Gulf and Caribbean Research Vol 25, 105-109, 2013

Manuscript received, September 24, 2012; accepted, November 28, 2012

SHORT COMMUNICATION

EFFECTS OF CLOSURE OF THE MISSISSIPPI RIVER GULF OUTLET ON
SALTWATER INTRUSION AND BOTTOM WATER HYPOXIA IN LAKE
PONTCHARTRAIN
Michael A. Poirrier
Department of Biological Sciences, University of New Orleans, Lakefront Campus, 2000 Lakeshore Drive, New Orleans, LA 70148
USA; email: mpoirrie@uno.edu

KEY WORDS: estuary, benthos, IHNC, MRGO, canal
INTRODUCTION
Saltwater intrusion from the Mississippi River Gulf Outlet (MRGO) through the Inner Harbor Navigation Canal
(IHNC) produced salinity stratification in Lake Pontchartrain which caused episodes of bottom water hypoxia and
anoxia from 1968—2009 (Poirrier et al. 2009). This resulted
in significant changes in benthic invertebrate community
structure which were more distinct and persistent in a 250
km2 (100 mi2) area north of the IHNC (Abadie and Poirrier
2001a, 2001b) where more saline water entered.
The MRGO was a deep draft shipping channel which provided a more direct route from the Gulf of Mexico (GOM) to
New Orleans. Construction began in 1958 and the MRGO
was completed and fully operational in 1968. However,
planned economic development along the outlet never materialized and maintenance costs greatly exceeded economic
benefits. The MRGO also impacted adjacent wetlands and
contributed to the storm surge from Hurricane Katrina that
flooded New Orleans (Shaffer et al. 2009). The need to improve hurricane protection and prevent additional damage
to coastal resources resulted in its de—authorization as a
navigation channel in 2008. The MRGO was closed in July
2009 following the construction of a rock dam across the
channel near Bayou La Loutre, about 38 km from the intersection with the Gulf Intracoastal Waterway (GIWW). Flow
from the IHNC into Lake Pontchartrain was also blocked
by a temporary rock cofferdam, which was completed on 31
October 2010. This was to allow construction of a larger cofferdam and a 30 x 5.5 m (95 x 18 ft) sector gate as well as
two 15 x 5.5 m (50 x 18 ft) vertical lift gates for hurricane
surge protection. The gates were opened to navigation in July
2012. Construction of the IHNC—Lake Borgne surge barrier, which included a sector gate on the GIWW, a vertical
lift gate at Bayou Bievenue and a concrete barrier wall across
the MRGO began in May 2009 and was completed in June
2011. It reduced tidal exchange through marshes between
Lake Borgne and the MRGO. The floodgates at the mouth
of the IHNC were opened on 3 August 2012. Details, including maps of the MRGO and other waterways and design

and status of recent U.S. Army Corps of Engineers projects
related to the IHNC and MRGO, are available at www.nolaenvironmental.gov as well as other internet sites.
The past occurrence of saltwater intrusion and associated
detrimental low dissolved oxygen (DO) from the MRGO
is well documented. Poirrier (1978) was first to report that
salinity stratification and associated low bottom DO from
MRGO saltwater intrusion occurred in Lake Pontchartrain
and discussed possible effects on biota. In this paper, it was
assumed that significant intrusion started when the MRGO
was completed in 1968. Sikora and Sikora (1982) found
through various measures of community structure that a
stressed benthic community was present in southern Lake
Pontchartrain. They attributed this stress mainly to chemical contamination. However, based on studies conducted
in 1976—1978, Junot et al. (1983) found that the principal
driver of community change was detrimental low DO associated with saltwater intrusion from the IHNC. Poirrier et al.
(1984) collaborated with Schurtz and St. Pè (1984) in a more
extensive study that included analysis of contaminants and
distribution of anoxia and hypoxia which confirmed that
low DO was the major source of stress. Further studies of
stratified water circulation were conducted by Georgiou and
McCorquodale (2000). Abadie and Poirrier (2000) found
that the density of large (> 21 mm) Rangia cuneata increased
after the cessation of shell dredging in 1990. However, they
also documented that large clams were absent from about
a 250 km2 (100 mi2) area north of the IHNC (Abadie and
Poirrier 2001a, 2001b). The absence of clams was attributed
to low DO. Additional information on the distribution of
anoxia and hypoxia and effects on benthos is summarized
by Poirrier et al. (2009) and Shaffer et al. (2009). The goal
of this study was to determine if MRGO closure by hurricane flood protection projects stopped saltwater intrusion,
salinity stratification, and associated low bottom water DO
in Lake Pontchartrain.
MATERIALS AND METHODS
The area of study is in Lake Ponchartrain, LA north of

105

 Poirrier

km); D3 30.08333N, 90.08333W (7.33 km); D5 30.0833N,
90.05000W (5.90 km); D7 30.08333N, 90.01667W (6.05
km) (Figure 1). Not all sites were sampled on each trip due to
bad weather and limited resources. Measurements were not
taken during winter months and after storm events that are
known to disrupt stratification (Poirrier 1978). Site B7 was
not sampled in 2010, because no stratification was detected
in 2009. Site D3 was sampled more often upon the elimination of site B7.

Figure 1. Map of study area showing the 7 sampling sites in Lake Pontchartrain, and location of the Inner Harbor Navigation Canal (IHNC) and
flood gates, the Mississippi River Gulf Outlet (MRGO), the Gulf Intracoastal
Waterway (GIWW) and the IHNC—Lake Borgne surge barrier. The MRGO
dam at Bayou La Loutre which was off the map scale was included and a gap
in the MRGO was used to indicate its true position 28 km SE of the arrow.

the outlet of the IHNC, an area impacted by changes in
water movement through the GIWW, MRGO and IHNC
due to flood control structures (Figure 1). Surface salinity
and DO (30 cm below surface) and bottom salinity and DO
(30 cm from bottom) were measured with either a YSI 85
meter or a YSI 600 sonde. Depth was measured with a calibrated, weighted chain. A benthic invertebrate sampling site
biD7 (Poirrier et al. 2009) located 5 km north of the IHNC
(30.07500N, 90.04916W) had representative stratification while the MRGO was open. With the exception of
fall 2007, it was sampled every fall from 1997 through
2011. Data from this typical site were used to present representative changes that occurred in long—term differences between surface and bottom salinity and dissolved oxygen as flood protection projects that closed the MRGO
were completed.
A sampling grid north of the IHNC established
in past studies (Poirrier et al. 2009) was used to determine the distribution of stratified water after the 2009
MRGO closure. Surface and bottom temperature, salinity, DO and depth were measured in most months
from July to November 2009, March to July 2010 and in
August 2012 after the gates at the mouth of the IHNC
were opened. Sites near the mouth of the IHNC were
sampled first, and sites away from the canal were added
until stratification was no longer detected. After closure
of the MRGO, it was determined that only 6 sites within 8 km of the IHNC needed to be sampled to confirm
the extent of stratification. Sites used with geographic
positions and distances from the IHNC were as follows:
B3 30.05000N, 90.08333W (4.98 km); B5 30.05000N,
90.05000W (2.44 km); B7 30.05000N, 90.01667W (2.77

RESULTS
A comparison of surface and bottom salinity and DO (Figure 2) from site biD7 indicated that following the closure of
the MRGO at Bayou La Loutre in July 2009, salinity differences were less than 0.2 and DO differences were less than
10%, illustrating that the closure stopped significant levels of
saltwater intrusion, stratification, bottom water anoxia and
hypoxia. Seasonal data from 6 sites near the mouth of the
IHNC following the closure of the MRGO supported these
results; no differences in surface and bottom salinity values
greater than one were present and there was no evidence of
hypoxia (DO < 2 mg/l) (Table 1). Differences between surface and bottom salinity ranged from 0.2—0.9. Dissolved oxygen % saturation differences ranged from 0—38 %. Surface
and bottom salinity differences greater than 0.1 (the accuracy
and resolution of the instrument) occurred on five sampling
events; site B5 in July 2009 (0.2), sites B5 and B7 in early
August 2009 (0.4 & 0.6), site B5 in late August 2009 (0.9)
and site B3 in October 2009. Surface and bottom dissolved

106

A

B

Figure 2. Comparison of (A) surface and bottom salinity and (B) surface and bottom dissolved oxygen % saturation values at site biD7 located 5 km north of the IHNC before and after the 2009 closure of the
MRGO.

 Mississippi River Gulf Outlet closure

TABLE 1. Surface and bottom salinity and dissolved oxygen measurements from 6 sites within 8 km of the Inner Harbor Navigation Canal (IHNC)
after closure of the Mississippi River Gulf Outlet (MRGO). Numbers in bold represent differences > 0.1 salinity unit or > 10% DO between surface
and bottom measurements.
Date
7/25/09

8/11/09

8/24/09
10/28/09
11/19/09
3/31/10

4/29/10
5/25/10

6/24/10

7/10/10

8/12/12

8/23/12

Site

Surface
Salinity

Bottom
Salinity

Salinity
Differance

Surface
DO%

Bottom
DO%

%DO
Difference

Bottom
DO mg/L

B3
B5
B7
D5
D7
B3
B5
B7
D5
B5
B3
B5
D5
B3
B5
B7
B3
B5
D3
D5
D7
B3
B5
D5
B3
B5
D3
D5
D7
B3
B5
D3
D5
D7
B3
B5
D3
D5
D7
B3
B5
D3
D5
D7
B3
B5
D3
D5
D7

5.1
5.1
5.1
4.9
4.9
5.2
5.3
5.2
5.3
6.1
6.6
6.7
7.1
5.9
6.2
6.4
1.6
1.6
1.6
1.6
1.7
1.9
1.9
2.0
1.7
1.8
1.9
1.8
1.8
2.3
2.3
2.1
2.3
2.3
2.4
2.4
2.4
2.5
2.5
2.5
2.6
2.7
3.0
2.6
3.0
3.0
3.2
3.0
2.9

5.2
5.3
5.1
4.9
4.9
5.2
5.7
5.8
5.4
7.0
6.8
6.8
7.1
5.9
6.2
6.4
1.6
1.6
1.7
1.6
1.7
1.9
1.9
2.0
1.7
1.8
1.9
1.8
1.8
2.3
2.3
2.1
2.3
2.3
2.4
2.4
2.5
2.5
2.5
2.5
2.7
2.7
3.0
2.6
3.0
3.0
3.2
3.0
2.9

0.1
0.2
0
0
0
0
0.4
0.6
0.1
0.9
0.2
0.1
0
0
0
0
0
0
0.1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.1
0
0
0
0.1
0
0
0
0
0
0
0
0

99
93
99
91
95
97
107
97
105
89
98
102
93
96
90
98
91
93
94
94
97
92
85
92
93
92
90
85
86
95
84
94
92
93
108
102
103
98
104
101
102
103
105
101
102
103
102
105
105

93
89
95
91
89
90
69
82
99
67
93
93
92
90
88
95
88
89
93
96
96
89
84
90
90
91
86
86
86
86
84
87
89
91
102
100
100
95
102
82
92
91
87
70
92
98
93
87
93

6
4
5
0
6
7
38
15
6
22
5
9
1
6
2
3
3
4
1
2
1
3
1
2
3
1
4
1
0
9
0
7
3
2
6
2
3
3
2
19
10
12
18
31
11
5
9
18
12

7.0
6.4
7.0
6.7
6.6
6.6
5.1
6.3
7.2
5.0
8.1
8.2
8.2
8.4
8.0
8.7
8.4
8.5
8.9
9.2
9.2
7.7
7.2
7.8
6.9
6.9
6.7
6.6
6.6
6.6
6.2
6.4
6.6
6.8
7.7
7.5
7.5
7.1
7.6
6.3
7.0
6.9
6.4
5.3
7.1
7.5
7.1
6.4
7.2

oxygen differences greater than 10% saturation were found
on 3 sampling events: 11 August 2009 (B5 [38%; 5.1 mg/l]
and B7 [15%; 6.3 mg/l]) and 24 August 2009 (B5 [22%; 5.0
mg/l]). Lower bottom DO, but not hypoxic DO, was associated with stratification. Mean depths for sites throughout
the sampling period were: 4.5 ± 0.1 m (B3), 4.9 ± 0.1 m (B5),
3.9 ± 0.1 m (B7), 4.7 ± 0.1 m (D3), 4.7 ± 0.1 m (D5), and

4.8 m ± 0.1 (D7). Low DO values were found at 2 of the
deeper sites north of the IHNC; the lowest values were at B5,
closest to the IHNC. No evidence of salinity stratification
or associated detrimental bottom DO was found in 2010.
Data obtained in August 2012 after the IHNC gates at Lake
Pontchartrain were opened indicated the continued absence
of salinity stratification and hypoxia. However, surface and

107

 Poirrier

bottom DO % saturation differences ranged from 5—31%.
Surface water temperatures ranged from 17.2—30.9oC.
Bottom temperatures ranged from 17.2—30.0oC. Water
temperature followed typical seasonal trends. Differences
between surface and bottom temperatures ranged from 0.0—
0.8oC. The temperature differences for the sites with relatively low DO were 0.1oC for B5, 0.2oC for D7 and 0.8oC for
D5 on 11 August 2009. Temperature differences within this
range were common at other sites that did not have relatively
low bottom DO values.
DISCUSSION
Data obtained in this study indicate that MRGO closure
at Bayou La Loutre in July 2009 has stopped saltwater intrusion and associated low DO through the IHNC into Lake
Pontchartrain. Episodes of extensive anoxia and hypoxia reported in previous studies (Schurtz and St. Pè 1984, Poirrier
et al. 2009) were not found after the closure. A trend toward
decreased mixing (Figure 2) appears to be present from 2006
to 2008, but this is due to lack of sampling in 2007 and a
weather event in 2008. Points on the graph were connected
with a line to compare surface and bottom values and do not
imply any rate of change over time before the 2009 closure.
Some evidence of residual stratification was detected during
July, August, and October 2009 (Table 1). No stratification
was detected in November 2009 or from March through July
2010. Construction of the temporary IHNC rock cofferdam
began in February 2010 and was completed in October 2010.
Sampling was discontinued after July 2010 because no stratification was detected and intrusion of higher salinity bottom
water appeared to be blocked by the cofferdam which was
in the final stages of construction. The IHNC—Lake Borgne
surge barrier was under construction as well, and should
have stopped any remaining exchange.
The residual stratification observed in 2009 was not surprising. Deep scour channels are present on both sides of
the mouth of the INHC. Saline bottom water was probably
present in the IHNC and MRGO after closure at Bayou La
Loutre. Closure at Bayou La Loutre greatly reduced tidal exchange between the GOM and Lake Pontchartrain through
the navigation canals. Any potential exchange was eliminated

after placement of the temporary cofferdam for construction
of flood protection gates. As in previous studies, temperature differences were not responsible for stratification.
Based on data obtained in August 2012, opening the gates
at the IHNC did not produce saltwater intrusion. This is an
expected result considering the MRGO was closed and the
depth and cross—sectional area of the IHNC at Lake Pontchartrain was reduced by the flood gates. The higher surface
to bottom DO differences in this data set may be due to
measurements taken only during hot August weather, or to
reduced water exchange due to control structures.
Closure of the MRGO has reversed north—to—south salinity gradients in eastern Lake Pontchartrain. Salinity measurement taken prior to closure of the MRGO were higher near
the south shore due to saltwater intrusion and decreased
with increasing distance from the mouth of the IHNC. Since
the closure, salinity is generally higher near the north shore
and decreases with decreasing distance to the IHNC.
After the MRGO closure, recovery of the benthic invertebrate community in the 250 km2 area north of the IHNC has
been slower than anticipated. In addition to the long—term
effects of the MRGO, Hurricane Katrina in 2005 caused
an acute disturbance that resulted in loss of R. cuneata and
other community dominants from 815 km2 (50% of the bottom) at depths > 3.7 m and limited recovery was observed
in 2006 (Poirrier et al. 2008). Fall 2008 benthic invertebrate
samples indicated an additional lake—wide disturbance,
which further reduced the density of R. cuneata and other
community—dominant species. This disturbance was more
widespread than Katrina and extended to depths < 3.7 m.
Ray (2009) studied effects of the 2008 Bonnet Carré Spillway opening on benthos and found similar results. He attributed the stressed community to hurricanes Ike and Gustav. Samples from fall 2009 and 2010 indicate some recovery
toward pre—hurricane Katrina conditions, as evidenced by
fewer annelids, and more mollusks and arthropods (M. Porrier, unpublished data). However, large R. cuneata clams were
rare or absent in 2009 and 2010 samples. Recovery studies
are still in progress and details will be submitted for publication after ongoing work is completed.

Acknowledgments
I thank R. Madison for her assistance and support in the preparation of this manuscript, R. Caillouet
for help with field surveys, and B. Spalding and A. Ferguson for their help with earlier studies. This project was supported by the NOAA, Pontchartrain Restoration Program and the Louisiana Department of
Wildlife and Fisheries.

108

 Mississippi River Gulf Outlet closure

literAture cited
Abadie, S.W. and M.A. Poirrier. 2000. Increased density of large
Rangia clams in Lake Pontchartrain after the cessation of shell
dredging. Journal of Shellfish Research. 19:481—485.

Poirrier, M.A., Z. Rodriguez del Rey, and E.A. Spalding. 2008.
Acute disturbance of Lake Pontchartrain benthic communities by Hurricane Katrina. Estuaries and Coasts 31:1221—1228.

Abadie, S.W. and M.A. Poirrier. 2001a. Recent trends in water
clarity and clam abundance. In: S. Penland, A. Beall, and J.
Waters, eds. Environmental Atlas of the Lake Pontchartrain
Basin. Lake Pontchartrain Basin Foundation, New Orleans,
LA, USA, p.165 .

Poirrier, M.A., E.A. Spalding, and C.D. Franze. 2009. Lessons
learned from a decade of assessment and monitoring studies
of submersed aquatic vegetation and benthic invertebrates in
Lake Pontchartrain. Journal of Coastal Research. SI 54:88—
100.

Abadie, S.W. and M.A. Poirrier. 2001b. Rangia Clams as indicators of Hypoxia in Lake Pontchartrain. In: S. Penland, A.
Beall, and J. Waters, eds. Environmental Atlas of the Lake
Pontchartrain Basin. Lake Pontchartrain Basin Foundation,
New Orleans, USA, p. 166.

Ray, G.L. 2009. Response of benthic invertebrate communities
following the 2008 Bonnet Carre Spillway Opening. Technical Report. U. S. Army Engineers, New Orleans District, New
Orleans, LA, USA, 150 p.

Georgiou, I.Y. and J.A. McCorquodale. 2000. Salinity stratification from a navigation canal in a shallow lake. In: G. A.
Laurence, R. Pieters, and N. Yonemitsu, eds. Stratified flows,
Volume 2. International Association for Hydraulic Research,
Madrid, Spain, 859—864 p.
Junot, J.A., M.A. Poirrier, and T.M. Soniat. 1983. Effects of saltwater intrusion from the Inner Harbor Navigation Canal on
the benthos of Lake Pontchartrain, Louisiana. Gulf Research
Reports 3:247—254.
Poirrier, M.A. l979. Studies of salinity stratification in Southern
Lake Pontchartrain near the Inner Harbor Navigation Canal.
Proceedings of the Louisiana Academy of Sciences 4l:26—35.
Poirrier, M.A., T.A. Soniat, Y.A. King, and L.E. Smith. 1984. An
evaluation of the southern Lake Pontchartrain benthos community. Technical report. Louisiana Department of Environmental Quality, Office of Water Resources, Water Pollution
Control Division, Baton Rouge, LA, USA, 80 p.

109

Schurtz, M.H. and K.M. St. Pè. 1984. Water Quality investigations
of Environmental conditions in Lake Pontchartrain. Technical Report on Interim findings. Louisiana Department of
Environmental Quality, Water Pollution Control Division,
Baton Rouge, Louisiana, USA, 85 p.
Shaffer, G. P., J.W. Day, S. Mack, G.P. Kemp, I. van Heerden,
M.A. Poirrier, K A. Westphal, D. FitzGerald, A. Melanes,
C.A. Morris, R. Bea, and P.S. Penland. 2009. The MRGO
Navigation Project: A massive human—induced environmental, economic and storm disaster. Journal of Coastal Research
SI 54:206—224.
Sikora W.B. and J.P. Sikora. 1982. Ecological characterization of
the benthic community of Lake Pontchartrain, Louisiana.
Technical Report. New Orleans District, U.S. Army Corps of
Engineers, New Orleans, LA, USA, 214 p.