment was 10 time stronger 

than at Bayou Chitigue. Drainage and desiccation tend to reduce void space, increase strength, 

and thereby resist subsequent depositional loads without deformation. The Day et al. study 

confirmed results of Holm et al. (2011) and Howes et al. (2010) by showing that root growth 

contributes to soil strength, even while contributing to void space, which otherwise translates 

into soil weakness. 

The mechanical strength of well-preserved peat deposits (fibric peat) derives from the 

preservation of plant biopolymers (mostly lignocellulose) in the deposited organic material, and 

extensive root structure from macrophytes living on the peat substrate. Degraded peats (sapric) 

and peats with less extensive rooted biomass have lower mechanical strength and may be subject 

to erosion (e.g. roll up and sloughing) during storm surge. There is limited evidence that 

nitrogen and phosphorus loading promotes peat degradation and thereby lowers soil strength 

24
below the rooting zone (Turner 2011b), but only under loading rates in excess of those used on 

highly fertilized agricultural fields. Similar levels of loading have not caused declines in soil 

strength in the rooting zone. 

Observed effects of diversions on soil strength

River diversions have introduced nutrients, sulfate, and other chemical species into freshwater 

wetlands that were previously low ionic strength oligotrophic environments (Meade 1995,

Goolsby et al. 1999, Goolsby 2000, Goolsby and Battaglin 2000). Decreases in organic soil 

strength resulting from such diversions has been suggested (Darby and Turner 2008a,b,

Swarzenski et al. 2008, Turner et al. 2009, 2011b) as one factor in the well documented loss of 

wetland area in this region (Boesch et al. 1994, Barras et al. 2003, 2008). Other studies suggest 

that river diversions promote peat growth and do not decrease the mechanical strength of organic 

soils (DeLaune et al. 1983, 2003, Nyman 2011). 

Water quality could impact soil strength in several ways. Nutrients stimulate macrophyte 

growth, but because the nutrients are available in the surface water, plant roots may not be as 

extensive or deep as in wetlands not affected by diversions (Darby and Turner 2008a). A 

wetland could have luxuriant macrophyte growth on the surface, but lack extensive belowground 

growth providing peat stability. Considering that root distribution and biomass correlate with 

soil strength, it is important to document changes not only in aboveground plant growth and 

root:shoot ratios, but also absolute changes in root mass and distribution when evaluating the 

effects of changes in river water and associated properties on soil strength. Nitrogen enrichment 

and eutrophication may change macrophyte community composition, which might also affect 

soil strength. Eutrophic-adapted species may replace species adapted to more oligotrophic 

conditions, and this change may impact soil strength. For example, in eutrophied portions of the 

Florida Everglades cattails have displaced sawgrass as the dominant macrophyte. Cattail peat is 

much more sapric compared to the fibric nature of the sawgrass peat, and therefore more subject 

to degradation and erosion. On the other hand, nutrients promote aboveground plant growth and 

additional biomass for accumulation, which would seemingly work to increase peat growth and 

the trapping of suspended solids when flooded. 

Nutrients may stimulate overall microbial activity leading to a greater rate of organic matter 

decomposition and hence loss of soil strength (Turner 2011b). Stimulation of nitrate and sulfate 

reduction following changes in diversion water flow may lead to enhanced nutrient release, 

further stimulating overall anaerobic microbial activity; but of course enhanced nutrient release 

could promote greater plant root growth. Furthermore, these terminal anaerobic processes may 

increase the degree of syntrophy in peat soils leading to increased overall biodegradation.

Are river diversions causing degradation of peat soil in south Louisiana through destabilization 

of soil strength, or are they a positive influence promoting plant growth and peat accretion with 

no adverse impact on organic soil strength? Unfortunately, there is no clear cut answer so 

further research will be needed to resolve this question. While differences in experimental 

design or measurement approaches could explain the different results from various research 

studies, perhaps the differences also reflect real differences in effects from different settings. 

This will need to be resolved.

25
Information needed

It is important to determine the suite of factors acting simultaneously that result in soil strength 

changes. For instance, if a change in soil strength is observed in wetlands receiving diversion 

materials (water, sediment, nutrients), is the change due to singular changes in salinity or nutrient 

levels, or to the combined effects of salinity and nutrients as well as any resultant changes in 

wetland plant composition? The effects of diversions on soil strength in wetlands undergoing 

severe and rapid successional change have not been clearly determined.

It is useful to consider idealized profiles of shear strength versus soil depth and how variations in 

those profiles contribute to degradation vulnerability and loss during storms. With uniform 

wetland soil composition over depth, we expect soil strength to increase with depth, primarily as 

a result of compaction by overburden loading. Surface soil drainage and desiccation enhances 

compaction, which can increase soil strength near the surface. Wetland ponding, regardless of 

cause, decreases soil strength and increases wetland vulnerability to storm scour and loss. Plant 

roots also change the depth profile of soil strength, resulting in increased soil strength in 

shallower soils where roots are present. Abrupt changes in soil strength slope vs. depth, 

especially from positive to negative slope, can result in failure planes - regions above which 

entire blocks of wetland soil can be scoured (Howes et al. 2010). Diversions can increase the 

scour vulnerability if they result in ponding or decreases in rooting depth or root tensile strength.

Specific information needs include:

1. Is soil strength a good indicator of the ability of marshes to resist scour or destruction 

from storm surge and waves?

2. What is the critical point between soil strength and the shear stress where the marsh 

plant/root system separates from the underlying sediment and how do soil strength vs.

depth profiles affect critical points?

3. How do measures of soil strength obtained using the field-vane torque meter compare to 

the Dutch penetrometer in spatial patterns of soil strength in Louisiana wetland soils? 

How do results of these instruments compare in providing mechanistic understanding of 

soil strength and its ability to predict wetland resilience?

4. What is the role of initial condition (e.g. elevation, degree of impoundment, soil strength, 

species composition, salinity regime, flooding/draining regime) on changes in soil 

strength following a river diversion?

5. To what extent is soil strength a reliable indicator of long-term marsh sustainability in 

coastal Louisiana; e.g. in the face of sea level rise and the ability of marshes to resist 

degradation from high storm surge shear stress?

6. When diversions cause rapid and strong changes in salinity, nutrient loading, and plant 

community composition, how does soil strength change over time? With the Mississippi 

River historically switching distributaries every several hundred years, the initial input of 

low salinity, low ionic strength, relatively nutrient-rich floodwaters into degraded 

wetland interdistributary basins mimics, in a sense, what happens with a diversion. Land 

building as a result of these historic switches was primarily due to new sediment inputs, 

which occurred regardless of changes in soil strength and hurricanes. How important 

26
were changes in soil strength during these events relative to susceptibility to hurricane 

shear and overall land building?

7. What is the relative importance of the factors that contribute to soil strength, including 

grain size, organic content, degree of organic matter decomposition, root type and 

distribution, bulk density, ionic strength, and water content? If we could predict the 

distribution of these factors affecting soil strength, could soil strength profiles be 

predicted in Louisiana wetlands?

8. How does soil strength change during geological and biological succession following the 

termination of a diversion program or a switch in river distributaries? 

9. What is the optimal flooding/drainage regime that promotes marsh accretion and building 

soil strength?

10. How do the effects of high level surface fertilization vs. low level floodwater fertilization 

on soil strength compare? 

11. Is the occasional loss of vegetation following severe hurricane passage significant in the 

long-term (> 5 to 10 y) as long as sediment remains within the system and thereby 

contributes to elevation gain elsewhere? Turner (2009) and Howes et al. (2010) have

shown the importance of hurricanes in redistributing sediments onto the marsh platform. 

12. What are the cumulative, long-term effects of vertical discontinuities in soil strength 

caused by inorganic layers deposited following storms or even planned diversions? 

Regions of low soil strength (weak layers) can increase the vulnerability of marshes (that 

develop over such deposits) to loss following the passage of storms with wave shear 

stress in excess of soil strength at those layers. 

Conclusions

• It is crucial to recognize the differences in diversions and their intended goals in retarding 

or reversing wetland loss in coastal Louisiana, specifically the few Sediment Diversions 

which are designed to deliver sediment, and the more numerous Freshwater Diversions, 

which deliver limited amounts of sediment but lots of river water of uncertain quality for 

marsh sustainability.

• The Sediment Diversions appear to be beneficial in building coastal wetlands and deltas, 

but are few in number.

• The effectiveness of the Freshwater Diversions, which deliver mostly river water 

containing high nutrients and other chemical constituents, in mitigating for marsh loss, is 

unclear from the available research.

• Some research suggests that the Freshwater Diversions deliver sediments and nutrients,

which benefit wetland plant growth and organic soil accumulation, while other research 

suggests that these Freshwater Diversions deliver nutrients and other chemical substances 

to the system, which negatively impact belowground biomass growth (root mat 

development) and cause organic soil deterioration.

• Some of the confusion in the previous research on the benefits/costs of diversions comes 

from invalid comparisons between Sediment Diversions, which deliver significant 

sediment loads, and Freshwater Diversions, which deliver mostly water, or between 

wetlands with mostly sediment substrate and those with mostly peat substrate.

• There is also a need to develop a measurement of soil strength that is widely accepted as 

accurate and is easily used by researchers. Intercomparisons should be made between 

27
penetrometer and vane-type meters – to what extent are they interchangeable?

• Research studies are needed on the following questions: (1) How do Freshwater 

Diversions of mostly river water and little sediment impact soil strength in fresh, 

brackish, and salt water wetlands with organic soil, (2) How do the Freshwater 

Diversions impact the factors that control soil strength including root and rhizome mass 

distribution, species distribution, organic and water content, and organic soil 

accumulation, and (3) Is soil strength a strong predictor of wetland sustainability?

• It is unclear what factors, if any, related to the operation of Freshwater Diversions for 

coastal restoration may serve to enhance or ameliorate the impacts of natural physical 

forces, such as flooding and storm surge, in shaping diverse deltaic landscapes.

Suggestions for moving forward

• Major Mississippi River distributaries have deposited tremendous loads of sediment onto 

the Louisiana continental shelf over the past 6,000 years resulting in a mosaic of deltaic 

lobes with sufficient soil strength and integrity to withstand storm surge over vast periods 

of time. Switching of river distributaries is a normal geologic process and future adaptive 

management that emulates this process should be encouraged. 

• Organize a panel of experts to recommend the best and most informative method for 

measuring soil strength, and then use this method consistently in future studies.

• Statistically determine how soil strength varies across the south Louisiana wetlands in 

relation to type of marsh (salt, brackish, fresh), type and vertical distribution of soil 

composition (e.g. grain size, organic content, uniformity over depth), elevation, plant 

species composition, flooding and draining regime, and relative damage from recent 

storm surge events.

• Develop a predictive understanding of the relation between the vertical and horizontal 

distribution of soil strength, wave shear stress, and marsh scalping, degradation and loss. 

• Fund studies of the effects of small Freshwater Diversions on marsh soil strength, organic 

soil accumulation, and vertical accretion in a way that enables the effects of sediment 

load, nutrient load, and flooding regime to be determined individually. These studies 

might include: (1) mesocosm studies, (2) field comparisons of different locations, (3) 

measurements of major factors influencing soil strength such as root density and depth, 

microbial activity, soil stratigraphy, etc.

• Prolonged flooding or ponding of all types of marshes has been demonstrated to cause 

marsh degradation and is correlated with low soil strength. It should be avoided through 

appropriate water management.

Monitoring and Adaptive Management

As evident from the present papers’ conclusions, predictions of the short- and especially longterm effects of constructed and planned Freshwater Diversions on Louisiana wetlands are limited 

28
– too few representative systems have been studied, ecologically relevant designs are lacking, 

and measurements of some key indices of vegetation and soil responses are rare. It is within this 

realm of uncertainty that formal adaptive management is most valuable because it is designed to 

support action when scientific knowledge is limited. Adaptive management functions when 

goals are agreed to among the actors, knowledge of pre-diversion conditions exist, monitoring is 

done to measure progress toward goals, and there is a process in place to adjust functions to 

improve the likelihood that goals can be met. 

The 2012 Coastal Master Plan contains an overview of an Adaptive Management Framework 

that will apply to the initial stages of Master Plan implementation (e.g. proposed diversions in 

Figure 1, Table 1) – a more comprehensive framework is in preparation by CPRA to be 

completed in 2013. The adaptive planning process has three steps leading to development of the 

2017 Coastal Master Plan. These include evaluation of the decision-making process and 

stakeholder acceptance of the 2012 Coastal Master Plan (Step 1), evaluation of the tools and 

models used to inform development of the 2012 Coastal Master Plan, and their key uncertainties 

(Step 2), and to incorporate feedback from Steps 1 and 2 in developing a strategy and budget for 

research and management in the next four years to inform revisions in the 2017 Coastal Master 

Plan.

Elements of the Adaptive Management Framework will include expansion and refinement of 

existing monitoring and modeling efforts, and development of new projects to fill information 

and management gaps. Specific tasks related to monitoring include:

• Bolster current monitoring and data collection efforts; appropriate spatial and temporal 

performance measures will be identified in later versions of the Adaptive Management

Framework;

• Determine when and where monitoring might dictate adjustments to the planning process 

and/or project implementation and operations that would result in a change to how 

projects interact, are sequenced or whether they are removed from the master plan.

Below, we provide some general suggestions for a comprehensive and effective monitoring 

program to inform management of diversions.

A well-designed monitoring program is essential to assess whether diversions are promoting 

marsh sustainability and to support adaptive management of diversions. For example, if an 

existing diversion is failing to produce marsh elevation gains sufficient to match rates of relative 

sea level rise, the information from a good monitoring program should point to where 

deficiencies lie and suggest ways to improve future diversions. Numerous books and articles 

describe appropriate sampling designs, particularly for the assessment of human modifications 

affecting natural ecosystems. Many of these resources emphasize the need to address spatial and 

temporal variation in response variables as well as any environmental gradients that might exist 

in the study area. Both spatial and temporal replication are needed to distinguish effects due to 

the modification under study from those due to natural gradients or cycles (Underwood 1994). 

In particular, replication of the main effect “diversion” is desired to test whether or in what 

direction diversions influence marsh sustainability. Equally important is the use of multiple 

reference sites (that encompass the range of habitats encountered in the diversion basin). A 

29
preliminary survey of vegetation and soils across the diversion basin would provide information 

to guide sampling design and potential locations of sampling stations. To answer questions 

about the spatial extent of diversion influence, it is important to establish multiple monitoring 

stations at suitable intervals (distances from discharge and in relation to discharge flows) and 

sufficiently replicated to detect significant differences. Preferably, pre-diversion measurements 

should be made over a suitable duration to provide baseline trends in elevation dynamics, 

including seasonal variation. Alternatively, measurements conducted before and after major 

discharge events might be used to assess effects of a diversion on marsh elevation trends. 

The existing network of CRMS sites across the MR Delta is an effective starting point for 

monitoring changes in marsh productivity, species composition, salinity, elevation, soil strength, 

and herbivory. The Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA) 

authorized funding in 2003 for CPRA and USGS to implement the CRMS as a mechanism to 

monitor and evaluate the effectiveness of CWPPRA projects across the region (Steyer et al. 

2003). A consequence of this monitoring program was the selection of reference sites that have 

been used to establish the status and trends of existing wetlands. A total of 390 sites with a fixed 

annual sampling design were approved and secured for CRMS data collection. These 390 

CRMS sites located within nine coastal basins covering the entire Louisiana coast are sampled 

annually using a fixed design. Sample collection from the ground began in 2005, and will be an 

important complement to remote sensing techniques. 

For comparison purposes, establishment of additional sampling sites within the footprint of 

diversions would ensure that sampling designs are sufficiently replicated. To accomplish this, 

the number of CRMS sites, for example, might be expanded in response to the start and 

placement of new restoration projects. Placement of monitoring sites is critical, e.g. along a 

gradient downstream of any diversion to record changes in salinity, nutrients, and species 

composition and biomass. Landscape features that reduce the footprint of land affected by 

diversions may need to be modified, e.g. remove spoil banks that, along with Hurricane Katrina 

folding, resulted in hydrologic isolation of one section downstream of the Caernarvon diversion. 

It is also desirable to alternate flooding and drainage of marshes receiving diversion runoff as 

this promotes retention of allochthonous materials and contributes to soil strength.

Changes that would enhance the existing and future diversion projects are:

• Transects downstream of diversions with detailed assessment of changes in community 

composition, detailed hydrology, and nutrient chemistry. 

• The addition of sentinel sites within different vegetation communities along nutrient and 

hydrologic gradients specifically geared to evaluate belowground production and root 

distribution.

• Because water quality effects are one of the key uncertainties associated with river 

diversions, add nitrogen and phosphorus to the list of parameters routinely measured in 

the coastal reference network soil and/or plants to assess accumulation and potential 

eutrophication.

Herbivory is a confounding issue in many studies and is an important factor to consider in future 

studies. Because one effect of nutria is to remove aboveground biomass, this could lead to 

30
reductions in soil organic matter, further reducing the ability of the marsh to maintain itself as 

sea level rises. It would be useful for studies examining how diversions affect soil organic 

matter accumulation and soil strength to have an explicit component addressing the effects of 

herbivory on these processes. Because many of the studies showing enhanced grazing of 

fertilized plots are based on experiments at a relatively small scale, herbivores can target these 

small areas. Experiments at larger scales comparing grazing within the diversion and in a 

reference area are necessary in order to address the impact of herbivory on accumulation of soil 

organic matter. Monitoring of nutria and alligator populations in diversions and reference sites 

would be useful. In adaptive management, managers may target nutria removal or limit hunting 

of large alligators in diversion areas to reduce nutria populations. 

With regard to marsh elevations, monitoring stations instrumented with Surface Elevation Tables 

(SETs) (elevation change), along with marker horizons (surface accretion), provide information 

about elevation dynamics. These are currently monitored at CRMS sites. Together, these 

measures can be used to estimate shallow subsidence. Guidelines for installation of SETs, data 

collection, and statistical analysis are available in the literature (Cahoon et al. 2002b) and online 

(http://www.pwrc.usgs.gov/set/). Measurements of soil texture (particle size distribution), soil 

strength, dry bulk density, and organic content conducted at each sampling station would allow 

more detailed monitoring of changes in soil texture and soil integrity. End of season standing 

live biomass (above- and belowground) would provide information about the health of the plant 

community and relationship to elevation changes. Monitoring of pore water chemistry (salinity, 

nutrients, sulfide) would assist in interpreting the influence of diversion discharges on 

environmental conditions affecting plant growth. 

Models are needed to predict shear stresses on marshes under varying hurricane and storm surge 

scenarios, and to spatially predict maps of marsh scour depths based on soil strength depth 

profiles as a function of shear stresses. There presently are not enough data on soil strength to 

come to any definitive conclusions about the impacts or benefits of river diversions with respect 

to the soil strength of Louisiana wetland soils. Short-term effects may be quite different than 

long-term ones. We also lack fundamental predictive understanding of what role soil strength 

plays in marsh rebuilding and long-term resilience. Some evidence indicates that an adverse 

outcome from Freshwater Diversions is more likely than benefits. Some of the experimental 

studies are of limited use for understanding the impacts of Freshwater Diversions on soil 

strength. For example, of the presentations given at the February 2011 “Workshop on Response 

of Louisiana Marsh Soils and Vegetation to Diversions”, Graham and Mendelssohn’s (2011) 

experimental field plots are of limited value for soil strength considerations (as pointed out in 

their talk) because ammonium rather than nitrate was used. Turner's (2011a) experiments also 

used ammonium in very high concentrations, making interpretation with respect to impacts on 

organic soil strength difficult. Hester and Fisher’s (2011) study provides perhaps the best 

experimental data available, on balance, as nitrate was used at approximately environmentally

relevant concentrations. This experiment suggests degradation of the organic substrate rather 

than build-up resulting from the nitrate additions. 

Monitoring is one way of evaluating Freshwater Diversions once they have been implemented, 

and using a scheme of iterative adaption and design modification as data become available is 

helpful in optimizing application of this technique. However, the cost of building even a small 

31
Freshwater Diversion can reach several hundreds of million dollars. It seems sensible and costeffective to engage in a comprehensive research program with both greenhouse and field studies, 

and with investigators with diverse fields of expertise, from organic geochemists to plant 

physiologists and wetland hydrologists, that address key uncertainties associated with the use of 

freshwater diversions for mitigation of marsh loss.

Summary

The 2012 Coastal Master Plan (Coastal Protection and Restoration Authority of Louisiana 2012), 

which was recently approved unanimously by the Louisiana Legislature, includes plans for major 

river diversions and channel realignment to divert sediment and freshwater from the Mississippi 

and Atchafalaya Rivers into adjacent receiving basins such as Breton Sound and Barataria Bay. 

The success of these proposed diversions will depend on a number of factors, but will provide 

new opportunities to establish comprehensive monitoring plans and to answer some of the 

questions raised in this document. One must distinguish between the large-scale Sediment 

Diversions and the typically much smaller Freshwater Diversions which deliver water but little 

sediment. The differences between these two types of diversions cannot be overemphasized. 

For example, some of the contradictions among datasets presented at the February 2011 

“Workshop on Response of Louisiana Marsh Soils and Vegetation to Diversions” regarding soil 

strength impacts arose from comparing results from these two types of diversions. If flows are 

too low, clays and silts cannot be carried into the marshes, and water chemistry interacting with 

organic matter buildup determines the success in marsh restoration. 

This Position Paper is an assessment by the Workshop Technical Panel on the state of knowledge 

and information gaps on expected soil and vegetation responses to Freshwater Diversions in 

coastal Louisiana. Freshwater Diversion effects were evaluated based on responses of wetland 

plant productivity, elevation, and soil strength. The Panel concluded that little evidence was 

available that any Freshwater Diversion in the Louisiana deltaic plain has significantly reversed 

the rate of marsh degradation and land loss. While there is evidence that the minor sediment 

load of freshwater diversions has enhanced accretion in localized areas, lack of uniform sheet 

flow across marshes, coupled with prolonged flooding and ponding of wetland vegetation, and 

rapid shifts in salinity have compromised the combined processes of macrophyte biomass 

accumulation, peat accumulation, building of soil strength and building wetland elevation and 

extent. These processes are necessary for reversing the high rates of wetland loss through most 

of the Louisiana deltaic plain. It is unlikely that any currently planned Freshwater Diversion 

without substantial sediment load will reverse wetland loss in Louisiana. Restoration of 

Louisiana wetlands may only be possible through significant inputs of sediment.

A number of “Sediment Diversions” are planned along lower portions of the Mississippi and 

Atchafalaya Rivers, but the science is still evolving to inform model predictions on the amount 

of sediment needed for adequate wetland rebuilding (not addressed here) and the role of high 

nutrient river water in contributing to or detracting from marsh restoration (addressed here). 

Effective adaptive management, therefore, will be paramount to ensure that diversions have their 

intended outcome. To this end, the Adaptive Management Framework being developed by 

CPRA as an element of the 2012 Coastal Master Plan emphasizes application of modeling and 

32
monitoring to inform management strategies and “course-correct” actions as needed. The 

Framework offers an excellent opportunity to develop a comprehensive research design that 

adequately tests the alternative hypotheses on wetland response to nutrient-rich river water 

characterized here, as a critical step to ensuring intended outcomes of restoring and sustaining

Louisiana wetlands. 

33
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