sediment infillin mississippi

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Sediment inlling and wetland formation dynamics in an active crevasse splay of the

Mississippi River delta

Donald R. Cahoon a,⁎,1, David A. White b, James C. Lynch a,1

a United States Geological Survey, National Wetlands Research Center, 700 Cajundome Boulevard, Lafayette, LA 70503, USAb Department of Biological Sciences, Loyola University, New Orleans, LA 70118, USA

a b s t r a c ta r t i c l e i n f o

Article history:

Received 15 April 2010Received in revised form 30 November 2010

Accepted 2 December 2010

Available online 13 December 2010

Keywords:

Crevasse splay

Mississippi River delta

Wetlands

Accretion

Shallow subsidence

Elevation

Crevasse splay environments provide a mesocosm for evaluating wetland formation and maintenance

processes on a decadal time scale. Site elevation, water levels, vertical accretion, elevation change, shallow

subsidence, and plant biomass were measuredat ve habitats along an elevationgradientto evaluate wetland

formation and development in Brant Pass Splay; an active crevasse splay of the Balize delta of the Mississippi

River. The processes of vertical development (vertical accretion, elevation change, and shallow subsidence)

were measured with the surface elevation table–marker horizon method. There were three distinct stages to

theaccrual of elevationcapital andwetlandformation in thesplay: sedimentinlling, vegetative colonization,

and development of a mature wetland community. Accretion, elevation gain, and shallow subsidence all

decreased by an order of magnitude from the open water (lowest elevation) to the forest (highest elevation)

habitats. Vegetative colonization occurred within the rst growing season following emergence of the mud

surface. An explosively high rate of below-ground production quickly stabilized the loosely consolidated sub-

aerial sediments. After emergent vegetation colonization, vertical development slowed and maintenance of

marsh elevation was driven both by sediment trapping by the vegetation and accumulation of plant organic

matterin thesoil. Continuedvertical development andsurvivalof themarsh then depended on thehealth and

productivity of the plant community. The process of delta wetland formation is both complex and nonlinear.

Determining the dynamics of wetland formation will help in understanding the processes driving the past

building of the delta and in developing models for restoring degraded wetlands in the Mississippi River deltaand other deltas around the world.

Published by Elsevier B.V.

1. Introduction

The 32,000 km2 Mississippi River delta consists of shallow estuaries,

wetlands, and distributaryridges that formed fromsix overlappingdelta

lobes during the past 6000 years (Coleman et al., 1998). This area is an

ecologic and economic engine for the Gulf of Mexico region and the

United Statesas a whole (Twilley, 2007). Thevast estuarine andwetland

area provides 30% of the US total sh catch and is a critical stopover

for neotropical migrating birds and waterfowl in the Central Flyway.

The wetlands store water, lter sediments and pollutants from the

water, help stabilize shorelines, and protect human settlements by

ameliorating storm surges associated with hurricanes. There is

extensive human settlement across the delta, including commercial

ports that handle more than 20% of the nation's foreign waterborne

commerce. However, during the past few centuries, human use of the

Mississippi River watershed and its delta ecosystem has dramatically

altered the delta, causing a shift to the destructive phase of the delta

cycle (Coleman et al., 2008; Blum and Roberts, 2009) resulting in

extensive wetland loss and a growing demand to restore the delta

environment (Day et al., 2007).

Multiple factors are contributing to the deterioration of the delta.

Dams on the Mississippi River and its tributaries have reduced the

sediment load in the River by 48% (Syvitski et al., 2009; Blum and

Roberts, 2009) and ood protection levees prevent annual spring

ooding of the delta plain and most of the sediment entrained in the

river from reaching the delta plain wetlands. Construction of an

extensive network of canals for oil and gas exploration and navigation

has altered local hydrology within delta plain wetlands that has

contributed to the impounding of water and related stresses to the

marsh vegetation (Day et al., 2000; Ko and Day, 2004). In addition,

extensive sub-surface uid extraction, particularly oil and gas, has led

to accelerated subsidence from reservoir compaction and fault

reactivation across the delta plain (Morton and Bernier, 2010; Morton

et al., 2006). The synergistic effect of reduced sediment load, altered

local marsh hydrology, and accelerated subsidence has resulted in an

Geomorphology 131 (2011) 57–68

⁎ Corresponding author. Tel.: +1 301 497 5523; fax: +1 301 497 5624.

E-mail addresses: [email protected] (D.R. Cahoon), [email protected]

(D.A. White), [email protected] (J.C. Lynch).1 Present Address: United States Geological Survey, Patuxent Wildlife Research

Center, c/o BARC-East, Building 308, 10300 Baltimore Avenue, Beltsville, MD 20705,

USA.

0169-555X/$ – see front matter. Published by Elsevier B.V.

doi:10.1016/j.geomorph.2010.12.002

Contents lists available at ScienceDirect

Geomorphology

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

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aggradation decit, where the delta plain wetlands are not keeping

pacewith current sea-level rise rates, resulting in historically high rates

of wetland loss (Barras et al., 2003). Besides reducing the natural

resource value of the delta ecosystem, wetland loss also has increased

the vulnerability of the remaining wetlands and human settlements to

storm surge impacts (Barras et al., 2008; Day et al., 2007). This story of

delta destruction as a result of human alterations has been repeated at

many deltasaround theworld(Ericson et al.,2006; Colemanet al., 2008;

Syvitski et al., 2009). The Mississippi along with the Ganges, Irrawaddy,Magdalena, Mekong, Niger, and Tigris deltas are considered in peril

of ooding as a result of reduction in aggradation plus accelerated

compaction overwhelming rates of global sea-level rise (Syvitski et al.,

2009).

A direct consequence of wetland loss in the delta plain is the loss of

wetlandelevation capital(sensu Reed, 2002; Cahoon and Guntenspergen,

2010) and an increase in accommodation space. Thus the restoration of

the wetland ecosystem will require the restoration of the wetland

elevation capital through the accumulation initially of mineral sediments

to restore elevations to a level suitable to support emergent vegetation,

and then organic matter provided by plant growth; the process by which

the delta wasformedhistorically.However,today thedelta plain is largely

disconnected from the river and hence the increased accommodation

space remains unlled and devoid of vegetated marsh. One approach

proposed for restoring the wetlands is to reconnect the delta plain to the

river with sediment diversions that will mimic historic crevasse splay

processes (Day et al., 2007). Although there is not suf cient sediment to

restore the historical extent of delta plain wetlands with this approach

because of the reduced sediment load of the river (Blum and Roberts,

2009), smaller scale restoration is feasible (Day et al., 2007; Boyer et al.,

1997). To this end we evaluated splay development processes near the

mouth of the Mississippi River where active crevassing still occurs on a

small scale.

This study describes wetland formation and elevation dynamics

along an elevation gradient of an individual lobe of a developing

crevasse splay within the degraded Cubits Gap sub-delta complex. The

approach of substituting space (location on the elevation gradient) for

time (ecogeomorphic succession) allows us to assess marsh develop-

ment processes from a comparison of subaqueous to recently emergent(herbaceous marsh), to mature emergent (forest) habitats. Specically,

we analyze the role of hydrogeomorphic processes in the vertical

development of open water substrates (sediment inlling), that lead to

the accumulation of elevation capital and marsh formation (plant

colonization thresholds), and marsh maintenance (sediment trapping

by vegetation and plant organic matter accumulation in the soil).

Our objectives were to determine the rate of inlling and vertical

development of the splay habitats, the elevationgradient among these

habitats, andthe rate at which shiftsin habitat type occur (i.e., gradual

versus abrupt). Such an accounting of the acquisition of elevation

capital (relative to local sea level), and the rates at which it can be

gained during wetland formation and evolution from open water to

emergent marsh, is fundamental to understanding wetland changein coastal Louisiana and to the development of wetland restoration

practices.

2. Regional setting

Subsidence rates in theBalize delta areconsideredto be thehighest

on the Louisiana and Mississippi coast as a result of compaction of

thick, young Holocene sediment deposits (~100 m) and faulting and

exure of the underlying Pleistocene sediments at the seaward margin

of thedelta(see reviews in Blum et al., 2008; Blum andRoberts, 2009).

However, there are few measurements of subsidence from this delta,

and those are not well constrained. Thus subsidence estimates range

from 10 to 30 mm/yr (e.g., Penland and Ramsey, 1990; Shinkle and

Dokka, 2004). When comparing theBalize delta to other regions of the

Louisiana coast, Blum and Roberts (2009, p. 489) do not provide an

estimate of the Balize subsidence rate. Rather they simply state it is

greater than the 6–8 mm/yr rate determined from the Grand Isle tide

gauge in the Lafourche delta. For the purposes of this study, we have

assigned to this region the conservative estimate of N10 mm/yr for

combined subsidence and eustatic sea-level rise.

The study region is located on the Cubits Gap sub-delta, which

roughly occupies the north-eastern quadrant of the MRD (Fig. 1). In

the 1800s sediment to this sub-delta gradually inlled the then

existing shallow seas between just north of Main Pass and Pass A

L'Outre (Otter Pass). Like all interior marshes in the MRD, this sub-

delta experienced deterioration and loss of its original marshes since

the 1950s as a result of high relative sea-level rise rates and elevation

decits that lead to the formation of huge shallow ponds in the

interior freshwater regions by the later 1970s (Coleman et al., 2008).During sub-delta degradation, a second wetland growth phase can

occur at a much smaller scale than the entire sub-delta complex

through the formation of crevasse splays, which are also called

overbank splays (Coleman, 1988). These splays are depositional

BA

10 km

N LA

Fig. 1. Illustrationof the activeBalizedeltaof the Mississippi Rivershowingland (stippledarea) present in1956 (A)and 1978 (B), modied fromWhite (1993). The starin (B) marks

the large shallow pond where sediment inlling created the Brant Pass splay. The pond is located adjacent to Brant Pass between Main Pass to the north and Pass A L'Outre to the

south.Thearrowin (A)pointsto theHead ofPasses atrivermile0 andshows themainchannel ofthe Mississippi Riversplitting into 3 principaldistributary passes, from west toeast,

Southwest Pass, South Pass, and Pass A L'Outre.

58 D.R. Cahoon et al. / Geomorphology 131 (2011) 57 –68


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forms that develop, depending on local conditions, from overtopping

or scouring of the natural levee resulting in a fan-shaped splay that

can mimic larger deltas (Coleman, 1988; Roberts, 1997). This study

took place in one of the largest shallow ponds formed in the sub-delta

complex on Brant Pass Splay, which began forming after a crevasse

opening along Brant Pass that appears in a 1978 photograph ( Fig. 2).

Brant Pass is located on Delta National Wildlife Refuge. Coleman

(1988, pp. 36–37) states the crevasse splay opened in 1975 and

rapidly began

lling the pond. The Brant Pass Splay grew to a totalmarshland area (excluding adjacent ponds) of 2 km2 by 1983 and to

approximately 5 km2 in 2000 (Fig. 2). Field measurements were

collectedin the southern-most and largest splay lobe, which is thesite

of a long-term vegetation study that was begun in 1983 (White, 1993,

2008; Fig. 3B). The southern lobe exists in the 1983 photograph

(Fig. 2).Thuswhen samplingbegan for this study in 1997the lobewas

more than 14 years of age.

Among the vegetation associations and habitat settings on the

southern lobe, we identied in 1997 ve habitat types that typically

occur on most lobes of the Brant Pass Splay. In order of decreasing

elevation and age, they are forest, high marsh, low marsh, pre-

emergent, and open water (Fig. 3). A woody community initially

dominated by Salix nigra (black willow) occurs on the highest

elevation, upstream region (i.e., at the head) of the lobe. As the

willows age and begin to die a mixed woody community composed

of species of Sesbania, Baccharis, and Iva develops with a ground

covering of several marsh species. From here on we refer to this as

forest habitat, which is the oldest splay habitat. Downstream from the

forest occurs the high marsh dominated by Schoenoplectus deltarum

(delta three-square), which is upslope from the adjacent low marsh

where the higher elevation areas are dominated by Sagittaria latifolia

(delta duck-potato) and the lower elevation areas by Sagittaria

platyphilla. When environmental conditions are right, an attenuated

elevation gradient and inundation pattern are created (White, 1993),

leading to the development of extensive monospecic marshes of

these 3 herbaceous species (Fig. 3A).

The habitats down slope from the low marsh are continually

ooded. We identi

ed two habitats in this sub-aqueous environment:pre-emergent and open water. The pre-emergent habitat represents

an older portion of the open water habitat where sediment inlling

has raised the substrate elevation to a level approximately half-way

between open water and low marsh (Fig. 3). The substrate of the open

water habitat is the lowest point on the elevation gradient and is the

youngest splay habitat in terms of crevasse sedimentation history.

Open water supports a submerged aquatic community of species in

the genera Najas, Ceratophyllum, Potomogeton, and Heteranthera

(Fig. 3). If the water is suf ciently deep and with signicant current,

the open water habitat lacks submerged aquatic vegetation.

The elevation of the substrate and its effect on ooding controls

development of each of these plant communities of the splay lobe.

Therefore, it is the degree to which the elevation changes over distance

that determines the extent of each community type along a lobe. This

elevation and community typedelityis notonly shown down thelobes

of each splay but also across the lobes, i.e., perpendicular to the general

low water direction of currentow. Adjacent to the main feeder passes

and even smaller channel meanders there is natural levee, the highest

elevation that declines perpendicularly away from the source of water

Fig. 2. A time series of false-color aerial winter photographs of the study region showing the large, shallow pond adjacent to Brant Pass in 1978 and the growing crevasse splay in

1983, 1995, and 2000. The southern lobe of the splay, where this study was conducted, grew to over 3 km in length between 1983 and 2000. The red color indicates willow (Salix

nigra) forest canopy and the white to lightest blue color indicates mud surfaces during the winter senescence when aboveground marsh vegetation is absent. The star indicates

where D. White collected accretion and vegetation data from 1984 to 1990. In 1983, the star marked the location of mudat habitat, which by 1988 had become high marsh

habitat.

59D.R. Cahoon et al. / Geomorphology 131 (2011) 57 –68


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ow (Fig. 4). This elevation change, however subtle, creates natural

‘shoreline-levee’ plant communities that would change into the typical

community down-slope or into the wetlands perpendicular from the

shoreline maintaining the same elevation/ ooding community delityfor the down-lobe communities. The kind of ‘shoreline levee’ commu-

nity would depend upon its elevation still adhering to the general

elevation and community type delity relationship. This means that

along a lobe an atypical vegetationtype along thedown-current axis can

sometimes be found along a shoreline if past sedimentation events

created the atypical conditions.So, sometimesmonospecic marshlands

of any of the dominant marsh species can be bordered along a higher

shoreline by a plant community typical of a higher elevation.

3. Materials and methods

In 1984, the environments of the young southern lobe included

an exposed mudat/pre-emergent habitat located adjacent and down

lobe of thehigh marsh habitat(star in the1983 image of Fig. 2). In 1984,

White (1993) placed out 15 feldspar marker horizon plots (Cahoon and

Turner, 1989) down and across this mudat/pre-emergent environ-

ment that developed into highmarsh by 1988and later becameSiteB of

this study. Above-ground plant biomass was collected annually from1984 to 1990 andbelowground biomass collectedannually from 1985 to

1989(Table1) along transects establishedon themudat/pre-emergent

habitat and reported in White (1993). At each transect across the

lobe, ve 0.25 m2 plots were harvested at peak aboveground biomass

(late August–early September) both for above-ground and below-

ground plant material. The aboveground live material was clipped at

ground level and sorted by species, dried and weighed in the lab. The

substrates in the clipped plotswere collected to a 30–40 cmdepth, well

below root and rhizome growth, washed of mud, and the remaining

below-ground material was dried and weighed with no attempt to sort

by plant species.

In 1997, ve sites were chosen for sampling (Fig. 3) to examine

marsh substrate elevation changes along the southern lobe and to

capture inter-site elevation relationships. Each of the ve sites was

Fig. 3. (A) Conceptual diagram (not to scale) of a longitudinal prole of a typical lobe of the Brant Pass Splay identifying the elevation gradient across the woody, herbaceous and

aqueous communities as conceptualized in White (1993) and represented by forest, highmarsh, low marsh, pre-emergent, and openwaterhabitats.Note thatthe vertical dimension

is extremely increased relative to the horizontal scale. (B) Year 2000 image of the Brant Pass Splay study region showing the attenuated southern lobe and the location of the ve

sampling sites: A—Forest habitat (Salix nigra overstory with mixed sparse understory of the herbs of the High Marsh); B—High Marsh habitat (Schoenoplectus deltarum-dominated);

C—Low Marsh habitat (Sagittaria latifolia-platyphilla dominated); D—Pre-emergent habitat (on the verge of becoming a marsh with scattered patches of Low Marsh dominants or

submerged aquatics depending upon the degree of ooding); E—Open Water habitat (with occasional patches of submerged aquatics). The High Marsh, site B, corresponds with the

location of the star in Fig. 2.


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located within a principal wetland community described above

(forest, high marsh, low marsh, pre-emergent, and open water). At

two random locations within each site, a single surface elevation table

bench mark and 3 marker horizon plots were established (Table 1).

The surface elevation table–marker horizon method was used to

collect multi-year, high resolution data on the relationship among

vertical accretion, elevation change, and shallow subsidence (Cahoon

et al., 1995, 1999, 2002). All stations were sampled approximately

three times per year over the course of the 5 year study until winter

2002. Additional marker horizons were added at some sites and an

A

B

C

2nd Channel Nascent Marsh Main Feeder Channel

Substrate Surface

SET-MH Pipe and Platform

Shoreline - Levee

Highest Marsh (Mixed grasses/forbes)

Base Water Level

Filling Back Channel

Low Marsh(Sagittaria)

High Marsh(Scheonoplectus)

Main Feeder Channel

lennahCredeeFniaMhsraMhgiHhsraMwoL

Fig. 4. A cross-section diagram (not to scale)of the southernlobe of Brant Pass splayat theHighMarsh site (B in Fig. 3 andstarin Fig. 2) showing a time seriesof habitat changes that

occurred beginning in 1984 (A) through 1997 and 2002 (B and C). The Nascent Marsh in A is mostly ooded and on the verge of becoming marsh. Note the early second feeder

channel on the backside of the lobe that became low marsh habitat (B and C). Note also the slight, but important, changes in substrate elevation that developed by 2002 (C),

especially the small natural berm habitat immediately adjacent to the shoreline that supports a mixed high marsh with only scattered patches of Schoenoplectus deltarum. Surface

elevation table–marker horizon stations were established in this habitat in 1997.

Table 1

Annual time line and locations of the methods.

Methodology Year

84a 85a 86a 87a 88a 89a 90a 91 92 93 94 95 96 97 98 99 00 01 02

Aboveground biomass X X X X X X X

Belowground biomass X X X X X X X X X

Laser elevation surveys X X X

Water level X X X

Soil analysis X X X

Marker horizon (accretion)

1984 Mudat X X X X

Forest 1b X X X X X X

Forest 2b X X X X X

High Marshb X X X X X

Low Marsh

b

X X X X XPre-emergentb X X X X X X

Open Water 1b X X X X

Open Water 2b X X X X X

Surface elevation table (elevation)c

Forest X X X X X X

High Marsh 1 X X X

High Marsh 2 X X X X X X

High Marsh 3 X X X

Low Marsh X X X X X X

Pre-emergent X X X X X X

Open Water X X X X X X

a Data were collected by White (1993) from mudat habitat indicated by the star in Fig. 2, 1983 photograph. The data were reanalyzed for this study.b For each habitat indicated in Fig. 3B, 6 marker horizons were established, 3 at each of 2 stations. Second sets of marker horizons were placed out at the Forest and Open Water

habitats after many of the original ones eroded away (i.e., Forest 2 and Open Water 2).c For each habitat indicated in Fig. 3B, a single surface elevation table was established at eachof 2 stations. In theHigh Marsh, one of the two stations(High Marsh 1) eroded away

and was abandoned by the end of 1999. The second station (High Marsh 2) remained stable for the entire study. In 2000, a third station (High Marsh 3) was established to replace

High Marsh 1.


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additional surface elevation table was added at the high marsh site

due to local erosion (Table 1). Consequently, some habitats have

multiple sets of marker horizons spanning different time periods such

that accretion measurement periods do not always match elevation

measurement periods.

Site elevation surveys were conducted in allve sites along the full

length of the lobe on three dates (January 14, 1997, January 12, 1999,

and May 25, 2000) using two Spectraphysics Laserplane 800

laser levels (Table 1). Since there is no permanent bench mark inthis remote region only relative elevations of the 5 sites could be

determined. The separate surveys were made to determine if there

were any relative elevation changes among the sites and to obtain

most accurate estimates of the differences among the sites. Since

there were no inter-site shifts over this time period the three survey

results were averaged to obtain mean relative elevations for each of

the ve habitats.

During the summer of 1997, a second below-ground biomass

study was begun at all ve sites along the southern lobe and carried

out until summer 2000. At the end of each of 4 summers, substrate

samples were collected from a single transect of ve 0.25 m2 plots at

each of the ve sites and processed in the same manner as in the

1980's study except the sample weights were not pooled by age. It is

noteworthy that substrate sample depth in this later study did not

sample to the fullest below-ground biomass depth in the forest and

high marsh habitats because the roots and rhizomes had grown

deeper into the substrate after 10 years. Therefore these data should

be viewedas shallow root values(they likelyare within75–85% of full

values) of late summer/early fall peak standing crop in g/m2.

In order to determine the characteristics of the sediments

delivered to the splay from the Mississippi River, soil samples were

collectedannuallyat each of theve sites from areas where vegetative

colonization had not occurred (Table 1). The procedures in Barnes

(1959) were used to determine particle size and loss on ignition of

the upper surface to 20 cm of each sample. Bulk densities were

determined by collecting a known volume of the substrate, then wet

and dry weight determined.

Hourly water level data was collected from January 1998 to

September 2000 (Table 1) using vented pressure transducers (5 psi)and a continuously recording data logger (Easylogger 900, Wescor

Inc., Logan, UT). The water level recorder was located on the shoreline

adjacent to the forest site.

3.1. Statistical analyses

Surface elevation change data were expressed as cumulative

changefor each pin of the surface elevation table over time, starting at

the initial, baseline measurement. Simple linear regressions were run

for each position of each surface elevation table, using the pins within

each position as replicates. Similarly, simple linear regressions were

run on accretion data from each individual marker surface, using

replicate observations as the error for each surface. Accretion data by

default is expressed as cumulative change. Linear slope estimates

were then used in mixed effects ANOVA models (SAS Proc Mixed) to

test two main hypotheses. The rst hypothesis involved comparing

elevation trends among the ve habitats. The second hypothesis

compared accretion trends to elevation trends for each habitat type

separately (

ve model runs), in order to quantify shallow subsidence.In both cases, two model structures were compared: 1) the “full”

model, specifying the nested error structure (individual bench

mark plots as samples, and positions within a surface elevation

table bench mark, or individual accretion surfaces, as subsamples),

and 2) the “reduced” model specifying only the positions within a

surface elevation table bench mark, or the individual accretion

surfaces. Since these two models are nested, their goodness of t

was compared using Akaike's Information Criterion adjusted for small

sample sizes. Data transformations of the slope data (log or square

root) were carried out as needed to resolve model assumptions. If a

data transformation was necessary, it was used for both models, to

enable comparisons of t. A non-linear change in elevation trajectory

was noted in several habitat sites over the last time interval (11/

2002). To estimate the inuence of this effect, separate elevation

change regressions were run on a truncated dataset which did not

include this last time interval (9/1997–7/2001). Fortesting thesecond

hypothesis, linear slope estimates corresponded to accretion and

elevation data during time periods over which both datasets were

intact. In other words, subsequent marker layer deployments were

not combined to create longer accretion datasets. Finally, since one

replicate surface elevation table bench mark in the high marsh site

was located in an area that later developedinto a small feeder channel

as the result of a natural breach of a shoreline levee, the bench mark

was abandoned. A replacement bench mark was installed at a later

time, but the data were not used in these statistical models since its

data spanned a later and shorter time period. The high marsh site was

thereforerepresented by only one surface elevation table bench mark.

4. Results

4.1. Sediment in lling

The sediment inlling the pond at Brant Pass and accumulating

throughout the southern splay consisted mostly of ne sand (N70%)

with the remainder consisting of silts, clays and ne organics. Average

soil bulk densities were higher in the more mature parts of the splay,

withvaluesb1 g cm−3 in thesubaqueous habitats(0.77to 0.87 g cm−3)

and N1 g cm−3 in the subaerial habitats (1.11 to 1.16 g cm−3). The

elevation surveys conducted 22 to 25 years after the crevasse breach

Table 2

Comparison of average linear accretion and elevation change rates for each of ve habitat types at a crevasse splay in southeast Louisiana.

Site Time seriesa Accretion

(cm yr−1±SE)

Elevation change

(cm yr−1±SE)

Shallow subsidence (A–E)

(cm yr−1±SE)

ProbNF Revised RSLR c

(cm yr−1)

Forest 9/97–7/01 0.64± 0.07 0.07± 0.06 0.57 ± 0.12 0.03⁎ N1.6

Forest 7/98–11/02 0.85± 0.20 0.64± 0.18 0.22 ± 0.06 0.51 N1.2

High Marshb 1/99–7/01 5.87± 0.66 5.97± 0.66 −0.10±0.00b 0.89 N1.0

Low Marsh 1/99–7/01 1.63± 0.2 0.05± 0.20 1.57 ± 0.34 b 0.0001⁎⁎⁎ N2.6

Pre-emergent§ 1/99–12/01 3.77± 0.01 3.55± 0.22 0.94 ± 0.51 0.71 N1.9

Pre-emergent§ 1/99–11/02 4.50± 0.02 3.88± 0.14 0.85 ± 0.87 0.12 N1.8

Open Water 1/97–10/00 8.77± 1.13 6.82± 0.99 2.10 ± 1.97 0.85 N3.1

Open Water§ 1/99–7/01 9.36± 3.07 3.83± 0.12 5.53 ± 2.95 0.13 N6.5

§ ANOVA analysis conducted on log-transformed slope estimates.⁎ Indicates signicance at Pb0.05.

⁎⁎⁎ indicates signicance at Pb0.0001.a The full statistical model was run for all time series except for the high and low marshes where the reduced model was applied.b Based on the one sampling station with a continuous 5-year record.c

RSLR=Relative Sea-level rise, which is the combined estimates of subsidence and eustatic sea-level rise = N

10 mm/yr. Revised RSLR=RSLR+Shallow Subsidence.



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revealed an average elevation difference of 79 cm between soil surfaces

in the forest and theopenwater (~3.25 kmdistance between A and E in

Fig. 3), a 62 cm difference from the forest to the pre-emergent surface

(~3.0 km distance between A and D in Fig. 3), and a 36 cm difference

fromtheforestto thelowmarshsurface (~1.2 kmdistancefromA toC in

Fig.3). Theelevation gradient,or slope,from theforest soil surface to the

open water bottom (A–E in Fig. 3) is about 24 cm/km, about 21 cm/km

from the forest to pre-emergent surfaces (A–D in Fig. 3), and about

30 cm/km for the emergent, vegetated wetland surfaces (B–

C in Fig. 3).A comparison of accretion and elevation change across the

different habitats of the lobe indicates that the rate of bay inlling

and elevation gain was not linear through time (Table 2, Fig. 5). The

rate of sediment inlling measured as vertical accretion over articial

soil marker horizons was inversely proportional to soil elevation

levels within the tidal frame of the splay, with annual rates of 9.4 cm

for open water, 3.8 cm for pre-emergent, 1.6 cm for low marsh, 5.9 cm

for high marsh, and 0.6 cm for forest habitats (Table 2, Fig. 5). The rate

for the highmarsh(Fig. 5B)doesnott the expected relationship with

relative site elevation (it is nearly 4-fold greater than the adjacent

downstream low marsh environment) because of the breach of the

shoreline levee that occurred shortly after initiating measurements in

this habitat. The breach scoured out a small feeder channel directly

through one of the high marsh sampling stations, which had to be

abandoned. Indeed, the elevation trend for both stations in the high

marsh was negative and marker horizons were eroded away during

channel formation (1997 to 1999). However, both accretion and

elevation were strongly positive after channel formation when

enhanced channel ows through the shoreline levee delivered

sediment directly to the interior marsh surface (1999 to 2002;

Fig. 5B). New marker horizons established at this site after channel

formation quickly became buried and remained stable for the

duration of the study. So even though the high marsh has a higher

relative elevation than the open water and mudat sites, its vertical

accretion rate is comparable to these lower elevation sites because of

the sediments shunted across lobe through the new channel

eventually past the low interior marsh to open water at the lowest

end of the elevation gradient. The associated sediment load of this

across lobe ow pattern is much less than the load carried down thelobe parallel to the shorelinebecause of the large difference in volume

of ow (size of feeder channels) between the two directions. These

realitiesare corroboratedby thevery lowelevation change(below the

trend at all other sites) at the low marsh site and the very high (above

the trend) elevation change at the high marsh site ( Table 2).

A plot of vertical accretion against relative site elevation

(excluding the high marsh site) reveals that for each 1 cm decrease

in site elevation there is a 9.8±0.20 mm/yr increase in vertical

accretion down the full length of the lobe (Fig. 6). Since suspended

sediment supply is not limiting in this environment, the variation

in sediment deposition is driven by variation in ooding levels that

are inversely correlated with elevation. In 1999, the forest site was

ooded only 6% of the time during the highest tides and thus had the

lowest accretion rate (Table 3, Fig. 5). The high and low marshes wereooded 50% and 94% of the time, respectively, and had intermediate

accretionrates. Theopen water site waspermanentlyooded and had

the highest accretion rate.

The open water and pre-emergent habitats showed different

temporal patterns of sedimentation (Fig. 5D, E). The open water

habitat exhibited uniformly high rates of sedimentation throughout

the study. In contrast, the pre-emergent habitat exhibited little to no

sedimentation for 1.5 years, then a springtime sedimentation event

(~1.4–1.5 cm), followed by a 2-year period of little to no sedimen-

tation, another springtime sedimentation event (~1.0–1.1 cm), and

then another period of little to no sedimentation.

Fig. 5. Surface elevation and accretion changes at the ve habitat sites of the southern

lobe: forest, high marsh, low marsh, pre-emergent, and open water.



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Of the 15 marker horizon plots established in October 1984, 7 plots

were never recovered or disappeared in less than 6 months and thus

were not analyzed. The results from the remaining plots reveal that

a major sedimentation event of about 13–14 cm occurred on the

mudat during the spring oods of 1985 (Fig. 7). This rapid sediment

deposition raised the soil elevation to a height suf cient to support

marsh plant species dominated by Schoenoplectus deltarum as

indicated by the rapid increase in aboveground plant biomass from

b100 g/m2 in 1984 to N800 g/m2 in 1987 and below-ground plant

biomass from b100 g/m2 in 1985 to 2200 g/m2 in 1986 (Fig. 7).

Below-ground biomass then declined during the next two years until

it leveled off by 1989. Following the conversion from mud at to

marsh habitat, the annualized rate of vertical accretion slowed from

21.5±1.7 cm/yr (R 2=0.99) in the spring of 1985 to 1.4±0.6 cm/yr(R 2=0.72) for the period from summer 1985 to winter 1987.

The loss of marker horizons in the 1980s and the need to re-

establishmarker horizons at several sites in the 1990s (Table 1, Fig. 5)

are indicative of the highly dynamic processes of sediment deposition

and erosion in this splay environment. The occurrence of erosion

indicates that our measurements of vertical accretion from marker

horizons are biased upward because eroded surfaces are not factored

into the accretion rate. Thus all accretion measures should be

considered maximum estimates for this environment.

4.2. Subsidence and surface elevation change

Similar to vertical accretion, surface elevation change measured

over the period for which vertical accretion data were available

(~3 years) was inversely proportional to soil elevation levels within

the tidal frame of the splay. Annual rates were 3.8 cm for open water,

3.6 cm for pre-emergent, 0.05 cm for low marsh, 5.9 cm for high

marsh, and 0.07 cm for forest habitats (Table 2, Fig. 5). For each 1 cm

decrease in site elevation there is a 0.84±0.19 mm/yr increase in

elevation change down the full length of the lobe ( Fig. 6). For the

entire 5-year study, surface elevation change was inversely propor-

tional to site elevation with annual rates of 5.5 cm for open water,

3.9 cm for pre-emergent, 0.3 cm for low marsh,4.4 cm forhigh marsh,

and 0.6 cm for forest habitats (Table 4). Surface elevation change at

the high marsh site did not t the relationship with site elevation

for the same reason as vertical accretion. However, surface elevation

change was consistently lower than vertical accretion at all sites, with

the exception of the atypical high marsh site where it was equal,indicating the occurrence of shallow subsidence in the lobe. Elevation

gain was signicantly lower than accretion for the vegetated forest

and low marsh sites (Table 2). Differences between accretion and

elevation were comparable or greater in the open water and pre-

emergent sites compared to vegetated sites, but were not signicant.

In general, the rate of shallow subsidence (calculated as accretion

minus elevation) decreased along the elevation gradient from the

aquatic (N5 cm/yr) to herbaceous to forested (b1 cm/yr) habitats

(Fig. 6, Table 2).

Below-ground biomass increased signicantly from the aquatic

sites (open water and pre-emergent) to the herbaceous marshes

to the forest (Fig. 8). Below-ground biomass was b80 g/m2 in the

sparsely vegetated open water and pre-emergent sites, ranged from

600 to 1600 g/m2

in the herbaceous marshes, and reached a peak of N3000 g/m2 in the forest.

Table 3

Hydroperiod for 5 sites on the southern lobe of Brant Pass splay, 1999.

Variable Site

Forest High

Marsh

Low

Marsh

Pre-emergent Open

Water

Total hoursa 8143 8143 8143 8143 8143

Flood events 50 180 66 3 1

Flooded hours 494 4090 7672 8134 8143

Time ooded (%) 6.07 50.23 94.22 99.89 100.00

Longest ood (h) 77 795 4656 7959 8143

Mean ood

depth (cm)

3.67 6.31 16.53 28.24 77.01

a

Start date = Jan. 26, 1999; stop date = Jan 1, 2000; length of record=340 days.

Fig.7. Accretion, andabove-ground andbelow-ground biomass at thesite indicated by the

star in Fig. 2—1983. In 1984 the site was mudat. After the spring 1985 sedimentation

event (inlling), the site was colonized by emergent vegetation and converted to low

marsh habitat, at which time the sedimentation rate decreased (maintenance). By 1988,

this site had become high marsh (site B in Fig. 3).

Table 4

Elevation trajectoriesover the5 years of thestudy foreach habitat type. Siteelevation is

relative to the level of the water bottom in the Open Water habitat.

Site Site elevation (cm) Elevation change (cm yr−1±SE)

Forest 79 0.6 ±0.3

High Marsh 73 4.4 ±0.6a

Low Marsh 43 0.3 ±0.2

Pre-emergent 17 3.9 ±0.6

Open Water 0 5.5 ±0.7

a

The elevation trajectory for the high marsh is based on only one sampling station.

Fig. 6. Accretion and surface elevation change regressed against relative site elevation.

The High Marsh habitat data were not included in the analysis. See Table 2 for a

description of the sampling intervals at each site and Table 4 for the actual site

elevations.


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5. Discussion

5.1. Elevation capital and wetland formation

Delta lobes, sub-deltas, and crevasse splays experience three phases

of growth and abandonment: rapid growth with stable to increasing

river discharge, relative stability as discharge begins to wane, and

abandonment, after which subsidence dominates vertical development

(Roberts, 1997). The Brant Pass Splay has experienced the rst two

phases, and is likely in the beginning of the third phase. The high

suspended sediment concentrations in the Mississippi River and the

rapid rate of sediment inlling through the 1975 crevasse at Brant Pass

more than compensated for both the high rates of regional subsidence

and the local shallow subsidence (Figs. 5 and 6, Table 2) related to

substrate compaction through sediment overburden and consolidation

of newly deposited sediments (Cahoon et al., 2000). The shallowopen water habitat of the original ~8 km2 pond of the 1970s converted

to a vegetated crevasse splay in less than 10 years, and expanded to

more than 5 km2 in size and lled much of the pond in less than

25 years. As the splay developed, the rate of accumulation of elevation

capital and the elevation gradient decreased as the hydrologic gradient

became reduced. This can be deduced from an evaluation of the aerial

photographs from the 1980s and our empirical data from the late

1990s. The negative feedback relationship between site elevation and

the rate of vertical development is similar to that reported for estuarine

(Temmerman et al., 2004) and back-barrier (Pethick, 1981) tidal

marshes, where young low marsh surfaces accumulate quickly and

asymptotically up to an equilibrium level around mean high water level

and higher marsh surfaces accumulate much slower. In the case of the

Brant Pass splay environment, the sequence is from shallow open waterto forested wetland habitats, and the sequence occurs on a sub-decadal

timescale. Indeed, Coleman (1988, p. 37) suggests that crevasse splays

like Brant Pass are rarely active for more than 10 to 15 years, Roberts

(1997, p. 614) suggests no more than 2–3 decades, and can result in up

to a 3-meter thick sedimentary deposit.

Initially, rapid sediment inlling raised the elevation of the

substrate to a level that supported willow forest in b8 years (compare

1978 and 1983 images in Fig. 2) and the gradient from forest to open

water spanned no more than 0.5 km.The steepness of thegradientled

to abrupt changes among plant communities and the absence of a low

marsh habitat (D. White, personal observation, 1984). The marsh was

situated between the forest and a young mudat community on the

accretingnewly emergent substrate of the elongating lobe. Thismarsh

became dominated by Schoenoplectus deltarum and the young mudat

community that had colonized the new sub-aerial surface became

dominated by a mixture of herbaceous species (White, 1993). On

this steep gradient, the large 1985 sedimentation event led to the

abrupt conversion of the mudat to high marsh by 1988. The large

1985 sedimentation event was related to an unusually high spring

Mississippi River discharge and concomitant high sediment load. This

springevent wasone of thefour largest duringthe 18 year periodfrom

1984 to 2002 (Fig. 2 in Blum and Roberts, 2009, p. 489). Additionally,

there was likely a greater accommodation space in 1985 on the steepgradient of the young splay, which allowed for the greater increase in

elevation. This area was still high marsh (i.e., had not become forest)

when we began sampling in 1997, and remained high marsh for the

duration of our study. So clearly the rate of elevation and habitat

changeon thehigher portions of thelobe slowedduring thelate 1980s

and the 1990s.

As the splay matured and the mid and outer reaches of the pond

lled, the slope became more attenuated and the gradient shallower

as the lobe grew to several kilometers in length (e.g., compare aerial

photos in Fig. 2). Coincident with the attenuation of the slope in the

1990s, a distinct low marsh community of Sagittaria spp. developed

between the high marsh and mudat. It later replaced the herbaceous

mixture of species as the gradient lessened even more. This gradual

leveling rst became evident as the near monospecic Sagittaria

latifolia low marsh community was replaced by Sagittaria platyphilla

on only the lower portions between the shallow water edges of the

open water, which we subsequently named pre-emergent. In 1997,

we identied only an extended pre-emergent habitat (the area

between sites C and D in Fig. 3B) fronting the low marsh habitat with

no mudat habitat present.

Not surprisingly, the rate of accumulation of elevation capital also

varies among the habitats. The high sediment concentrations in the

Mississippi River ensurethat sedimentation in theopen water areas of

the pond remains comparatively high year after year. The high, linear

rates of sediment inlling in the open water habitat contrast with the

pre-emergent habitat where the accretionary response slows and

becomes more variable. This shallower sub-aqueous habitat is

apparently more sensitive to changes in Mississippi River discharge

and sediment load than the open water habitat as indicated by thespring sedimentation events separated by 1–2 year periods of little

sediment deposition (Fig. 5D). Thus, in the 25-year old splay, the

conversion of pre-emergent to low marsh habitat apparently occurs

through the cumulative effects of modest biennial or triennial spring

sedimentation events. This is not to say that the conversion to

low marsh could not occur abruptly as a result of a single, large

sedimentation event (e.g., a storm or historic river ood) as occurred

in the 10-year old splay in 1985 when the mudat habitat converted

to low marsh after a 13–14 cm spring sedimentation event. In

addition to the down-lobe pattern of increasing elevation capital

with decreasing elevation (Fig. 6), the data from the high marsh

(Site B), where a breach in the shoreline levee introduced sediments

to the back-marsh that caused a sharp increase in the accretion rates,

suggests that the accretion rate decreases with distance from theshorelinelevee, as hasbeen reported for other estuarinetidal marshes

(Reed et al., 1999; Temmerman et al., 2004).

The development of vegetated marsh marks an important tran-

sition point in the accretionary development of the splay. Although

river-born sediment inlling caused the initial increase in substrate

elevation that led eventually to wetland formation, now both mineral

sediment deposition and plant organic matter accumulation in the

soil contribute to further increases in elevation. The rate of accrual of

elevationcapital in allof theemergentvegetated habitats (i.e., thelow

and high marsh, the forest habitat) slows dramatically compared to

the rates of the subaqueous habitats, in large part because of their

higher elevation and the subsequent reduction in the frequency,

depth and duration of ooding. Vertical development of the substrate

isnowinuencedby the trapping of sediment by the vegetation when

Fig. 8. Below-ground biomass at each of the ve habitats along the southern splay lobe,

1997–2000.


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the marsh surface is ooded and the accumulation in the soil of plant

organic matter that consists primarily of roots and rhizomes in this

high energy environment where surface litter is washed away ( Reed,

2002; Rybczyk and Cahoon, 2002).

Given the reduction in ooding and sediment delivery to these

higher-elevation habitats, the role of below-ground biomass in

maintaining soil volume and elevation capital in these habitats is

critical (Figs. 7 and 8). During the rst year of vegetative colonization

from 1985 to 1986, below-ground biomass increased more than oneorder of magnitude (Fig. 7) as a result of root growth by a rapidly

increasing number of new plants (as deduced from the increase in

above-ground biomass) invading an unoccupied niche (i.e., a bare

sediment surface at an elevation suitable for emergent plant

establishment and growth). We propose the following mechanism

to explain the initial phase of marsh formation (i.e., conversion of

mudat to low marsh).

During the rst year of colonization of this empty niche, plant

density is increasing rapidly and root growth rates are high because the

plants are allocating resources to root growth in order to scavenge for

nutrients.At thesame time, mortality rates of thenew, young root tissue

are low. This unique circumstance accounts for the explosive rate of

accumulationof below-ground biomass, whichquickly helpsto stabilize

the sub-aerial muddy substrate vulnerable to erosion by waves and

storms. As the niche lls and the plant community matures during the

next 2–3 years, plant colonization slows and root mortality and

decomposition increase, resulting in a gradual decline in below-ground

biomass. Over time, the low and high marsh habitats continue to accrue

elevation relative to sea level, albeit at a much slower rate. Indeed, the

marsh that began to form in 1985 was not fully developed until 1988

(White, 1993). We hypothesize that the high marsh replaces the low

marsh, and the forested wetland replaces the high marsh by a process

Morris (2006) refers to as geomorphological displacement. That is, the

invading species modies its environment and raises the elevation to a

levelthat excludesthe original species. Theupper limit to theacquisition

of elevation capital is determined by the optimum growth range of

the vegetation at a site (Morris et al., 2002), which is directly related

to the tidal range at that site (McKee and Patrick, 1988; Kirwan and

Guntenspergen, 2010).The forest site represents the highest elevation and level of maturity

for this lobe environment. At this mature stage of development, the

maintenance of intertidal elevations is controlled by the feedbacks

and adjustments among plant biomass density, sedimentation, and

the local rate of relative sea-level rise (Reed, 2002; Morris, 2007). Has

the acquisition of elevation capital ceased at this mature site such that

vertical development of the substrate only counterbalances the local

processes of subsidence and sea-level rise, which conservatively is

N10 mm/yr? We hypothesize that the forest site is still acquiring

elevation capitalfor tworeasons. Therate of elevationchange (6 mm/yr,

Table 4) lags behind the relative sea-level rise rate and the forest is

beginning to convert to herbaceous marsh, indicating that the marsh

elevation is becoming lower relative to local sea level. In addition,

relative sea-level rise rates estimated from tide gauges do not includethe subsidencethat occurs above the base of the piling to which the tide

gauge is attached. However, it is for that portion of the substrate above

the base of the piling that the SET-MH technique measures shallow

subsidence (Cahoon et al., 1999; Rybczyk and Cahoon, 2002). Adding

ourestimates of shallow subsidence to theN10 mm/yrrate derived from

the tide gauges effectively makes the revised relative sea-level rise rate

at the forest site N12–16 mm/yr (Table 2). These data indicate that

relative sea-level rise is outpacing wetland vertical development in

themid to outer reaches of thesplay,suggesting thataccrualof elevation

capital is continuing in an attempt to ll the accommodation space.

Given the reduction in hydrologic ef ciencies of this mature crevasse

splay, it is unlikely that the mid to outer reaches of the splay will attain

an elevation where the rate of accrual of elevation capital is zero (i.e.,

the accommodation space is lled completely) before the degradation

phase of the delta cycle begins, if it has not already begun. In contrast,

the forest habitats on the oldest lobes of the splay adjacent to the

crevasse opening (see 2000 image in Fig. 2) were stable and healthy at

this time. This suggests that these oldest forest settings are still accruing

elevation capital or are in equilibrium (i.e., zero accrual) with relative

sea-level rise.

5.2. Implications for wetland sustainability

The processes of growth and development of a crevasse splay

provide an analog for the key factorscontrolling wetlandformation and

sustainability acrossmajordelta lobes. In a maturemarsh, theimportant

role of vegetative biomass in maintaining elevation capital has several

implications for wetland sustainability. A mature vegetated substrate

provides some resistance to erosion caused by high-magnitude, low-

frequency (acute) events (e.g., hurricanes) through the properties of

vegetated substrates to bind soil and baf e storm surges (Cahoon,

2006), and resilience to low-magnitude, high frequency (chronic)

events (e.g., sea-level rise) through vegetation trapping of sediments

(Gleason et al., 1979) and accumulation of organic matter in the soil

(Nyman et al., 2006). Also, models predict that if inorganic sediment

supply decreases or sea-level rise accelerates in an already sediment-

poor marsh, some marshes can maintain elevation by increasing plant

production andorganic carbonstorage, at least until the rate of sea-level

rise exceeds some critical threshold (Mudd et al., 2009).

Stabilization of elevation capital by vegetation means that wetland

elevation is vulnerable to environmental factors that inuence plant

production, including eutrophication of the local water body, elevated

concentrations of atmospheric CO2, herbivory, re, and increased

ooding. These factors can have either a positive or negative effect on

elevation. For example, increased above-ground growth related to N

and P fertilization can lead to increased sediment trapping capacity

(Morris et al., 2002), and elevated atmospheric concentrations of CO2can lead to increased root growth and elevation gain in some species

(Langley et al., 2009; Cherry et al., 2009). Conversely, nitrogen

enrichment can negate, at least in part, the stimulatory effect of CO2on root growth and elevation gain (Langley et al., 2009), and cause a

reduction in below-ground production and negative elevationtrajectories in mangrove forests where elevation gain occurs primarily

through accumulation of root matter (McKee et al., 2007). Grazing by

nutria in a brackish marsh can cause a reduction in below-ground

production that leads to decreased elevation (Ford and Grace, 1998).

Burning of a brackish marsh can cause an increase in the volume of

root biomass that leads to increased elevation (Cahoon et al., 2004).

Plant production by Spartina alterni ora increases as the elevation

of the salt marsh becomes lower within its optimum growth range,

and then decreases as elevations approach the minimum depth at

which vegetation can grow (Morris et al., 2002). Lastly, death of the

vegetation leads to root decomposition and rapid peat collapse, and

rapid conversion of the wetland back to mudat or shallow open

water habitat (Cahoon et al., 2003; Morris et al., 2002).

6. Conclusions

Small crevasse splay environments provide a mesocosm for

evaluating wetland formation and maintenance processes (from

subtidal through to mature marsh habitats) on a decadal time-scale.

There are three distinct phases to the accrual of elevation capital and

wetland formation: sediment inlling, vegetative colonization,

and development of a mature wetland community. Mineral sediment

inlling initially leads to rapid increases in elevation relative to sea

level and creates a distinct elevation gradient. Rates of inlling slow

and become non-linear as elevation increases. When elevation

increases to within the intertidal zone and the mud surfaces become

sub-aerial, vegetative colonization occurs within the rst growing

season. An explosively high rate of belowground root production



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quickly stabilizes the loosely consolidated sub-aerial sediments.

The accrual of elevation capital slows dramatically in this marsh

environment. Vertical development is now driven by sediment-

trapping by the vegetation and accumulation of plant organic matter

in the soil. The rate of accrual of elevation capital continues to slow as

elevation increases and marsh converts to forest. All three phases

occur rapidly (b8 years) during initial splay formation. Then as the

pond lls and the hydrologic gradient becomes reduced, the rate of

accrual of elevation capital and the maturation of the wetlandcommunity slows. Indeed, the high marsh that formed in 1988 just

13 years after the crevasse opening was still high marsh in 2003.

Given the mature stage of the splay (i.e., the size of the splay relative

to the receiving pond), this high marsh environment may never

become a forested wetland.

The formation of vegetated communities marks an important stage

in splay development and stability. The presence of vegetated marsh

stabilizes the loosely consolidated sediment. Furthermore, vertical

development and survival of the marsh now depend upon the health

and productivity of the plant community. This may not be of critical

importance in a short-lived crevasse splay (b30 years) where subsi-

dence soon dominates vertical development after marsh formation.

However, in large delta lobes andother non-deltaic wetlands, therole of

plant productivity in vertical development can be critical for wetland

sustainability. To improve projections of wetland sustainability, further

research is needed on the environmental factors (e.g., storms,

disturbance, eutrophication, increased temperatures, and changes in

precipitation and river ows) and their interaction with the plant

community dynamics and physical processes within the marsh that

control elevation change throughout all landscape scales (single lobe to

full delta) in thelife of a delta. Both wetlandformation andmaintenance

processes are complex and non-linear.

Acknowledgements

We thank T. Crane, L. Wilkinson, R. Holland, B. Segura and B. Perez

for their help with eld work and sample processing in the lab.

J. Harris and the staff of the Delta National Wildlife Refuge provided

invaluable logistical support for this project. P. Hensel providedstatistical advice and support. Use of trade, product, or rm names

does not imply endorsement by the U.S. Government.

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