U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2014–5077

Prepared in cooperation with the U.S. Army Corps of Engineers

Effects of Hydrologic Modifications on Salinity and 

Formation of Hypoxia in the Mississippi River-Gulf Outlet and 

Adjacent Waterways, Southeastern Louisiana, 2008 to 2012
Front cover:

Top, Looking southward along the Mississippi River-Gulf Outlet at the rock barrier. 

Middle, Looking westward along the Gulf Intracoastal Waterway towards the Inner Harbor Navigation 

Canal-Lake Borgne Surge Barrier. 

Bottom, Looking southward from Lake Pontchartrain to the Seabrook floodgate complex and Inner 

Harbor Navigation (Industrial) Canal. 

All photographs courtesy of the U.S. Army Corps of Engineers.
Effects of Hydrologic Modifications on 

Salinity and Formation of Hypoxia in the 

Mississippi River-Gulf Outlet and Adjacent 

Waterways, Southeastern Louisiana, 2008 

to 2012

By Christopher M. Swarzenski and Scott V. Mize

Prepared in cooperation with the U.S. Army Corps of Engineers

Scientific Investigations Report 2014–5077

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

Suggested citation:

Swarzenski, C.M., and Mize, S.V., 2014, Effects of hydrologic modifications on salinity and formation of hypoxia in the 

Mississippi River-Gulf Outlet and adjacent waterways, southeastern Louisiana, 2008 to 2012: U.S. Geological Survey 

Scientific Investigations Report 2014–5077, 21 p., http://dx.doi.org/10.3133/sir20145077. 

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ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3803-6
iii

Acknowledgements

The authors thank Laura Lee Wilkinson and Barbara Kleiss of the U.S. Army Corps of Engineers 

for their assistance with the execution of the project.

v

Contents

Acknowledgements .....................................................................................................................................iii

Abstract ..........................................................................................................................................................1

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

Purpose and Scope .............................................................................................................................2

Description of the Study Area ...........................................................................................................2

Methods ..........................................................................................................................................................4

Effects of Hydrologic Modifications ..........................................................................................................6

Salinity ...................................................................................................................................................6

Hypoxia Formation .............................................................................................................................16

Summary .......................................................................................................................................................20

References Cited .........................................................................................................................................20

Figures

1. Map showing location of data-collection sites in the Mississippi River-Gulf 

Outlet and adjacent waterways, southeastern Louisiana ....................................................3

2. Graph showing continuous salinity in the Inner Harbor Navigation (Industrial) 

Canal (IHNC), the Mississippi River-Gulf Outlet (MRGO), Lake Pontchartrain, and 

Lake Borgne in southeastern Louisiana, 2009–12 ..................................................................7

3. Salinity contours, in parts per thousand, between miles 30 and 59 in the 

Mississippi River-Gulf Outlet in southeastern Louisiana for August 15, 2008, 

July 30, 2009, and August 19, 2010 ............................................................................................9

4. Salinity contours, in parts per thousand, between miles 30 and 59 in the 

Mississippi River-Gulf Outlet in southeastern Louisiana for July 08, 2011, and 

July 10, 2012 ................................................................................................................................10

5. Extent of hypoxic layer, between miles 20 and 59 in the Mississippi River-Gulf 

Outlet in southeastern Louisiana on October 8, 2009; hypoxia occurs when 

dissolved oxygen concentrations are less than 2 milligrams per liter .............................16

6. Vertical profiles of salinity and dissolved oxygen at transects in the Mississippi 

River-Gulf Outlet (MRGO) and the Gulf Intracoastal Waterway (GIWW) in 

southeastern Louisiana, June to October 2010 ....................................................................17

7. Vertical profiles of salinity and dissolved oxygen at transects in the Mississippi 

River-Gulf Outlet (MRGO) and the Gulf Intracoastal Waterway (GIWW) in 

southeastern Louisiana, May to October 2011 .....................................................................18

8. Vertical profiles of salinity and dissolved oxygen at transects in the Mississippi 

River-Gulf Outlet (MRGO) and the Gulf Intracoastal Waterway (GIWW) in 

southeastern Louisiana, May to October 2012 .....................................................................19
vi

Tables

1. Data-collection sites in the Mississippi River-Gulf Outlet (MRGO) and adjacent 

waterways, and in Lake Borgne and Lake Pontchartrain in southeastern 

Louisiana .......................................................................................................................................5

2. Water depth below which dissolved oxygen concentrations are less than 

2 milligrams per liter, indicating hypoxic conditions, in the Mississippi River-Gulf 

Outlet (MRGO) and adjacent surface waters in southeastern Louisiana, August 

2008 to October 2012 .................................................................................................................11

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeters (mm)

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

Area

square foot (ft2) 0.09290 square meter (m2)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8x°C)+32

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). 
Effects of Hydrologic Modifications on Salinity and 

Formation of Hypoxia in the Mississippi River-Gulf Outlet 

and Adjacent Waterways, Southeastern Louisiana, 2008 

to 2012

By Christopher M. Swarzenski and Scott V. Mize

Abstract 

The Mississippi River-Gulf Outlet (MRGO) was 

constructed between 1958 and 1968 to provide a safer and 

shorter route between the Gulf of Mexico and the Port of New 

Orleans for ocean-going vessels. In 2006, the U.S. Congress 

directed the U.S. Army Corps of Engineers (USACE) to 

develop and implement a plan to deauthorize a portion of 

the MRGO ship channel from its confluence with the Gulf 

Intracoastal Waterway to the Gulf of Mexico. In 2009, in 

accordance with plans submitted to Congress, the USACE 

built a rock barrier across the MRGO near Hopedale, 

Louisiana. Following Hurricane Katrina, Congress also 

authorized the USACE to implement the Hurricane Storm 

Damage Risk Reduction System (HSDRRS) by building 

structures in the MRGO and adjacent surface waters, to 

reduce vulnerability of this area to storm surge. The HSDRRS 

includes the Gulf Intracoastal Waterway-Lake Borgne Surge 

Barrier and Gate Complex near mile 58 of the deauthorized 

portion of the MRGO and the Seabrook Gate Complex on 

the Inner Harbor Navigation (Industrial) Canal (IHNC). By 

blocking or limiting tidal exchange in the MRGO, these 

barriers could affect water quality in the MRGO and nearby 

waters including Lake Pontchartrain, the IHNC, and Lake 

Borgne. In 2008, the U.S. Geological Survey, in cooperation 

with the USACE, began a study to document the effects of 

the construction activities on salinity and dissolved oxygen in 

these surface waters. Data were collected from August 2008 

through October 2012. 

Completion of the rock barrier in the vicinity of mile 35 

in July 2009 reduced hydrologic circulation and separated the 

MRGO into two distinct salinity regimes, with substantially 

fresher conditions prevailing upstream from the rock barrier. 

The rock barrier also contributed to a zone of hypoxia 

(dissolved oxygen less than 2 milligrams per liter) that 

formed along the channel bottom during the warmer summer 

months in each year of this monitoring; the zone was much 

more developed downstream from the rock barrier. The most 

extensive hypoxic zone was measured in October 2009 when 

it extended at least 34 miles in the MRGO, from mile 20 to 

mile 54. Construction of the surge barrier and floodgates did 

not affect salinity or dissolved oxygen in any comparable 

manner. 

The factors that contributed the most to hypoxia in 

the MRGO were the reductions in tidal water movement 

there after completion of the rock barrier combined with the 

channel depth in the MRGO, in places 10 to 30 feet deeper 

than surrounding surface water bodies. These factors helped 

to stratify salinity by reducing vertical mixing in the water 

column.

Introduction

The Mississippi River-Gulf Outlet (MRGO) was 

authorized by a March 29, 1956, Act of the U.S. Congress 

(Public Law 84-455) to provide an emergency outlet from 

the Mississippi River in the interest of National defense and 

general commerce and as a safer and shorter route between 

the Port of New Orleans and the Gulf of Mexico (U.S. Army 

Corps of Engineers, 2010). Construction of the MRGO began 

in 1958, and the channel was completed in 1968.

In 2006, the U.S. Congress directed the Secretary of 

the Army, acting through the Chief of Engineers, to plan for 

deauthorization of the MRGO from the Gulf Intracoastal 

Waterway (GIWW) to the Gulf of Mexico. The USACE 

began a study to develop a comprehensive plan to deauthorize 

deep-draft navigation, evaluate any navigation functions that 

should be maintained, and to be consistent with Louisiana’s 

Comprehensive Master Plan for a Sustainable Coast (Coastal 

Protection and Restoration Authority of Louisiana, 2007). The 

final Deep-Draft Deauthorization Report was sent to Congress 

on June 5, 2008, officially deauthorizing the channel from the 

GIWW to the Gulf of Mexico and authorizing the construction 

of a rock closure structure (barrier) across the channel near 

Hopedale, Louisiana (U.S. Army Corp of Engineers, 2010). 
2 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

Following Hurricane Katrina, Congress also authorized 

the USACE to implement the Hurricane Storm Damage Risk 

Reduction System (HSDRRS) by building structures in the 

MRGO and adjacent surface waters to reduce vulnerability 

of this area to storm surge. The HSDRRS included the Lake 

Borgne Surge Barrier and Gate Complex, hereafter referred 

to as the “surge barrier,” and included a 2-mile long surge 

barrier crossing the marshland just east of the Inner Harbor 

Navigation (Industrial) Canal (IHNC), a navigable sector 

gate along the GIWW, an adjacent bypass barge gate, and a 

navigable lift gate at Bayou Bienvenue (U.S. Army Corps 

of Engineers, 2013a). This combination of surge barrier 

and navigable gates serves to reduce risk from hurricane 

and tropical event storm surge entering the area from Lake 

Borgne and the Gulf of Mexico. The Seabrook Gate Complex, 

hereafter referred to as the “Seabrook floodgates,” is located 

just south of where the IHNC enters Lake Pontchartrain. The 

Seabrook floodgates include a navigable sector gate and two 

nonnavigable lift gates. In coordination with the surge barrier 

and gates, they reduce the risk of hurricane and tropical 

event storm surge entering the area from Lake Pontchartrain. 

By blocking or limiting tidal exchange in the MRGO, these 

barriers could affect water quality in the MRGO and nearby 

surrounding waters such as Lake Pontchartrain, the IHNC, and 

Lake Borgne. 

In 2008, in cooperation with the Louisiana Coastal Area 

Science and Technology Program (a collaboration between 

the USACE and the Coastal Protection and Restoration 

Authority of Louisiana), the U.S. Geological Survey (USGS) 

began a study to document the impacts of the rock barrier 

before, during, and after its construction. Water-quality, bedsediment, and discharge data collected during 2008–9 were 

published in Swarzenski and others (2012). In 2010, the study 

was continued solely in cooperation with the USACE. The 

focus of the data collection shifted to the waters north of the 

rock barrier more directly affected by the surge barrier and 

the Seabrook floodgates. Only water-quality information was 

collected. Additional sampling stations were added in the 

vicinity of the surge barrier and near construction activities 

(fig. 1). Sampling stations downstream from the rock barrier 

were discontinued.

Purpose and Scope

This report presents salinity and dissolved oxygen data 

collected in the MRGO and adjacent waterways and discusses 

effects of hydrologic barriers on patterns of hypoxia that 

occurred in these waterways from August 2008 to October 

2012. Hypoxia refers to conditions when dissolved oxygen 

concentrations in the water column drop below 2 milligrams 

per liter (mg/L) (Committee on Environment and Natural 

Resources, 2003). The data represent conditions in the MRGO 

before, during, and after construction of the rock barrier across 

the MRGO near Hopedale. The data also represent conditions 

before, during, and after construction of the surge barrier and 

the Seabrook floodgates. Discrete measurements of dissolved 

oxygen and salinity and continuous salinity data collected in 

the MRGO upstream, in between, and downstream from the 

two barriers, as well as in Lake Borgne, Violet Canal, and 

the IHNC, are presented in tables and figures. Additionally, 

starting in 2010, 5 sites in smaller waterways that had 

historically been sampled by the Louisiana Department of 

Environmental Quality (DEQ) and 1 site in Lake Pontchartrain 

near the IHNC were added to the study. 

Description of the Study Area

The MRGO, located in southeastern Louisiana, 

extends eastward from a turning basin in the IHNC to the 

confluence with the GIWW near Michoud, La., and then in 

a southeasterly direction through Breton Sound into the Gulf 

of Mexico (fig. 1). All reaches of the MRGO were authorized 

to be 36 feet (ft) deep with a bottom width of 500 ft except in 

Breton Sound, where it was authorized to be 38 ft deep, with 

a 600-ft bottom width. Construction of the channel began in 

March 1958 near the confluence of the IHNC and the GIWW. 

By July 1963, an interim channel had been completed to 

the full design depth but only half of the design width, and 

the first vessel had traveled down the channel. By 1968, all 

dredging had been completed, and the channel was at the full 

design dimensions. By 2006, erosion had widened the channel 

from the original 500 ft to more than 1,500 ft in many places, 

and shoaling caused by Hurricane Katrina in August 2005 

had reduced the channel depth to 22 ft in places, restricting 

access of deep-draft vessels (U.S. Army Corp of Engineers, 

2007; 2010). 

The MRGO was originally dredged through shallow 

bays, coastal marshes, and cypress swamps. The channel 

changed local and regional hydrologic circulation patterns and 

allowed saltwater from the Gulf of Mexico to enter wetlands 

that were dominated by vegetation with a low tolerance to 

salinity. The MRGO created a direct route for the freshwater 

held in these wetlands to escape towards estuarine bays and 

salt marshes. In addition to the direct loss of wetlands caused 

by the channel construction, the increased salinity exacerbated 

wetland loss and conversion of freshwater marsh to brackish 

or saltwater marsh in areas adjacent to the MRGO. The 

MRGO also altered salinity in Lakes Pontchartrain and Borgne 

(U.S. Army Corps of Engineers, 2007). 

A portion of the MRGO, from the southern edge of the 

GIWW to the Gulf of Mexico, was deauthorized in 2008. 

As a result of deauthorization, a rock barrier was placed 

across the MRGO near mile 35 just south of the Bayou 

La Loutre Ridge near Hopedale to prevent navigation on an 

unmaintained channel. Construction of the rock barrier began 

in February 2009 with the placement of a wide rock base 

along the channel bottom. The rock barrier first extended 

above the water surface for the entire channel width on June 4, 

2009. The rock barrier, reaching a height of 7 ft above average 

water level, was completed at the end of July 2009. Details of 

the deauthorization and closure can be found at http://www.

mrgo.gov.
Introduction 3

Mile 20

Mile 30

Mile 40

Mile 50

20

14

13

15

16

18

21

17

12

9

11

10

2

1

5

4

6

7

8

3

19

LAKE

BORGNE

LAKE

BORGNE

BRETON

SOUND

LAKE

PONTCHARTRAIN

Violet Canal

Gulf Intracoastal Waterway

Gulf Intracoastal Waterway

MISSISSIPPI RIVER

MISSISSIPPI RIVER

Mississippi River-Gulf Outlet

Mississippi River-Gulf Outlet

Meraux

Chalmette

New

Orleans

Michoud

New

Orleans

Hopedale

Meraux

Yscloskey

Chalmette

Violet

Michoud

I-510

bridge

Bayou La Loutre

and ridge

Inner Harbor

Navigation

Canal

Rock

barrier

Bayou Bienvenue

Seabrook Gate 

Complex and bridge

Inner Harbor Navigation Canal

Lake Borgne Surge Barrier

EXPLANATION

Upland

Marsh

1

5 Water-quality monitor site and

site number

Water-quality transect or point

and site number

GULF OF MEXICO

LOUISIANA

Map

M

area ississippi R

30°

90° 89°50’ 89°40’ 89°30’ 89°20’ 89°10’

30°

29°

50’

29°

40’

29°

30’

0 10 20 KILOMETERS

0 10 20 MILES

0 10 KILOMETERS

0 10 MILES

90° 89°50’

Map Credit: Modified from Louisiana Oil Spill Coordinator, Office of the Governor,

Louisiana GIS Digital Map, May 2007, Compilation DVD

Area of

inset map

Figure 1. Location of data-collection sites in the Mississippi River-Gulf Outlet and adjacent waterways, southeastern Louisiana.
4 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

To reduce the risk of storm surge in the IHNC, a surge 

barrier was constructed across the MRGO and GIWW, 

connecting the MRGO and GIWW hurricane levees (fig. 1). 

The main pilings for the surge barrier were completed in 

October 2009 and 6-inch (in.) gaps between pilings were 

closed by February 11, 2010 (U.S. Army Corps of Engineers, 

2012). All segments of the surge barrier wall were in place 

by September 13, 2010. Floodgates were constructed in 

the surge barrier at the GIWW and Bayou Bienvenue to 

permit navigation. A cofferdam was constructed in Bayou 

Bienvenue during construction of the floodgates, but 

hydrologic exchange was maintained in the bayou during 

construction by using flow-through pipes. In addition, the 

Seabrook floodgates were constructed to prevent entry of 

surge water from Lake Pontchartrain into the IHNC during a 

storm. During construction of these floodgates, a cofferdam 

in the IHNC prevented exchange of water between the IHNC 

and Lake Pontchartrain. Hydrologic exchange between Lake 

Pontchartrain and the IHNC ceased on October 31, 2010, 

and was reestablished in July 2012 (U.S. Army Corps of 

Engineers, 2012).

Methods

Discrete and continuous data were collected during this 

study and stored in the U.S. Geological Survey’s National 

Water Information System (http://waterdata.usgs.gov/nwis). 

Data collection sites and the types of data-collection activity 

are listed in table 1. For discrete data, a multiparameter waterquality meter was used to measure specific conductance, pH, 

temperature, dissolved oxygen (D.O.), and salinity (calculated 

from specific conductance, Schemel, 2001) at various depths 

along transects in the MRGO, GIWW, and at selected points 

in adjacent waterways. Measurements along transects were 

made at three or five evenly spaced points across the full width 

of the channel. Channel widths and the distances from the 

banks were determined by using a laser range finder. Vertical 

profiles of water quality were measured at three evenly spaced 

points where the channel was about 900 ft wide and at five 

evenly spaced points where the channel width was greater than 

about 900 ft. At each point, vertical water-quality profiles were 

made by taking measurements at every 10 ft of depth starting 

at 0.5 ft from the channel bottom and ending at 0.5 ft from 

the water surface. If there was a rapid change in conditions 

within a 10-ft interval, additional measurements were 

made every 5 ft. The water-quality meter was calibrated for 

specific conductance, pH, and D.O. on each day prior to data 

collection; measurements were made following USGS field 

methods presented in Wilde (variously dated). 

Specific conductance, pH, water temperature, D.O., and 

salinity were measured at seven transects (sites 3, 4, 6, 7, 9, 

10, and 11) in the MRGO and at a single point in Lake Borgne 

(site 12). Regular measurements were made at sites 3, 4, and 

6 from August 2008 to August 2010 and at sites 7, 9, 10, 11, 

and 12 from August 2008 to October 2012 (fig. 1). Sites 3 

and 6 also were sampled in July in 2011 and 2012. Site 4 was 

sampled in July 2012 as well. Measurements generally were 

made over 1- or 2-day periods every 4 to 6 weeks from June 

through October and about every 8 weeks in the intervening 

time period. Measurements made in August and November 

2008 and in January and March 2009 documented conditions 

before the rock barrier near Hopedale breached the water 

surface or construction of the surge barrier and the Seabrook 

floodgates began, when there was still considerable water 

movement in the MRGO. For this report, concentrations of 

salinity and D.O. from all vertical profiles along a transect 

were averaged by depth to create a single vertical profile for 

each transect for each sampling event. These average values 

are used herein to represent salinity and D.O. conditions at 

each transect site.

Hypoxia was documented downstream from the rock 

barrier at transects near miles 30 (site 3) and 34 (site 4) 

during June 2009, just as construction was being completed 

(Swarzenski and others, 2012); consequently, water quality 

was measured at two additional transects, located farther 

downstream towards Breton Sound near miles 20 (site 1) and 

27 (site 2) (fig. 1), from June 2009 to July 2010 to document 

the southern extent of hypoxic conditions. These were 

sampled occasionally, as conditions in the field dictated and 

time permitted.

In September 2010, sampling downstream from the 

rock barrier was discontinued after the focus of the waterquality monitoring shifted to the effects of the surge barrier 

construction. Five stations historically sampled by the DEQ 

were added. These new stations (sites 15 through 19) were 

in shallower and narrower waterways, including Bayou 

Bienvenue and the borrow canal running parallel to and on 

the landward side of the MRGO levee (fig. 1). At these sites, 

a single vertical water-quality profile was made in the center 

of the channel. Two additional transects (sites 13 and 14) were 

established in the GIWW (fig. 1), on either side of the gated 

structure being constructed.

 Temperature and specific conductance were recorded 

at 60- or 120-minute intervals by continuous water-quality 

meters suspended in the water column at selected monitoring 

sites. The meters were suspended in stilling wells attached to 

existing structures or galvanized steel pipes driven into the 

channel bottom. The stilling wells were constructed of 2-in. 

polyvinyl chloride (PVC) pipe with 18 in. of perforated copper 

pipe at the bottom to inhibit the growth of algae on the meters’ 

sensors. The meters were serviced and calibrated during 

routine site visits by using methods described in Wagner and 

others (2006). 
Methods 5

Table 1. Data-collection sites in the Mississippi River-Gulf Outlet (MRGO) and adjacent waterways, and in Lake Borgne and Lake 

Pontchartrain in southeastern Louisiana.

[NAD 27, North American Datum of 1927; Data-collection activity codes: T, water-quality transect; M, water-quality monitor; P, water-quality point; ddmmss is 

degrees, minutes, and seconds; GIWW, Gulf Intracoastal Waterway]

Site 

number 

(see fig. 1)

Station 

number Station name

Latitude 

ddmmss

(NAD 27)

Longitude 

ddmmss 

(NAD 27)

Datacollection 

activity 

code

Period 

sampled

1 294047089232700 MRGO near mile 20 near 

Hopedale

294047 892327 T June 2009–July 2010

2 294414089282400 MRGO near mile 27 near 

Hopedale

294414 892824 T June 2009–July 2010

3 294608089310500 MRGO near mile 30 near 

Hopedale

294608 893105 T August 2008–July 2012

4 294805089335200 MRGO near mile 34 near 

Hopedale

294805 893352 T August 2008–July 2012

5 294900089350800 MRGO near mile 35 near 

Hopedale

294900 893508 M March 2009–September 2010

6 294946089361900 MRGO near mile 37 near 

Hopedale

294946 893619 T August 2008–July 2012

7 295157089423200 MRGO near mile 44 near 

Yscloskey

295157 894232 T August 2008–October 2012

8 295311089445100 MRGO near mile 46 near 

Yscloskey

295311 894451 M January 2009–September 2012

9 295556089494400 MRGO near mile 52 near Violet 295556 894944 T August 2008–October 2012

10 295709089511200 MRGO near mile 54 near Violet 295709 895112 T August 2008–October 2012

11 300018089551200 MRGO near mile 59 near 

Michoud

300108 895512 T,M August 2008–October 2012

12 295655089495600 Lake Borgne near MRGO near 

Violet

295655 894956 P,M August 2008–October 2012

13 300018089561600 GIWW west of I-510 bridge near 

Michoud

300018 895617 T June 2010–October 2012

14 300058089534300 GIWW east of I-510 bridge near 

Chalmette

300058 895343 T August 2010–October 2012

15 295716089523800 Borrow canal west of MRGO 

near Meraux

295716 895238 P August 2010–October 2012

16 295534089502000 Violet Canal west of MRGO near 

Violet

295534 895020 P August 2010–October 2012

17 295916089525900 Bayou Bienvenue east of MRGO 

near Michoud

295916 895259 P August 2010–October 2012

18 295434089491100 Borrow canal west of MRGO 

near Violet

295434 894911 P August 2010–October 2012

19 295855089564400 Bayou Bienvenue at Paris Rd 

bridge near Chalmette

295855 895644 P August 2010–October 2012

20 300156090021100 Lake Pontchartrain at Seabrook 

bridge

300156 900211 M August 2010–October 2012

21 738023321 Industrial Canal at I-10 at New 

Orleans

300016 900132 M January 2009–October 2012
6 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

Six continuous water-quality meters measuring 

temperature and conductance were deployed at 3 sites in 

the MRGO near miles 35, 46, and 59 (sites 5, 8, and 11) 

and 1 site each in Lake Borgne (site 12) and in the IHNC 

(site 21) in late December 2008 (fig. 1). The meters were 

suspended at a depth of 6–10 ft below the water surface at 

sites in the MRGO; at a depth of 4 ft at the site in the IHNC 

(site 21); and at a depth of 6 ft at the site in Lake Borgne 

(site 12) near the MRGO. An additional water-quality meter 

was suspended 20 ft below the water surface at a site in 

the MRGO near mile 46 (site 8) for evaluation of salinity 

stratification in the MRGO. In September 2010, the waterquality meter downstream from the rock barrier (site 5) was 

discontinued. The water-quality meter in the MRGO near mile 

46 (site 8) at a depth of 20 ft was discontinued in September 

2010. In August 2010, a water-quality meter was placed at 

the intersection of the IHNC and Lake Pontchartrain, near 

Seabrook (site 20; fig. 1).

Placement of continuous water-quality meters was 

primarily limited to existing structures in the channel because 

of concerns about boat traffic and cost and time constraints 

on the construction of new data-collection platforms. Existing 

structures in the MRGO included navigation markers located 

in relatively shallow water near the sides of the deeper 

shipping lane. Hence, the maximum depths at which the 

meters could be placed in the MRGO generally were limited 

to about 10 ft below the water surface even though the depth 

of the shipping lane generally ranged from 35 to over 40 ft 

in most places. During the project, vertical profiles of water 

quality along these transects provided a more comprehensive 

indication of channel salinity stratification, whereas the 

continuous water-quality meters indicated shorter term 

temporal fluctuations.

Effects of Hydrologic Modifications

Data on salinity and dissolved oxygen from August 2008 

to October 2012 are presented in the following sections to 

document how the restrictions in water movement caused by 

the rock barrier, the surge barrier, and the Seabrook floodgates 

affected salinity and the formation of hypoxia in the MRGO, 

the GIWW, and other surface waters. Salinity and hypoxia 

are key parameters affecting habitat quality of estuarine biota. 

The more than 4 years of sampling measured conditions 

before, during, and after activities related to the construction 

and operation of the rock and surge barriers, and the 

floodgates.

Salinity

The completion of the rock barrier in July 2009 resulted 

in the sharp reduction of velocities and discharge upstream 

and downstream from the barrier in the MRGO (Swarzenski 

and others, 2012). Prior to completion, salinity measured by 

the continuous water-quality meter in the MRGO at site 5 

downstream from the rock barrier was only moderately higher 

than salinity at all sites upstream from the barrier as far north as 

at mile 59, near the junction with the GIWW (fig. 2). Also prior 

to completion of the rock barrier, fluctuations of 5 or even 10 

parts per thousand (ppt) over short time periods were common 

at sites in the MRGO south of mile 59 (fig. 2). After completion, 

salinities in the MRGO upstream and downstream from the 

rock barrier immediately diverged—salinity downstream from 

the barrier increased rapidly and remained approximately 10 to 

20 ppt higher than salinity at sites upstream from the barrier. 

In addition, short-term salinity fluctuations upstream from the 

rock barrier became more moderate (fig. 2). The reduction in 

tidal exchange and hydrologic circulation that resulted from the 

completion of the rock barrier induced a major shift in salinity 

patterns in the MRGO. 

Completion of the surge barrier in October 2010 did not 

result in similarly substantial changes to salinity regimes in the 

MRGO. Salinities in Lake Borgne (site 12) and in the MRGO 

at mile 46 (site 8) tracked each other closely, with the exception 

that site 12 had slightly greater fluctuations. Salinities in the 

MRGO at mile 59 (site 11) and in the IHNC at I-10 (site 21) 

also tracked each other (fig. 2), though during periods of high 

salinity in the area, site 21 was usually slightly fresher than 

site 11. Site 20 in Lake Pontchartrain at the mouth of the IHNC 

did not record sufficient data for thorough interpretation, but 

salinity values were similar to values in the IHNC at I-10 

(site 21). Salinity in the MRGO at mile 46 (site 8) and in Lake 

Borgne (site 12) dropped very quickly and steeply in late May 

and June 2011 and then rebounded quickly as well (fig. 2). 

This drop coincided with the opening of the Bonnet Carre 

Spillway, located about 20 miles upstream from New Orleans 

on the Mississippi River, by the U.S. Army Corps of Engineers 

between May 9 and June 20, 2011, to ease flooding concerns on 

the Mississippi River (U.S. Army Corps of Engineers, 2013c). 

Mississippi River water was released through the Bonnet Carre 

Spillway into Lake Pontchartrain. Some of the water eventually 

flowed through the IHNC and other waterways into the GIWW 

and Lake Borgne. The limited data from site 20 suggested that 

salinity in the lake at that location was slightly lower than in the 

IHNC at I-10 (site 21) during the time the cofferdam was built 

(October 2010–July 2012), a period when hydrologic exchange 

was limited (fig. 2).
Rock barrier

completed

Surge barrier

completed Period when there was a cofferdam acro

05

101520253035

Salinity, in parts per thousand

J F M A M J J A S O N D

2009

J F M A M J J A S O N D

2010

J F M A M J J A S O N D

2011

J F 

Industrial Canal at I-10 at New Orleans MRGO near mile 59 near Michoud (site MRGO near mile 46 near Yscloskey (sitLake Pontchartrain at Seabrook bridge Lake Borgne near MSRGO near Violet (MRGO near mile 35 near Hopedale (site 

Sampling sites

EXPLANATION

Figure 2. Continuous salinity in the Inner Harbor Navigation (Industrial) Canal (IHNC), the Mississippi River-Gulf Outlet (MRGO), Lake Pontchartrain, southeastern Louisiana, 2009–12. 
8 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

Salinity data collected at the transects showed patterns 

similar to those seen in the continuous data. Figures 3 and 

4 show contours of salinity between miles 30 and 59 in the 

MRGO during midsummer from 2008 to 2012. The effect of 

the rock barrier is pronounced and is evident in June 2009 

even before its completion in July 2009 (table 2). Contours 

for August 15, 2008, indicate a continuous layer of water with 

salinities ranging from 18 to 20 ppt extending from mile 30 

to mile 59 (fig. 3). This layer occupied the bottom 20 ft of the 

channel at mile 59 (site 11) and the upper 20 ft of the MRGO 

at mile 30 (site 3). After completion of the rock barrier, a layer 

of water of near-equal salinity extending that distance was no 

longer observed. Salinity downstream from the rock barrier 

became elevated without the inflow of lower salinity water. 

Salinity upstream from the rock barrier freshened rapidly once 

saltwater intrusion was essentially blocked (figs. 3 and 4).

The effects of the surge barrier on salinity in the MRGO 

were less obvious. Discrete sampling at transect sites indicated 

that completion of the surge barrier did not result in the 

divergent salinity regimes at upstream and downstream sites 

that had resulted after the completion of the rock barrier. 

Salinity in the MRGO at site 10 at mile 54 may have become 

slightly more stratified after the surge barrier was completed 

(figs. 3 and 4); however, there were other factors influencing 

contour patterns.

The lowest salinities measured at sites both upstream and 

downstream from the rock barrier were recorded on August 

19, 2010 (fig. 3), during the time when controlled diversions 

of the Mississippi River into coastal estuaries had been 

maximized to prevent the Deepwater Horizon oil spill from 

moving upstream into the deltaic wetlands (Louisiana Office 

of Coastal Protection and Restoration, 2010). Furthermore, 

there was little stratification of salinity in the MRGO 

upstream the rock barrier. Salinity in the MRGO upstream 

and downstream from the rock barrier and the surge barrier 

differed substantially on July 8, 2011 (fig. 4), unlike on 

August 19, 2010, when upstream and downstream salinities 

were more similar (fig. 3). This difference was due in part to 

influx of freshwater into the upstream reach which resulted 

from the opening of the Bonnet Carre Spillway during May 

and June 2011. On July 8, 2011, salinity downstream from 

the rock barrier was very stratified (fig. 4). Salinity upstream 

from the rock barrier was moderately stratified and only 

along the bottom 20 ft of the channel between the barrier 

and mile 54 (site 10) near the opening to Lake Borgne. 

Salinity in the uppermost 20 ft of the channel on the upstream 

side of the rock barrier was fairly homogenous and fresh. 

Salinity on both sides of the surge barrier in the MRGO was 

below 2 ppt. 
Effects of Hydrologic Modifications 9

59 54 52 44 37 34 30 Miles

Surge

barrier

Rock

barrier

Violet

Canal

0

Water depth, in feet

-10

-20

-30

-40

4

4

8

6

16

18

14

August 19, 2010

0

-10

-20

-30

-40

July 30, 2009

14

12

10

16

18

20

22

30

28

32

0

-10

-20

-30

-40

22

20

24

26

16

18

August 15, 2008 Channel bottom

Channel bottom

Channel bottom

Salinity, in parts

per thousand

EXPLANATION

4

2

0

8

6

14

12

10

16

30

28

32

34

22

20

24

26

18

Figure 3. Salinity contours, in parts per thousand, between miles 30 and 59 in the Mississippi River-Gulf Outlet in southeastern 

Louisiana for August 15, 2008, July 30, 2009, and August 19, 2010. 
10 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

59 54 52 44 37 34 30 Miles

Surge

barrier

Rock

barrier

Violet

Canal

0

-10

-20

-30

-40

0

-10

-20

-30

-40 Channel bottom

Salinity, in parts

per thousand

EXPLANATION

4

2

0

8

6

14

12

10

16

30

28

32

34

22

20

24

26

18

22

4

4

24

8

8

26

16

6

6

28

18

10

30

12

32

14

2

22

24

8

16

6

10

12

14

59 54 52 44 37 34 30 Miles

Surge

barrier

Rock

barrier

Violet

Canal

Water depth, in feet

0

July 10, 2012

0

July 08, 2011

Channel bottom

No

data

4

2

0

8

6

14

12

10

16

30

28

32

34

22

20

24

26

18

Salinity, in parts

per thousand

EXPLANATION

Figure 4. Salinity contours, in parts per thousand, between miles 30 and 59 in the Mississippi River-Gulf Outlet in southeastern 

Louisiana for July 08, 2011, and July 10, 2012. 
Table 2. Water depth below which dissolved oxygen concentrations are less than 2 milligrams per liter, indicating hypoxic conditions, in the Mississ(MRGO) and adjacent surface waters in southeastern Louisiana, August 2008 to October 2012.[n.d., no data; “none” in green shading indicates no hypoxia in the water column; brown shading indicates hypoxic conditions; GIWW, Gulf Intracoastal Waterway]

Site 

number 

(see 

fig. 1) 

Station name

Maximum 

channel 

depth

(feet)

Water depth, in feet, below which dissolved oxygen concentrations are les

8/15/2008 11

1 MRGO near mile 20 near Hopedale 32 n.d. 2 MRGO near mile 27 near Hopedale 37 n.d. 3 MRGO near mile 30 near Hopedale 40 none 4 MRGO near mile 34 near Hopedale 37 none 6 MRGO near mile 37 near Hopedale 42 none 7 MRGO near mile 44 near Yscloskey 37 none 9 MRGO near mile 52 near Violet 34 none 

10 MRGO near mile 54 near Violet 36 none 

11 MRGO near mile 59 near Michoud 42 none 

12 Lake Borgne near MRGO near Violet 8 none 13 GIWW west of I-510 bridge near Michoud 40 none 14 GIWW east of I-510 bridge near Chalmette 27 n.d. 15 Borrow canal west of MRGO near Meraux 5 n.d. 16 Violet Canal west of MRGO near Violet 10 n.d. 17 Bayou Bienvenue east of MRGO near Michoud 7 n.d. 18 Borrow canal west of MRGO near Violet 5 n.d. 19 Bayou Bienvenue at Paris Rd bridge near Chalmette 15 n.d. 
Table 2. Water depth below which dissolved oxygen concentrations are less than 2 milligrams per liter, indicating hypoxic conditions, in the Mississ(MRGO) and adjacent surface waters in southeastern Louisiana, August 2008 to October 2012.—Continued [n.d., no data; “none” in green shading indicates no hypoxia in the water column; brown shading indicates hypoxic conditions; GIWW, Gulf Intracoastal Waterway] Site number (see fig. 1) Station name Maximum channel depth (feet) Water depth, in feet, below which dissolved oxygen concentrations are les

1/23/2009 3/1/2009 6/1/2009 6/17/2009 7/30/2009 9/2/2009 

1 MRGO near mile 20 near Hopedale 32 n.d. n.d. n.d. 5 n.d. n.d. 2 MRGO near mile 27 near Hopedale 37 n.d. n.d. n.d. 5 15 33 3 MRGO near mile 30 near Hopedale 40 none none 10 n.d. 15 25 4 MRGO near mile 34 near Hopedale 37 none none 4 n.d. 15 7 6 MRGO near mile 37 near Hopedale 42 none none 34 n.d. 35 35 7 MRGO near mile 44 near Yscloskey 37 none none 30 n.d. 15 28 9 MRGO near mile 52 near Violet 34 none none 32 n.d. 30 31 

10 MRGO near mile 54 near Violet 36 none none none n.d. none none 

11 MRGO near mile 59 near Michoud 42 none none 37 n.d. none none 

12 Lake Borgne near MRGO near Violet 8 none none none n.d. none none 13 GIWW west of I-510 bridge near Michoud 40 n.d. n.d. n.d. n.d. n.d. n.d. 14 GIWW east of I-510 bridge near Chalmette 27 n.d. n.d. n.d. n.d. n.d. n.d. 15 Borrow canal west of MRGO near Meraux 5 n.d. n.d. n.d. n.d. n.d. n.d. 16 Violet Canal west of MRGO near Violet 10 n.d. n.d. n.d. n.d. n.d. n.d. 17 Bayou Bienvenue east of MRGO near Michoud 7 n.d. n.d. n.d. n.d. n.d. n.d. 18 Borrow canal west of MRGO near Violet 5 n.d. n.d. n.d. n.d. n.d. n.d. 19 Bayou Bienvenue at Paris Rd bridge near Chalmette 15 n.d. n.d. n.d. n.d. n.d. n.d. 
Table 2. Water depth below which dissolved oxygen concentrations are less than 2 milligrams per liter, indicating hypoxic conditions, in the Mississ(MRGO) and adjacent surface waters in southeastern Louisiana, August 2008 to October 2012.—Continued[n.d., no data; “none” in green shading indicates no hypoxia in the water column; brown shading indicates hypoxic conditions; GIWW, Gulf Intracoastal Waterway]

Site 

number 

(see 

fig. 1)

Station name

Maximum 

channel 

depth

(feet)

Water depth, in feet, below which dissolved oxygen concentrations are les

2/13/2010 3/24/2010 5/17/2010 6/16/2010 7/22/2010 8/19/2010 

1 MRGO near mile 20 near Hopedale 32 n.d. n.d. 30 n.d. 15 n.d. 2 MRGO near mile 27 near Hopedale 37 n.d. n.d. none n.d. 25 n.d. 3 MRGO near mile 30 near Hopedale 40 none none 27 5 22 13 4 MRGO near mile 34 near Hopedale 37 36 none 27 5 25 8 6 MRGO near mile 37 near Hopedale 42 none 40 37 n.d. 30 35 7 MRGO near mile 44 near Yscloskey 37 none none 27 33 30 28 9 MRGO near mile 52 near Violet 34 none none none 28 n.d. none 

10 MRGO near mile 54 near Violet 36 none 35 31 none none 33 

11 MRGO near mile 59 near Michoud 42 none none none none none none 

12 Lake Borgne near MRGO near Violet 8 none none none none none none 13 GIWW west of I-510 bridge near Michoud 40 n.d. n.d. n.d. 33 none 27 14 GIWW east of I-510 bridge near Chalmette 27 n.d. n.d. n.d. n.d. n.d. none 15 Borrow canal west of MRGO near Meraux 5 n.d. n.d. n.d. n.d. n.d. none 16 Violet Canal west of MRGO near Violet 10 n.d. n.d. n.d. n.d. n.d. none 17 Bayou Bienvenue east of MRGO near Michoud 7 n.d. n.d. n.d. n.d. n.d. none 18 Borrow canal west of MRGO near Violet 5 n.d. n.d. n.d. n.d. n.d. none 19 Bayou Bienvenue at Paris Rd bridge near Chalmette 15 n.d. n.d. n.d. n.d. n.d. none 
Table 2. Water depth below which dissolved oxygen concentrations are less than 2 milligrams per liter, indicating hypoxic conditions, in the Mississ(MRGO) and adjacent surface waters in southeastern Louisiana, August 2008 to October 2012.—Continued [n.d., no data; “none” in green shading indicates no hypoxia in the water column; brown shading indicates hypoxic conditions; GIWW, Gulf Intracoastal Waterway] Site number (see fig. 1) Station name Maximum channel depth (feet) Water depth, in feet, below which dissolved oxygen concentrations are less 

1/22/2011 3/31/2011 5/9/2011 6/23/2011 7/8/2011 8/12/2011 9/20/2

1 MRGO near mile 20 near Hopedale 32 n.d. n.d. n.d. n.d. n.d. n.d. n.d2 MRGO near mile 27 near Hopedale 37 n.d. n.d. n.d. n.d. n.d. n.d. n.d3 MRGO near mile 30 near Hopedale 40 n.d. n.d. none n.d. 5 n.d. n.d4 MRGO near mile 34 near Hopedale 37 n.d. 33 35 n.d. n.d. n.d. n.d6 MRGO near mile 37 near Hopedale 42 none 35 none n.d. 34 n.d. n.d7 MRGO near mile 44 near Yscloskey 37 none 33 none 30 25 27 27 9 MRGO near mile 52 near Violet 34 none 28 none n.d. 23 27 27 

10 MRGO near mile 54 near Violet 36 none 32 none n.d. 25 23 27 

11 MRGO near mile 59 near Michoud 42 none none none none 33 none non

12 Lake Borgne near MRGO near Violet 8 none none none none none none non13 GIWW west of I-510 bridge near Michoud 40 none none none n.d. 37 none 32 14 GIWW east of I-510 bridge near Chalmette 27 none none none none none none non15 Borrow canal west of MRGO near Meraux 5 none none none none none none non16 Violet Canal west of MRGO near Violet 10 none none none none none none non17 Bayou Bienvenue east of MRGO near Michoud 7 none none none none none none non18 Borrow canal west of MRGO near Violet 5 none none none none none 2 non19 Bayou Bienvenue at Paris Rd bridge near Chalmette 15 none none none none none none non
Table 2. Water depth below which dissolved oxygen concentrations are less than 2 milligrams per liter, indicating hypoxic conditions, in the Mississ(MRGO) and adjacent surface waters in southeastern Louisiana, August 2008 to October 2012.—Continued[n.d., no data; “none” in green shading indicates no hypoxia in the water column; brown shading indicates hypoxic conditions; GIWW, Gulf Intracoastal Waterway]

Site 

number 

(see 

fig. 1) 

Station name

Maximum 

channel 

depth

(feet)

Water depth, in feet, below which dissolved oxygen concentrations are les

3/8/2012 5/11/2012 6/15/2012 7/10/2012 8/

1 MRGO near mile 20 near Hopedale 32 n.d. n.d. n.d. n.d. 2 MRGO near mile 27 near Hopedale 37 n.d. n.d. n.d. n.d. 3 MRGO near mile 30 near Hopedale 40 n.d. n.d. n.d. 12 4 MRGO near mile 34 near Hopedale 37 n.d. n.d. n.d. 5 6 MRGO near mile 37 near Hopedale 42 n.d. n.d. n.d. 38 7 MRGO near mile 44 near Yscloskey 37 none 30 32 28 9 MRGO near mile 52 near Violet 34 none n.d. none 28 

10 MRGO near mile 54 near Violet 36 none none none 28 

11 MRGO near mile 59 near Michoud 42 none n.d. none none 

12 Lake Borgne near MRGO near Violet 8 none none none none 13 GIWW west of I-510 bridge near Michoud 40 none none none none 14 GIWW east of I-510 bridge near Chalmette 27 none none none none 15 Borrow canal west of MRGO near Meraux 5 none none none none 16 Violet Canal west of MRGO near Violet 10 none none none none 17 Bayou Bienvenue east of MRGO near Michoud 7 none none none none 18 Borrow canal west of MRGO near Violet 5 none none 2 none 19 Bayou Bienvenue at Paris Rd bridge near Chalmette 15 none none none none 
16 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

Hypoxia Formation

In every year of sampling following completion of 

the rock barrier in 2009, many parts of the MRGO became 

hypoxic for some or most of the channel depth during 

summer months. Hypoxia generally was more severe in the 

section of the MRGO downstream from the rock barrier. 

Hypoxia was not found in the MRGO during any of the 

four preconstruction sampling events, but preconstruction 

sampling was limited to the 10 months prior to the rock barrier 

completion and included only one event during a summer 

month, the time period when hypoxia generally was observed 

after construction. If hypoxia occurred in the MRGO prior to 

completion of the rock barrier, it was most likely of limited 

extent and duration because of the strong daily tidal exchange 

which moved water throughout the MRGO (Swarzenski 

and others, 2012). Completion of the surge barrier did not 

contribute to the new formation of extensive hypoxia in the 

MRGO or adjacent surface waters.

Hypoxia was first measured in the MRGO in June 2009 

(table 2). On occasion, hypoxia extended to within 5–10 ft 

of the channel surface for at least 16 mi downstream from 

the rock barrier, the farthest point sampled in this study. A 

hypoxic zone frequently extended upstream from the rock 

barrier to mile 52 (site 9). Hypoxia developed mainly along 

the bottom 5–15 ft. The most extensive hypoxia measured in 

the MRGO was on October 8, 2009, when it extended from 

mile 20 (site 1) to mile 54 (site 10) (fig. 5). This was the only 

year that hypoxic conditions were extensively documented in 

the MRGO as late as October. From 2010 to 2012, hypoxia 

generally formed from May to September (table 2), but in 

2011, extensive hypoxia was found in March and again 

starting in July, but not in May and June. 

When a hypoxic zone was observed in the MRGO, 

the horizontal and vertical extent of the zone was typically 

greater downstream from the rock barrier than it was 

upstream. This pattern appeared to be linked to the potential 

for lateral hydraulic exchange with adjacent surface waters. 

Downstream from the barrier, hydraulic gradients can push 

water in a northwesterly direction in the MRGO and trap the 

water against the rock barrier. The fairly contiguous marshes 

and spoil banks lining the MRGO inhibit the movement and 

exchange of water between the MRGO and surrounding 

surface waters. Upstream from the rock barrier, there are 

several lateral openings along the MRGO to adjacent surface 

waters, including Bayou La Loutre, Lake Borgne, and the 

GIWW, which could allow easier water circulation and reduce 

the occurrence of hypoxia in the MRGO upstream from the 

rock barrier.

Vertical profiles of salinity and D.O. at transect sites in 

the MRGO and the GIWW were plotted for summer months 

of 2010, 2011, and 2012 to more closely examine the role of 

the surge barrier and the cofferdam on the development of 

hypoxia in surrounding surface waters (figs. 6–8). Transect 

sites in the MRGO at mile 54 (site 10, downstream from the 

surge barrier), at mile 59 (site 11, upstream from the surge 

barrier), and in the GIWW (site 13, west of the I-510 bridge) 

were used.

Figure 5. Extent of hypoxic layer, between miles 20 and 59 in the Mississippi River-Gulf Outlet in southeastern Louisiana on October 8, 

2009; hypoxia occurs when dissolved oxygen concentrations are less than 2 milligrams per liter. 

27 20 Miles

October 08, 2009

Water depth, in feet

Hypoxic layer Hypoxic layer

59 54 52 44 37 34

Surge

barrier

Rock

barrier

Violet

Canal

0

-10

-20

-30

-40

30

Channel bottom
Effects of Hydrologic Modifications 17

Vertical profiles of salinity at all three transect sites 

generally were fairly uniform during 2010 and ranged from 

2 to 5 ppt. In 2010, hypoxia was measured along the very 

bottom of the channel on August 19 in the MRGO near mile 

54 (site 10), downstream from the surge barrier. Surprisingly, 

salinity on this date did not show much stratification. On 

June 16 and October 28, 2010, D.O. concentrations dropped 

to near hypoxic conditions in the MRGO near mile 54 (site 

10). For these times, salinity was higher along the bottom. In 

the GIWW west of the I-510 bridge (site 13), hypoxia was 

found along the channel bottom both on June 16 and August 

19, 2010. In 2011, hypoxia was found at the channel bottom 

in the MRGO near mile 54 (site 10) in the July, August, and 

September sampling. Salinity at this site increased near the 

bottom, and markedly so for the 3 months when hypoxia 

was found. Upstream from the surge barrier, vertical salinity 

profiles were more uniform. Hypoxia also was found along 

the channel bottom in the MRGO at mile 59 (site 11) and 

in the GIWW west of the I-510 bridge (site 13) on July 8, 

2011, and at the latter site again on September 20, 2011. 

In 2012, hypoxia was measured only downstream from the 

surge barrier, in the MRGO at mile 54 (site 10). This finding 

occurred on July 10 and August 21, when there was an 

increase in salinity along the channel bottom.

The surge barrier may have contributed to localized 

hypoxia formation. Once the barrier was in place, water 

movement in the MRGO between the GIWW and the opening 

to Lake Borgne was further reduced. This reduction in water 

EXPLANATION

Salinity, in parts per thousand, or dissolved oxygen, in milligrams per liter

Salinity

Dissolved oxygen

June 16, 2010

Water depth, in feet

-40

-30

-20

-10

0

-40

-30

-20

-10

0

-40

-30

-20

-10

0

July 22, 2010 Aug. 19, 2010 Sept. 9, 2010 Oct. 28, 2010

0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10

0 2 4 6 8 10

MRGO near mile 54 near Violet (site 10)

MRGO near mile 59 near Michoud (site 11)

GIWW west of I-510 bridge near Michoud (site 13)

No data

Figure 6. Vertical profiles of salinity and dissolved oxygen at transects in the Mississippi River-Gulf Outlet (MRGO) and the Gulf 

Intracoastal Waterway (GIWW) in southeastern Louisiana, June to October 2010.
18 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

movement could allow higher salinity water to enter from the 

opening to Lake Borgne and move along the bottom northward 

in the MRGO, which could establish salinity stratification and 

eventually result in hypoxic conditions forming near mile 54 

(site 10). Upstream from the surge barrier, hypoxia appeared 

to have formed temporarily, possibly in response to the 

blocking of hydrologic exchange between Lake Pontchartrain 

and the GIWW that ended in 2012. 

In the waterways adjacent to the MRGO and the GIWW, 

conditions became hypoxic only once, at a site on Bayou 

Bienvenue (site 17), on the flood side of the surge barrier on 

October 13, 2011 (table 2). No other site in or adjacent to the 

MRGO and the GIWW showed hypoxia on this date. On the 

inside (sites 15, 16, 18, and 19) of the MRGO flood control 

levee, hypoxia formed several times but never more than at 

one site per unique sampling date (table 2).

Factors that contributed the most to development of 

hypoxia in the MRGO likely were the reduction of hydrologic 

circulation and tidal exchange in the MRGO upon completion 

of the rock barrier (Swarzenski and others, 2012) and to a 

lesser extent the surge barrier, and the depth of the MRGO, 

which was 10 to 30 ft deeper than surrounding water bodies. 

These conditions effectively created areas with minimal water 

movement that were conducive to stratification of salinity, 

reduced vertical mixing, and likely facilitated development 

of hypoxic conditions. Hypoxia primarily formed during the 

summer months when high water temperatures lower the 

oxygen-holding capacity of water. 

EXPLANATION

Salinity, in parts per thousand, or dissolved oxygen, in milligrams per liter

Salinity

Dissolved oxygen

May 9, 2011

Water depth, in feet

-40

-30

-20

-10

0

-40

-30

-20

-10

0

-40

-30

-20

-10

0

July 8, 2011 Aug. 12, 2011 Sept. 20, 2011 Oct. 13, 2011

0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10

MRGO near mile 54 near Violet (site 10)

MRGO near mile 59 near Michoud (site 11)

GIWW west of I-510 bridge near Michoud (site 13)

Figure 7. Vertical profiles of salinity and dissolved oxygen at transects in the Mississippi River-Gulf Outlet (MRGO) and the Gulf 

Intracoastal Waterway (GIWW) in southeastern Louisiana, May to October 2011.
Effects of Hydrologic Modifications 19

In contrast, hypoxic events in the shallow, low-flow 

bayous surrounding the MRGO and the GIWW were localized 

and occurred in isolated episodes independent of construction 

activities on MRGO. Hypoxia in these waterways can result 

from nonpoint source pollution, for example through organic 

and nutrient enrichment after rainfall events. Two construction 

activities proposed for environmental mitigation of the 

MRGO impacts (U.S. Army Corps of Engineers, 2011) may 

affect salinity and the occurrence and severity of hypoxic 

conditions in the MRGO and adjacent waterways in the future. 

A proposed controlled diversion of Mississippi River water 

near Meraux, La., and Violet Canal will have to cross the 

MRGO to reach its target wetlands to the east. New patterns 

of hydrologic circulation and salinity will result. Restoration 

of wetlands affected by the construction of the MRGO also 

calls for extensive excavation of borrow material from the 

bottom at various locations in Lake Borgne, to be used to 

armor wetland shorelines. This excavation of bottom material 

will create pits with reduced hydrologic circulation that could 

increase the prevalence of localized hypoxia. 

EXPLANATION

Salinity, in parts per thousand, or dissolved oxygen, in milligrams per liter

Salinity

Dissolved oxygen

May 11, 2012

Water depth, in feet

-40

-30

-20

-10

0

-40

-30

-20

-10

0

-40

-30

-20

-10

0

June 15, 2012 Aug. 12, 2011 Aug. 21, 2012 Oct. 4, 2012

0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10

MRGO near mile 54 near Violet (site 10)

MRGO near mile 59 near Michoud (site 11)

GIWW west of I-510 bridge near Michoud (site 13)

Figure 8. Vertical profiles of salinity and dissolved oxygen at transects in the Mississippi River-Gulf Outlet (MRGO) and the Gulf 

Intracoastal Waterway (GIWW) in southeastern Louisiana, May to October 2012.
20 Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways

Summary

The Mississippi River-Gulf Outlet (MRGO) was 

constructed between 1958 and 1968 to provide a safer and 

shorter route between the Gulf of Mexico and the Port of 

New Orleans for ocean-going vessels. In 2006, the U.S. 

Congress directed the U.S. Army Corps of Engineers 

(USACE) to develop and implement a plan to deauthorize 

a portion of the MRGO ship channel from its confluence 

with the Gulf Intracoastal Waterway to the Gulf of Mexico. 

In 2009, in accordance with plans submitted to Congress, 

the USACE built a rock barrier across the MRGO near 

Hopedale, Louisiana. Following Hurricane Katrina, Congress 

also authorized the USACE to implement the Hurricane 

Storm Damage Risk Reduction System (HSDRRS) by 

building structures in the MRGO and adjacent surface 

waters, to reduce vulnerability of this area to storm surge. 

The HSDRRS included a Gulf Intracoastal Waterway-Lake 

Borgne surge barrier near mile 58 of the MRGO and a 

Seabrook Gate Complex on the IHNC. Construction of these 

structures affected hydrologic circulation in the MRGO and 

in adjacent waterways, in some locations permanently and in 

others temporarily. In 2008, the U.S. Geological Survey, in 

cooperation with the USACE, began a study to document the 

effects of the construction activities on salinity and dissolved 

oxygen in these surface waters. Data were collected from 

August 2008 through October 2012. 

Of all the modifications made to the MRGO and adjacent 

waterways because of deauthorization and flood control, 

completion of the rock barrier near mile 35 had by far the most 

effect on salinity and formation of hypoxia in surface waters 

in the study area. The rock barrier, which was completed in 

July 2009, restricted tidal exchange in the MRGO and divided 

the MRGO into an upstream section with lower salinity and 

a downstream section, open to Breton Sound and the Gulf 

of Mexico, with higher salinity. Rapid and large salinity 

fluctuations in much of the MRGO were typical before the 

barrier was completed. These fluctuations were dampened 

along most of the MRGO and even in Lake Borgne after 

completion of the rock barrier. The continuous salinity data 

indicated that stations Lake Borgne near MRGO (site 12) and 

MRGO near mile 46 (site 8) responded more quickly to the 

opening of the Bonnet Carre Spillway in 2011 (going from 

more saline to fresher conditions), but also reverted to more 

saline conditions faster than stations Industrial Canal at I-10 

(site 21) and MRGO near mile 59 (site 11). 

The completion of the rock barrier also contributed to the 

formation of hypoxic conditions in a large part of the MRGO 

during summer months when water temperatures were high. 

Hypoxic conditions extended throughout much of the water 

column downstream from the rock barrier, but upstream from 

the barrier, hypoxic conditions generally only occurred in the 

bottom half of the channel or just near the bottom. The most 

extensive hypoxic zone was measured in October 2009 when 

it extended at least 34 miles in the MRGO, from mile 20 to 

mile 54. Similarly large shifts in salinity regimes or formation 

of extensive hypoxia did not occur once the surge barrier was 

completed nor during construction activities related to the 

construction of floodgates, which also restricted hydrologic 

exchange even if only temporarily. 

The factors that contributed the most to hypoxia in the 

MRGO were the reductions in tidal water movement there 

after completion of the rock barrier combined with the channel 

depth in the MRGO, which is 10 to 30 ft deeper in places 

than surrounding surface water bodies. These factors helped 

to stratify salinity by reducing vertical mixing in the water 

column. 

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Swarzenski and Mize—Effects of Hydrologic Modifications on Salinity and Formation of Hypoxia in the MRGO and Adjacent Waterways—SIR 2014–5077

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ISSN 2328-0328 (online)

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