U.S. Department of the Interior

U.S. Geological Survey Printed on recycled paper Fact Sheet 2013-3017March 2013

Trends and Causes of Historical Wetland Loss in Coastal Louisiana

Geologic Setting

The Mississippi River delta plain 

was constructed from fluvial sediments 

that were deposited in overlapping delta 

lobes. Thick peat deposits that accumulated in interdistributary areas underlie 

the modern delta-plain wetlands. In comparison, the chenier plain was constructed 

by the alongshore progradation of broad 

mudflats capped by thin peat deposits and 

wetland vegetation with intervening narrow, sandy beach ridges (cheniers). 

The 10 delta-plain study areas encompassed several different physiographic 

settings, including (1) an upper deltaplain levee flank of the Mississippi River, 

(2) a preexisting upper delta-plain interior 

channel, (3) four upper delta-plain 

interdistributary areas, (4) three lower 

delta-plain interdistributary areas, and (5) 

a lower delta-plain beach-ridge margin 

area. The six western chenier-plain study 

areas were located in Sabine National 

Wildlife Refuge (SNWR), which occupies a broad, shore-parallel, low-lying 

area that formed between the higher 

elevation beach ridges along the gulf 

shoreline to the south and upland areas to 

the north.

Historical Wetland Loss

In the upper delta plain, extensive 

areas of formerly emergent marsh 

converted to open water before 1978, 

with little significant land-area change 

since (fig. 2). Analysis of historical aerial 

photography showed that the most rapid 

wetland loss at many of these study areas 

occurred during the late 1960s and 1970s. 

In comparison, at most of the lower deltaplain study areas, some areas of wetland 

loss were apparent by the middle to late 

1970s, but the majority of historical land 

loss occurred between 1978 and 1990. 

Project Summary

Wetland losses in the northern Gulf Coast region of the United States are so extensive that they represent critical concerns to government environmental agencies and natural resource managers. In Louisiana, almost 3,000 square kilometers 

(km2) of low-lying wetlands converted to open water between 1956 and 2004 (fig. 1), and billions of dollars in State and 

Federal funding have been allocated for coastal restoration projects intended to compensate for some of those wetland 

losses. Recent research at the St. Petersburg Coastal and Marine Science Center (SPCMSC) focused on understanding the 

physical processes and human activities that contributed to historical wetland loss in coastal Louisiana and the spatial and 

temporal trends of that loss.

The physical processes (land-surface subsidence and sediment erosion) responsible for historical wetland loss were 

quantified by comparing marsh-surface elevations, water depths, and vertical displacements of stratigraphic contacts at 10 

study areas in the Mississippi River delta plain and 6 sites at Sabine National Wildlife Refuge (SNWR) in the western chenier plain (fig. 1). The timing and extent of land loss at the study areas was determined by comparing historical maps, aerial 

photographs, and satellite imagery; the temporal and spatial trends of those losses were compared with historical subsidence 

rates and hydrocarbon production trends. 

Figure 1. Satellite image of coastal Louisiana showing extent of historical (1956 to 2004) wetland loss (in red). White boxes encompass study-area 

locations in the western chenier and delta plains. Landsat 5 Thematic Mapper image acquired December 13, 2006.

Chenier Plain

Delta Plain

TX

LA MS AL GA

FL

Gulf of Mexico

0 50 KILOMETERS

0 50 MILES
At some sites, areas of formerly continuous marsh were alternately emergent or 

partially submerged on historical aerial 

photographs acquired prior to 1978, 

depending on whether the images were 

acquired under low or high water conditions, respectively. Between 1978 and 

1990, most of these “wet marsh” areas 

became permanently submerged. In the 

western chenier plain, most of the historical wetland loss occurred prior to 1978; 

however, some expanses of wet marsh in 

the western part of SNWR persisted into 

the 1980s.

Wetland Subsidence and 

Erosion

The two primary physical processes 

responsible for historical wetland loss in 

coastal Louisiana are land-surface subsidence and erosion. These processes create 

accommodation space, which is described 

by the difference between the emergent 

marsh-surface elevation and the existing 

water depth at a core site (fig. 3). The 

predominance of subsidence or erosion at 

the study areas varied by physiographic 

and geologic setting. Compared with the 

upper delta-plain interdistributary study 

areas, where subsidence greatly exceeded 

erosion, erosion was about equal to or 

greater than subsidence at most of the 

lower delta-plain study areas. At all of 

the delta-plain study areas, organic-rich 

marsh deposits (peats) were preserved 

where formerly emergent wetlands were 

converted to open water. In contrast, erosion generally exceeded subsidence at the 

western chenier plain study areas, and the 

thinner peat deposits were mostly eroded 

at the open-water sites.

Induced Subsidence 

Related to Hydrocarbon 

Production

Historical delta-plain subsidence rates 

were assessed by analyzing rates of relative sea-level rise derived from tide-gage 

records, elevation changes at established 

benchmarks, and Global Positioning System vertical velocities at Continuously 

Operating Reference Stations. Integration of these data sources indicated that 

subsidence rates were slow (≤ 5 millimeters per year, mm/yr) from 1947 until the 

mid-1960s, relatively fast (≥ 10 mm/yr) 

from the mid-1960s until the early 1990s, 

1958 1974

1989 2004

0 10 KILOMETERS

0 10 MILES

EXPLANATION

Core location

1956 - 1978 land loss

1978 - 1990 land loss

1990 - 2004 land loss

Figure 2. Pre- and post-land-loss aerial photographs overlaid with historical wetland-loss 

trends at selected upper delta-plain interdistributary study areas. Image sources are: Edgar 

Tobin Aerial Surveys controlled aerial mosaics (1958), National Aeronautics and Space 

Administration color-infrared aerial photography (1974), and U.S. Geological Survey Digital 

Orthophoto Quarter Quadrangles (1989 and 2004). Data sources from which the land-loss 

trends were derived are: National Wetlands Inventory habitat data (1956 and 1978) and 

Landsat 5 Thematic Mapper satellite imagery (1990 and 2004).

and then slow since the early 1990s. Subsidence rates for the slow periods were 

comparable to rates that are attributed to 

natural processes such as shallow sediment compaction. 

Some areas of wetland loss in the 

Gulf Coast region coincide with some of 

the Nation’s largest oil-and-gas fields. 

Several lines of evidence support the 

hypothesis that long-term, large-volume 

hydrocarbon production resulted in landsurface subsidence and wetland loss in 

coastal Louisiana. There is close temporal 

and spatial correlation between rates of 

historic wetland loss, subsidence, and 

hydrocarbon production on the Louisiana 

coastal plain. Peak annual production 

from onshore Louisiana oil-and-gas fields 

coincided with the period of greatest wetland loss (fig. 4). In addition, historical 

subsidence rates measured at benchmarks 

were generally higher over oil-and-gas 

fields (fig. 5). Numerical modeling by 

SPCMSC collaborators at Stanford 

University confirmed that reservoir 

depletion and pore-pressure reductions 

at large oil-and-gas fields in the southern Mississippi River delta plain were 

capable of inducing land-surface subsidence. If accelerated historical subsidence 

rates and wetland loss were induced by 

hydrocarbon production, then the recent 

reductions in subsidence rates likely 

reflect a balancing of subsurface stresses 

after production decreased. 

Marsh

core

Marsh

core

Open-water

core

Sediment - water interface

Elevation datum

Reference contact

Reference emergent-marsh surface

E

TW

S

A

TR

Organics Mud and sand

EXPLANATION

Figure 3. Conceptual diagram showing the 

stratigraphic relations used to estimate 

subsidence and erosion at an open-water 

core site. Abbreviations: A, accommodation; E, erosion; S, subsidence; TR

, reference 

stratigraphic thickness; TW, stratigraphic 

thickness in the open-water core.
1930 1940 1950 1960 1970 1980 1990 2000 2010

Year

0

50

100

150

200

250

Annual production, natural gas (mcf) x 106

0

2

4

6

8

10

12

14

16

18

20

Annual production, oil and water (bbls) x 106

0

5

10

15

20

25

Land loss (km2/yr)

Wetland loss

Oil

Gas

Water

EXPLANATION

Figure 4. Composite annual fluid production (oil, gas, and formation 

water) from delta-plain oil-and-gas fields with wetland-loss rates 

measured at five representative delta-plain wetland-loss hotspots. 

Abbreviations: mcf, millions of cubic feet; bbls, barrels; km2

/yr, square 

kilometers per year.

RACELANDVALENTINE

CUT OFF - 

BULLY CAMP

GOLDEN 

MEADOW

LEEVILLE

LAROSE

-20

-15

-10

-5

0

0 25 50 75 100

Distance (km)

Subsidence rate (mm/yr)

1952-1965: average -7.8 mm/yr

1965-1982: average -7.9 mm/yr

1982-1993: average -11.1 mm/yr

Figure 5. Historical subsidence rates calculated by the National 

Geodetic Survey from repeat leveling surveys at benchmarks along 

Bayou Lafource in the delta-plain study area. Shaded areas delineate 

approximate subsurface extents of nearby oil-and-gas fields. Abbreviations: mm/yr, millimeters per year; km, kilometers.

Historical 

Accommodation 

Formation

Accommodation is the space available 

for sediment accumulation as a result of 

sea-level rise and (or) land-surface subsidence. Historical conversion of coastalplain wetlands of the northern Gulf of 

Mexico to open water is an example 

of accommodation that formed at the 

decadal scale. Bathymetric data acquired 

at the study areas (fig. 6) were integrated 

with the areal extent of historical wetland 

loss between 1956 and 2004 to estimate 

the total volume of accommodation space 

that formed historically on the western 

chenier and delta plains.

Magnitudes of historical accommodation that formed locally at the western 

chenier plain study areas (about 20 × 

106

 cubic meters, m3) were significantly 

less than formed at the delta plain study 

areas (about 108 × 106 m3). In part, this 

reflects the much greater spatial extent 

of wetland loss and total area in the delta 

plain (fig. 1); however, normalized magnitudes of accommodation that formed 

in the western chenier plain are generally 

less than formed in the delta plain and 

are significantly less than at some upper 

delta-plain study areas. These differences 

are attributed to greater subsidence and, 

consequently, deeper water bodies and 

greater accommodation in the delta plain 

(figs. 6 and 7).

Despite differences in geologic setting, 

similarities in temporal and spatial trends 

of wetland loss indicate that historical 

accommodation formation was likely 

initiated by similar processes in both the 

delta and western chenier plains. The 

importance of land-surface subsidence 

to initiating delta-plain wetland loss is 

demonstrated by the fact that erosion was 

totally contained within the peat section 

and did not penetrate into the underlying 

mud and sand, even at sites where erosion 

exceeded subsidence and extant water 

depths were greater than the thicknesses 

of the organic-rich marsh sediments. It is 

physically impossible to erode to those 

depths and still preserve some of the 

marsh deposits. In addition, the expanses 

of wet marsh that were identified on the 

historical aerial photography also indicate 

that subsidence was the process that initiated historical wetland loss. Wet marsh is 

an intermediate stage in the progression 

from emergent wetlands to open water 

and represents nearly uniform drowning 

of large sections of marsh. In the western 

chenier and lower delta plains, initial 

subsidence likely lowered the emergent 

marshes to a position where they were 

more susceptible to erosion. 

0 1,000 METERS 0 500 METERS

EXPLANATION

Core location

Analysis extent

Elevation (cm)

 0 to 15

-20 to 0

-40 to -20

-60 to -40

-80 to -60

< -80

Figure 6. Example bathymetric grids with 

superimposed core locations from the lower 

delta plain (left) and western chenier plain 

(right). Image sources are U.S. Geological 

Survey Digital Orthophoto Quarter Quadrangles (2004). Abbreviation: cm, centimeters.
Implications and Future 

Considerations

This study provides the first comprehensive analyses of the three-dimensional 

aspects of historical wetland loss in 

coastal Louisiana and characterizes the 

likely causes of those losses. Calculated 

accommodation volumes provide estimates of the new sediment that would be 

needed just at the study areas to restore 

the coastal-plain wetlands to their pre1956 areal extent and elevations. Incorporating these results into project plans may 

improve wetland restoration strategies. 

For example, the volume of sediment 

used to create vegetated earthen terraces 

around Greens Lake in SNWR (about 0.7 

× 106 m3) is an order of magnitude less 

than the total accommodation that formed 

in the same area between 1956 and 2004. 

Similarly, at sites where wetland loss was 

driven primarily by subsidence, structures 

designed to mitigate shoreline erosion 

may not be effective. 

Considering recent and historical 

land-loss trends, and given that historical subsidence was not driven entirely by 

natural processes that would be expected 

to persist in the future, rates of future 

accommodation formation in coastal 

Louisiana will likely be low except for 

losses associated with episodic storm 

impacts. For example, about 850 km2 of 

new water areas formed in coastal Louisiana since 2004 following the passage 

of Hurricanes Katrina and Rita in 2005 

and Hurricane Ike in 2008; however, not 

enough time has passed to assess which 

of these changes are transitory and (or) 

which changes will persist as areas of 

permanent land loss and new accommodation space.

Increased storm intensities and sealevel rise due to climate change will 

likely have a significant effect on the 

resilience of the coastal wetlands along 

the northern Gulf of Mexico. Understanding the processes driving landscape 

change, as well as measurements of rates 

of change, is necessary to characterize 

the resiliency and fate of these ecosystems and provide scientific support for 

mitigation and restoration activities. Data 

from this study characterize historical 

processes that contributed to wetland 

change and establish baseline conditions 

for comparing future change. 

Peat thickness

Subsidence

Erosion

Marsh elevation

Water depth

Accommodation

-450

-350

-250

-150

-50

50

150

250

350

Value (cm)

Average and range, delta-plain study areas

Average and range, western chenier-plain study areas

EXPLANATION

Figure 7. Comparison 

of one-dimensional 

(vertical) accommodation parameters 

and measurements 

in the western 

chenier and delta 

plains. Abbreviation: 

cm, centimeters.

Both episodic events, such as storms, 

and gradual sea-level changes leave a 

signature within the sediment record that 

can be used to reconstruct environmental 

conditions through time (fig. 8). Storm 

deposits and magnitudes of sediment 

erosion and compaction in near-surface 

marsh sediments can be measured 

directly in sediment cores. Temporal 

elevation measurements can provide rates 

of change. Together these data provide 

a comprehensive characterization of 

the environment over several centuries 

and can be used to improve predictive 

capability in modeling future change. In 

addition, coastal managers are recognizing the need to incorporate subsidence 

rates into management practices and 

project designs. These methodologies can 

be applied to regions across the Gulf of 

Mexico to develop standardized protocols for monitoring and measurement of 

coastal and wetland change.

Figure 8. Top: Example of a push core collected at SNWR. Top of the core is to the 

right. Photograph by Bob Morton (2008). 

Bottom: Shoreline-protection structure 

under construction in the upper delta plain. 

Photograph by Julie Bernier (2007).

• Barras, J.A., Bernier, J.C., and Morton, 

R.A., 2008, Land area change in coastal 

Louisiana — A multidecadal perspective 

(from 1956 to 2006): U.S. Geological 

Survey Scientific Investigations Map 

3019, scale 1:250,000, 14 p. pamphlet. 

• Bernier, J.C., Morton, R.A., and Kelso, 

K.W., 2011, Trends and causes of historical wetland Loss, Sabine National Wildlife Refuge, Southwest Louisiana: U.S. 

Geological Survey Open-File Report 

2011–1169, 36 p., plus app.

• Morton, R.A. and Bernier, J.C., 2010, 

Recent subsidence-rate reductions in 

the Mississippi Delta and their geological implications: Journal of Coastal 

Research, v. 26, no. 3, p. 555-561.

• Morton, R.A., Bernier, J.C., Kelso, K.K., 

and Barras, J.A., 2010, Quantifying 

large-scale historical formation of accommodation in the Mississippi Delta: Earth 

Surface Processes and Landforms, v. 35, 

no. 14, p. 1625-1641.

• Morton, R.A., Bernier, J.C., and Kelso, 

K.W., 2009, Recent subsidence and 

erosion at diverse wetland sites in the 

southeastern Mississippi delta plain: U.S. 

Geological Survey Open-File Report 

2009–1158, 39 p., plus app. (p. 41-221).

• Morton, R.A., Bernier, J.C., Barras, J.A., 

and Ferina, N.F., 2005, Rapid subsidence 

and historical wetland loss in the Mississippi delta plain: Likely causes and future 

implications: U.S. Geological Survey 

Open-File Report 2005–1216, 116 p.

Selected References

Contact Information

Julie Bernier

U.S. Geological Survey

600 4th Street South

St. Petersburg, FL 33701

jbernier@usgs.gov

http://coastal.er.usgs.gov/gc-subsidence/