U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5132

Discharge, Suspended Sediment, and Salinity in the Gulf 

Intracoastal Waterway and Adjacent Surface Waters in 

South-Central Louisiana, 1997–2008
Cover, Gulf Intracoastal Waterway near Bay Wallace east of Morgan City.
Discharge, Suspended Sediment, and 

Salinity in the Gulf Intracoastal Waterway 

and Adjacent Surface Waters in SouthCentral Louisiana, 1997–2008

By Christopher M. Swarzenski and Scott M. Perrien

Scientific Investigations Report 2015–5132

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: 2015

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod/.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Swarzenski, C.M., and Perrien, S.M., 2015, Discharge, suspended sediment, and salinity in the Gulf Intracoastal 

Waterway and adjacent surface waters in south-central Louisiana, 1997–2008: U.S. Geological Survey Scientific 

Investigations Report 2015–5132, 21 p., http://dx.doi.org/10.3133/sir20155132.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)
iii

Contents

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

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

Purpose and Scope .............................................................................................................................3

Description of Study Area ..................................................................................................................3

Approach and Methods ...............................................................................................................................3

Discharge, Suspended Sediment, and Salinity ........................................................................................7

Discharge ..............................................................................................................................................7

Interannual Variability ..............................................................................................................10

Trends .........................................................................................................................................12

Suspended Sediment ........................................................................................................................14

Salinity .................................................................................................................................................14

Implications for Coastal Louisiana ...........................................................................................................18

Summary .......................................................................................................................................................19

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

Figures

1. Map showing surface-water data collection sites in the Gulf Intracoastal 

Waterway and adjacent water bodies in south-central Louisiana .....................................2

2. Graph showing monthly average stage of the Lower Atchafalaya River at 

Morgan City, Louisiana (average [1997–2008]; sustained high water [1993]; low 

water [2000, 2006]; and peak flood [2008]) ...............................................................................5

3. Graph showing relation between monthly average discharge in the Gulf 

Intracoastal Waterway (GIWW) at Bayou Sale Ridge near Franklin and monthly 

average discharge of the Wax Lake Outlet at Calumet .........................................................9

4. Graphs showing relation between monthly average discharge in the Gulf 

Intracoastal Waterway (GIWW) near Bay Wallace east of Morgan City and 

monthly average discharge of the Lower Atchafalaya River at Morgan City or 

Bayou Boeuf at Railroad Bridge near Amelia (panels A and B); graphs showing 

relation between monthly average discharge in the GIWW at Houma or west 

of Bayou Lafourche at Larose and monthly average discharge of the GIWW 

near Bay Wallace east of Morgan City (panels C and D). Correlation in panel B 

is only for a Lower Atchafalaya River stage of 3 feet or less North American 

Vertical Datum of 1988 (NAVD 88) ...........................................................................................10

5. Graph showing percent time the Lower Atchafalaya River at Morgan City 

exceeded indicated stage. Stage was averaged in 10-year increments starting 

in January 1940 and extending through December 2008 ....................................................14

6. Graphs showing comparison of instantaneous discharge measurements made 

in 1997 and 1998 and measurements made in 2007 and 2008 in relation to stage 

of the Lower Atchafalaya River at Morgan City ...................................................................15

7. Boxplots showing statistical summary of suspended-sediment concentrations 

collected between 1997 and 2008 at selected locations in the Gulf Intracoastal 

Waterway (GIWW) and adjacent surface-water bodies. Site numbers in 

parentheses.................................................................................................................................16
iv

8. Graphs showing monthly average salinity at selected stations in south-central 

Louisiana under average, sustained high water, low water, and peak flood 

conditions of the Atchafalaya River. Not all stations were operational in 1993; 

months with more than 50 percent missing data are not shown .......................................17

9. Graphs showing monthly average salinity in parts per thousand at selected 

stations in Barataria Basin under average and low water conditions of the 

Atchafalaya River. The Davis Pond Freshwater Diversion Structure was not 

operational in 2000 .....................................................................................................................18

Tables

1. Data-collection sites and types of data collected along the Gulf Intracoastal 

Waterway and adjacent surface-water bodies in south-central Louisiana, 

1997–2008 ......................................................................................................................................4

2. Monthly average discharge of the Atchafalaya River at Simmesport, Louisiana, 

and monthly average precipitation at Morgan City, Louisiana, for indicated time 

periods (average [1997–2008]; sustained high water [1993]; low water [2000; 2006]; 

and peak flood [2008]) .................................................................................................................6

3. Summary of instantaneous discharge measurements at selected sites along or

 near the Gulf Intracoastal Waterway in south-central Louisiana, 1997–2008 ..................7

4. Discharge at selected locations in the Gulf Intracoastal Waterway and adjacent 

waterways averaged for indicated stage increments of the Lower Atchafalaya 

River at Morgan City, 1997–2008 ................................................................................................8

5. Estimated discharge in the Gulf Intracoastal Waterway at stations west of the 

Lower Atchafalaya River at Morgan City; data presented are for averages from 

1997 to 2008, for a sustained high water year (1993), for a low water year (2006), 

and for a peak flood year (2008) ..............................................................................................11

6. Estimated discharge in the Gulf Intracoastal Waterway at stations east of the 

Lower Atchafalaya River at Morgan City; data presented are for averages from 

1997 to 2008, for a sustained high water year (1993), for a low water year (2006), 

and for a peak flood year (2008) ..............................................................................................11

7. Monthly average continuous discharge at stations with continuous index 

velocity measurements along the Gulf Intracoastal Waterway and at the Davis 

Pond and Caernarvon freshwater diversions; data presented are for averages 

from 2003 to 2008, for a low water year (2006), and for a peak flood year (2008) ............13
v

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Flow rate

cubic foot per second (ft3

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

cubic foot per second (ft3

/s) 1.9835 acre-foot per day (acre-ft/d)

mile (mi) 1.609 kilometer (km)

Datum

Vertical coordinate information is referenced to North American Vertical Datum of 1988 

(NAVD 88).

Supplemental Information

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

or micrograms per liter (μg/L).

Abbreviations

ADCP acoustic Doppler current profiler

GIWW Gulf Intracoastal Waterway

HNC Houma Navigation Canal

LAR Lower Atchafalaya River

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

WLO Wax Lake Outlet

Discharge, Suspended Sediment, and Salinity in the Gulf 

Intracoastal Waterway and Adjacent Surface Waters in 

South-Central Louisiana, 1997–2008

By Christopher M. Swarzenski and Scott M. Perrien

Abstract 

Discharge, suspended sediment, and salinity data 

collected between 1997 and 2008 indicate that the Gulf 

Intracoastal Waterway (GIWW) is an important distributary 

of river water and suspended sediments to coastal wetlands in 

south-central coastal Louisiana. Following natural hydraulic 

gradients, the GIWW passively distributes freshwater and 

suspended sediments from the Atchafalaya River to areas at 

least 30 to 50 miles west and east, respectively, of Morgan 

City. The magnitude and reach of the discharge in the GIWW 

increase as stage of the Wax Lake Outlet at Calumet and 

Lower Atchafalaya River (LAR) at Morgan City increase. 

The magnitude and duration of discharge vary from year 

to year depending on the flow regime of the Atchafalaya 

River. Annual discharge of water in the GIWW was greater 

during years when stage of the LAR remained anomalously 

high throughout the year, compared with average and peak 

flood years. During years when Atchafalaya River flow is 

low, Bayou Boeuf, a waterway draining the Verret subbasin, 

becomes a major source of water maintaining the eastward 

flow in the GIWW. The GIWW is the only means of getting 

river water to some parts of coastal Louisiana. 

The length of time stage of the LAR at Morgan City 

exceeds a given height has increased from the 1940s to 

2008. This shift has increased the length of time the GIWW 

functions as a predictable distributary of river water each 

year. Similar shifts in the future could be expected to increase 

the duration and amounts of river water reaching coastal 

Louisiana wetlands through the GIWW. 

Median suspended-sediment concentrations in the GIWW 

to the west of Morgan City were around 160 milligrams 

per liter (mg/L). In the GIWW east of Morgan City, median 

concentrations were 120–160 mg/L, except in Bayou Boeuf 

at Railroad Bridge in Amelia and the parts of the GIWW 

between Bayou Boeuf and the Houma Navigation Canal; 

median concentrations here were around 100 mg/L.

River water penetrates much of the Louisiana coast, as 

demonstrated by the large year-to-year fluctuations in salinity 

regimes of intradistributary basins in response to differences 

in flow regimes of the Mississippi and the Atchafalaya Rivers. 

This occurs directly through inflow along the GIWW and 

through controlled diversions and indirectly by transport 

into basin interiors after mixing with the Gulf of Mexico. 

The GIWW plays an important role in moderating salinity 

in intradistributary basins; for example, salinity in surface 

waters just south of the GIWW between Bayou Boeuf and the 

Houma Navigation Canal remained low even during a year with 

prolonged low water (2000).

Introduction

The Gulf Intracoastal Waterway (GIWW) is a major ship 

channel traversing the entire Louisiana coast along the interface 

between wetlands and upland (fig. 1). Following natural 

hydraulic gradients, the GIWW captures water from the Lower 

Atchafalaya River (LAR) and the Wax Lake Outlet (WLO) and 

distributes it west and east to many parts of Louisiana’s coastal 

wetlands (Paille, 1997; Swarzenski, 2003). Most of the water 

flowing in the GIWW comes from the Atchafalaya River, with 

additional inflow from the Charenton Drainage and Navigation 

Canal and Bayou Boeuf. Discharge in the Atchafalaya River is 

composed of about 30 percent of the combined discharge of the 

Red River and the Mississippi River at the Old River Control 

Structure. This amount is about 25 percent of the total discharge 

of the Mississippi River above the Old River Control Structure 

and auxiliary structures. On average, about 80–90 percent of the 

discharge in the Atchafalaya River comes from the Mississippi 

River (Wells and Demas, 1977).

Prior to the construction of the GIWW, the upper, lowsalinity reaches of intradistributary basins in the Mississippi 

River Delta were hydrologically isolated from one another; 

freshwater entered each basin through local streamflow, runoff, 

and precipitation. By cutting across natural levees of former 

Mississippi River distributaries in the upper reaches of these 

basins, the GIWW has enabled direct hydrological exchange 

between and among intradistributary basins of the Mississippi 

River Delta. This feature, along with the connecting northsouth ship channels dug to service the offshore oil industry (for 

example, the Houma Navigation Canal [HNC] and the Barataria 

Bay Waterway), represents a radical departure from natural 

delta plain hydrology. The north-south trending ship channels 

have allowed the frequent, rapid northward movement of 

saltwater from offshore into fresher parts of basins. These salt 

pulses, generated during storm surges associated with tropical 
30° ́ 29 30 ́ 92° 91°30 ́ 91° 90°30 ́ 90° Cypremort Point Verret subbasin I II III 221 Instantaneous or continuous discharge gage location and map number (table 1) Continuous salinity gage location and map number (table 1) Basin Vermilion-Teche Terrebonne Barataria II IIII Upland Wetland GULF OF MEXICO BAYOU BOEUF LOCK GIWW GIWW GIWW EXPLANATION Simmesport Stud

Constructed levee Basin boundary Subbasin boundary

GIWW Gulf Intracoastal Waterway 

0 10 20 30 40 50 KILOMETERS0 10 20 30 40 50 MILES

4

2

3

5

6

9

19

20

10

11

12

14

13

21

8

7

22

23

24

17

16

15

18

26

25

27

1

Modified from Louisiana Oil Spill Coordinator, Office of the Governor, Louisiana, GIS Digital Map, May 2007, compilation DVD

Baldwin

Morgan

City

Amelia

Houma

Larose

Calumet

Franklin

Grand

Isle

NewOrleans

Cutoff

Vermilion

Bay

Jaws Bay Missis

Barataria B

Caillou

Lake

Caillou

Bay

Lower

Atchafalaya

River

Charenton 

Drainage and 

Navigation Canal

Avoca

IslandCutoff

Davis PondFreshwater Diversion Structure

Lake Salvador

LittleLake

CaernaDivers

Bayou

Penchant

Bayou Boeuf

Atchafalaya River

Wax Lake Outlet Barataria Waterway Houma Navigation Canal Baratari

Bayou Lafourche Bayou Sale Ridge

Figure 1. Surface-water data collection sites in the Gulf Intracoastal Waterway and adjacent water bodies in south-central Louisiana. 
Approach and Methods

3

storms and hurricanes, can then travel laterally in the GIWW 

(Swarzenski, 2003). In combination with the lateral distribution 

of fresh river water from upland runoff and the Atchafalaya 

River along the prevailing hydrologic gradients during the 

remainder of the time, the original, natural north-south estuarine 

salinity gradient of the intradistributary basins has been 

disrupted. The hydrologic alterations have had and continue to 

have broad implications for basic estuarine ecology and also for 

restoration activities in the Mississippi River Delta. 

The GIWW provides an escape route for floodwaters 

carried through the Atchafalaya River Basin. Understanding 

discharge along the GIWW is helpful in evaluating flooding 

threats to surrounding communities. The freshwater, nutrients, 

and suspended sediments flowing in the GIWW are important 

resources that could support coastal Louisiana restoration 

efforts with minimal cost. 

In 1997, the U.S. Geological Survey (USGS) began 

operating a network of stations that monitor velocity, stage, and 

discharge in the GIWW and several connecting ship channels. 

This effort was initiated in cooperation with Federal agencies 

concerned with flood control and environmental quality in 

the GIWW, principally the U.S. Army Corps of Engineers 

(USACE) but also including the U.S. Fish and Wildlife 

Service, Natural Resources Conservation Service, the U.S. 

Environmental Protection Agency, and the Coastal Wetlands 

Protection, Planning, and Restoration Act Task Force. An initial 

report described surface hydrology of the GIWW by using the 

initial 3 years of data collection from 1997 to 1999 (Swarzenski, 

2003). 

Purpose and Scope

The purpose of this report is to summarize the discharge 

and suspended sediment in the GIWW and to describe effects 

of the discharge on salinity in surrounding surface waters and 

wetlands. Mineral sediments are an important component of 

wetland soils, and their delivery to wetlands is essential for 

wetland health. This report characterizes both discharge and 

suspended sediment in the GIWW and selected adjacent surface 

waters between Cypremort Point, Louisiana, and Larose, La., 

from October 1997 through December 2008 and how the fresh 

water flowing in the GIWW affected salinity in adjacent surface 

waters. Instantaneous discharge and suspended-sediment 

data collected during synoptic surveys and discharge data 

computed from continuous velocity gages were used to evaluate 

streamflow in the GIWW in relation to varying flow conditions 

of the Atchafalaya River, an important source of water to the 

GIWW. Flow regimes evaluated for this study included average, 

low water, sustained high water, and peak flood years. How 

this variable distribution of freshwater via the GIWW affected 

salinity in major Louisiana coastal basins was evaluated by 

using continuously recording salinity stations. Quantification 

of the flow and the flux of suspended sediments in the GIWW 

and how this relates to salinity in coastal Louisiana basins could 

help State and Federal agencies in implementing flood-control 

measures and designing coastal wetland restoration projects. 

Description of Study Area

The study area includes the GIWW and adjacent water 

bodies in south-central coastal Louisiana from Cypremort 

Point in the west to Bayou Lafourche in the east (fig. 1). The 

study area also includes the Verret subbasin to the north and 

east of Morgan City. The Barataria Basin, the Terrebonne 

Basin, and the Vermilion-Teche Basin receive various amounts 

of river water inflow and also are included within the study 

area. Houma and Morgan City are regional population centers. 

West of Morgan City, the GIWW trends westward across 

the WLO, past Jaws Bay and Cypremort Point to southwestern 

Louisiana (fig. 1). East of Morgan City, the GIWW is 

hydraulically disconnected from the LAR by the Bayou 

Boeuf navigation lock and the Avoca Island levee (fig. 1). The 

levee was completed in the early 1950s. At the termination 

of the levee, about 10 miles south of Morgan City, the Avoca 

Island Cutoff captures water from the LAR and carries it in 

a northeasterly direction back towards the GIWW. Bayou 

Penchant, which flows easterly captures some of the water in 

the Avoca Island Cutoff before it intersects the GIWW. Bayou 

Boeuf drains the Verret subbasin, flowing into the GIWW just 

east of the Bayou Boeuf Lock at Morgan City. The GIWW 

extends eastward to Houma, crossing the HNC and Bayou 

Lafourche before continuing on towards New Orleans, where 

navigation locks separate the GIWW from the Mississippi 

River. 

West of the WLO continuing to east of Jaws Bay, the 

GIWW has well-defined, higher elevation spoil banks that 

confine the streamflow and direct it towards Jaws Bay, a 

shallow bay contiguous with the Gulf of Mexico (fig. 1). To 

the east, the GIWW crosses forested and emergent wetlands 

near the upland-wetland interface between Morgan City 

and the HNC. Along this segment, the GIWW has poorly 

defined spoil banks with many breaks and openings providing 

substantial leakage to the interior wetlands. Easterly flow is 

increasingly confined towards the confluence with the HNC, 

and some of the leaked flow is funneled back into the GIWW 

before reaching the HNC.

Approach and Methods

Discrete (instantaneous) discharge measurements were 

used to describe flow at 12 stations on the GIWW or part of 

the primary conveyance system and 2 stations connected to 

the GIWW (fig. 1, table 1). Measurements were made during 

synoptic surveys, lasting from 2 to 3 days. Between 52 and 

62 measurements were made at most stations. Computed 

hourly discharge derived from relations using instantaneous 

discharge and continuously recording point-velocity and stage 

measurements (Morlock and others, 2001) was determined 

for the Charenton Drainage Canal at Baldwin (site 1), Bayou 

Boeuf at Railroad Bridge at Amelia (site 11), Bayou Penchant 

south of Morgan City (site 10), and for the GIWW at Bayou 

Sale Ridge near Franklin (site 4), at Houma (site 14), and west 

of Bayou Lafourche at Larose (site 15). 
4

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

Table 1. Data-collection sites and types of data collected along the Gulf Intracoastal Waterway and adjacent surface-water bodies in 

south-central Louisiana, 1997–2008.

[X, parameter sampled; --, not measured]

Station 

number

Map 

number 

(fig. 1)

Station name

Continuous Instantaneous

Salinity Discharge Velocity Stage Discharge Sediment

07385790 1 Charenton Drainage Canal at 

Baldwin

-- X X X X X

294551091422600 2 Gulf Intracoastal Waterway west of 

Jaws Bay near Cypremort

-- -- -- -- X X

07381680 3 Gulf Intracoastal Waterway east of 

Jaws Bay west of Franklin

-- -- -- -- X X

07381670 4 Gulf Intracoastal Waterway at 

Bayou Sale Ridge near Franklin

-- X X X X X

07381590 5 Wax Lake Outlet at Calumet -- X X X X X

073816614 6 Gulf Intracoastal Waterway east of 

Wax Lake Outlet near Calumet

-- -- -- -- X X

293931091145200 7 Gulf Intracoastal Waterway at mile 

99 south of Morgan City

-- -- -- -- X X

07381600 8 Lower Atchafalaya River at 

Morgan City

-- X X X X --

073816501 9 Avoca Island Cutoff south of 

Morgan City

-- -- -- -- X X

073816503 10 Bayou Penchant south of 

Morgan City

-- X X X X X

073814675 11 Bayou Boeuf at Railroad Bridge 

at Amelia

-- X X X X X

073816505 12 Gulf Intracoastal Waterway near 

Bay Wallace east of Morgan City

-- -- -- -- X X

293400090432000 13 Gulf Intracoastal Waterway west 

of Houma Navigation Canal at 

Houma

-- -- -- -- X X

07381331 14 Gulf Intracoastal Waterway 

at Houma

-- X X X X X

07381235 15 Gulf Intracoastal Waterway west of 

Bayou Lafourche at Larose

-- X X X X X

293408090230400 16 Gulf Intracoastal Waterway east of 

Bayou Lafourche at Larose

-- -- -- -- X X

295501090190400 17 Davis Pond Freshwater Diversion 

near Boutte

-- X X X -- --

295124089542100 18 Caernarvon Outfall Channel at 

Caernarvon

-- X X X -- --

07387040 19 Vermilion Bay near Cypremort Point X -- -- -- -- --

20 TV04-04R1 X -- -- -- -- --

21 TE28-07R1 X -- -- -- -- --

07381349 22 Caillou Lake (Sister Lake) 

southwest of Dulac

X -- -- -- -- --

073813498 23 Caillou Bay southwest of Cocodrie X -- -- -- -- --

073802375 24 Lake Salvador near Lafitte X -- -- -- -- --

07380335 25 Little Lake near Cutoff X -- -- -- -- --

07380251 26 Barataria Bay north of Grand Isle X -- -- -- -- --

073802516 27 Barataria Pass at Grand Isle X -- -- -- -- --

1

Gages operated by the Coastal Protection and Restoration Authority. Data can be accessed at http://cims.coastal.la.gov/DataDownload/DataDownload.

aspx?type=hydro_hourly under hourly hydrographic data.
Approach and Methods

5

Instantaneous measurements were made with a 600- 

or 1,200-kilohertz boat-mounted acoustic Doppler current 

profiler (ADCP, RD Instruments, San Diego, California, 

USA) by following standard USGS protocol (Simpson and 

Oltmann, 1993). Concurrent with the instantaneous discharge 

measurements, suspended sediments at the same location in 

the GIWW were sampled at points in the center, left, and right 

of the channel cross-section with a depth-integrating sampler 

(Edwards and Glysson, 1999). There are corresponding 

suspended-sediment measurements for most but not every 

discharge measurement. 

Flow direction is highly variable in this low-gradient 

coastal area; a sign convention was used to indicate 

streamflow direction. To the west of Morgan City, positive 

flow is to the west and south towards the Gulf of Mexico. To 

the east of Morgan City, positive flow is to the east and south, 

with the exception of the Avoca Island Cutoff where positive 

flow is to the northeast.

Salinity data were tabulated from stations located in the 

upper, middle, and lower parts of the hydrological basins with 

sondes recording salinity continuously at a single location in 

the water column. These stations are part of long-standing 

monitoring programs not associated with this study. Some of 

the stations are operated by USGS, and others are operated 

by Coastal Protection and Restoration Authority (Coastal 

Protection and Restoration Authority, 2014). Not all stations 

were operational for all of the selected years; some had 

incomplete records. 

Stage and discharge of source waters were used to 

understand factors driving flow in the GIWW. To determine 

how the stage of the LAR influenced flow along the GIWW, 

the instantaneous discharge measurements at a given 

station made between 1997 and 2008 were averaged for 

four classes of stage of the LAR at Morgan City (site 8), 

all measured against the North American Vertical Datum of 

1988: (1) 2.99 feet (ft) or less, (2) between 3.00 and 3.99 

ft, (3) between 4.00 and 5.49 ft, and (4) 5.50 ft or greater. 

The majority of measurements were made when the stage 

of LAR at Morgan City (site 8) was less than 4 ft, but a 

substantial number of measurements were made above this 

stage. To determine how discharge of source waters influence 

flow along the GIWW, monthly average discharge in source 

waters was correlated with monthly average discharge at sites 

recording discharge continuously by using Pearson correlation 

coefficients.

To estimate the interannual variability of the streamflow 

in the GIWW, the long-term average conditions and years 

exhibiting extreme variations in LAR streamflow, with 

contrasting stage hydrographs, were selected for analysis 

(fig. 2). Stage of the LAR at Morgan City (site 8) was 

averaged for the period 1997–2008 to represent long-term 

averages. The year 1993 represented conditions of sustained 

high water with higher than average water levels throughout 

most of the year, not just during the spring. The spring stage 

was not unusually high in 1993. The years 2000 and 2006 

were examples of low water years when stage remained below 

average for most of the year, including the normal spring 

flood period. The year 2008 was a year with a peak flood, a 

period when stage of the LAR was exceptionally high during 

the spring. Monthly average discharge of the Atchafalaya 

River at Simmesport (U.S. Army Corps of Engineers, 2009) 

and precipitation in the south-central climate division of 

Louisiana at Morgan City (National Oceanic and Atmospheric 

Administration, 2009) for these years are shown in table 2 to 

provide context for background conditions. 

Discharge is measured continuously at only three stations 

along the GIWW. For these stations it was possible to directly 

compute monthly average discharge for typical and unusual 

water years. Monthly average discharge data for these stations 

were calculated for the period 2003–8 and for 2006 and 2008. 

Continuous data collected prior to 2003 were not tabulated 

because almost no velocity gages were operating before the 

fall of 1999, and continuous data were not collected at most 

stations until after 2002. 

For the majority of locations along the GIWW, only 

instantaneous discharge measurements were available. An 

indirect method was needed to estimate the interannual 

variability in discharge along the GIWW. Daily 8 a.m. stage 

data of the LAR at Morgan City (site 8) (U.S. Army Corps of 

Engineers, 2009) for 1993 and for all years between 1997 and 

2008 were compiled and assumed to be representative of daily 

1

2

3

4

5

6

7

8

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Stage, in feet above North American Vertical Datum of 1988 

1997–2008 

1993 

2000 

2006 

2008 

EXPLANATION

Month

Figure 2. Monthly average stage of the Lower Atchafalaya River 

at Morgan City, Louisiana (average [1997–2008]; sustained high 

water [1993]; low water [2000, 2006]; and peak flood [2008]). 
6

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

stage. The percentage of time stage remained within each of 

the previously described four classes for which instantaneous 

discharge was averaged and was calculated for the January 

through June and July through December periods for the 

years 1993, 2006, and 2008 and for the period 1997–2008 

(the average period). Average discharge was estimated at each 

location for each of these time periods by using the percentage 

of time the stage of the LAR at Morgan City (site 8) remained 

within a class for that time period and 6-month interval and 

the average measured instantaneous discharge for the same 

stage class at that location. Average discharge was determined 

for the January–June (spring flood) and for July–December 

(seasonal low water) periods. 

To evaluate possible long-term trends in water flowing 

in the GIWW, stage exceedance curves for daily stage of the 

LAR at Morgan City (site 8) were prepared by averaging 

the daily 8 a.m. stage in 10-year intervals starting in 1940 

and continuing through 2008. The percent time a specified 

stage was exceeded was calculated in 1-ft increments. 

Because streamflow in the GIWW is related to stage of the 

LAR at Morgan City (site 8), this analysis made it possible 

to determine whether the amount of water flowing from the 

Atchafalaya River into the GIWW has changed over time.

Suspended-sediment concentrations at selected sites were 

examined by making boxplots of the individual measurements. 

Median values; the 10th, 25th, 75th, and 90th percentiles; and 

outliers were determined.

The influence of freshwater flowing in the GIWW 

on adjacent wetlands and surface waters was evaluated by 

comparing surface-water salinity at select locations in the 

intradistributary basins that the GIWW crosses in south-central 

Louisiana. Intradistributary basins are the basins bordered 

by current and historic former channels of the Mississippi 

River. Natural, higher elevation levees from the former river 

channels provided hydrologic barriers to cross-basin flow prior 

to construction of the GIWW. In each basin, 2 to 4 stations 

where salinity was collected continuously through routine 

monitoring programs were selected for evaluation (fig. 1, 

table 1). Average monthly salinity for the period 1997–2008, 

the year with sustained high water (1993), a low water year 

(2000), and a peak flood year (2008) were tabulated. The 

low water year in 2000, rather than in 2006, was selected to 

show patterns that existed prior to operation of a freshwater 

diversion in the Barataria Basin. The influence the Davis Pond 

Freshwater Diversion Structure had on salinity in the Barataria 

Basin was evaluated by comparing salinity for low water years 

Table 2. Monthly average discharge of the Atchafalaya River at Simmesport, Louisiana, and monthly average precipitation at Morgan 

City, Louisiana, for indicated time periods (average [1997–2008]; sustained high water [1993]; low water [2000; 2006]; and peak flood 

[2008]).

[ft3/s, cubic feet per second]

Average 

(1997–2008)

Sustained high

water year

(1993)

Low water year

(2000)

Low water year 

(2006)

Peak flood year 

(2008)

Discharge

(ft3

/s ×

1,000)

Precipitation

(inches)

Discharge

(ft3/s ×

1,000)

Precipitation

(inches)

Discharge

(ft3/s ×

1,000)

Precipitation

(inches)

Discharge

(ft3/s ×

1,000)

Precipitation

(inches)

Discharge

(ft3/s ×

1,000)

Precipitation

(inches)

Jan. 248 5.00 365 3.61 97 2.68 125 2.65 207 6.75

Feb. 282 4.10 293 2.81 94 0.88 213 3.87 267 2.95

Mar. 326 3.74 369 5.84 221 4.03 188 0.34 381 2.6

Apr. 338 3.77 472 8.76 252 0.92 184 6.54 586 2.11

May 315 3.71 492 8.65 168 0.72 208 1.71 472 5.48

June 262 8.10 321 4.32 179 6.05 129 1.75 345 4.4

July 211 5.86 290 6.02 211 5.36 98 9.07 302 3.19

Aug. 141 5.04 342 3.07 109 3.37 76 6.23 177 8.69

Sept. 120 6.35 231 4.38 78 5.23 80 4.08 198 11.13

Oct. 122 4.72 254 3.82 79 1.25 131 7.18 144 0.66

Nov. 133 4.47 212 4.00 102 13.43 179 2.36 99 2.26

Dec. 195 4.22 325 3.28 139 3.09 173 10.87 144 4.24

Jan.–June1 295 28.42 386 33.99 168 15.28 174 16.86 376 24.29

July–Dec. 153 30.67 276 24.57 120 31.73 123 39.79 177 30.17

1

Discharge data are 6-month averages; precipitation data are 6-month totals.
Discharge, Suspended Sediment, and Salinity

7

in 2000 and 2006. Operation of the freshwater diversion began 

in 2004; the diversion inflow influenced basin salinity in 2006. 

Not all stations were operational for all of the selected years; 

some had incomplete records.

Discharge, Suspended Sediment, and 

Salinity

Analysis of more than 10 years of instantaneous and 

continuous data demonstrates that the GIWW functions 

as a distributary of river water laterally across a wide area 

of coastal Louisiana. The GIWW conveys freshwater and 

suspended sediments originating from the LAR and WLO to 

intradistributary basins and wetlands at least 30 and 50 miles 

west and east, respectively, of Morgan City. Predictably, 

discharge along the GIWW is highest at stations closest to the 

LAR and WLO and then decreases with increasing distance 

from these sources. In general, discharge increased steadily 

as stage of the source waters increased; discharge at many 

locations in the GIWW becomes predictable when stage of the 

LAR at Morgan City is between 3 and 4 ft or higher. As shown 

in Swarzenski (2003), flow in the GIWW is passive, following 

naturally occurring hydraulic gradients. These are generated 

by differences in water-surface elevation between the source 

waters and the GIWW. For some parts of coastal Louisiana too 

far away from the Mississippi River, the GIWW is the only 

means of getting river water to adjacent surface waters.

Discharge

Discharge measurements made between 1997 and 2008 

are summarized in table 3; mean and median values and 

maximum and minimum values are included. Discharge in 

the GIWW decreased with distance from source waters. The 

highest single discharge was 61,100 cubic feet per second 

(ft3/s) at Avoca Island Cutoff south of Morgan City (site 9). 

Substantial reverse discharge was measured at least once at all 

stations, with the exception of the GIWW at Bayou Sale Ridge 

near Franklin (site 4) and east of Jaws Bay west of Franklin 

(site 3) where the lowest measurements were 20 and about 

2,000 ft3/s, respectively (table 3), but discharge was still to the 

west. 

Table 3. Summary of instantaneous discharge measurements at selected sites along or near the Gulf Intracoastal Waterway in southcentral Louisiana, 1997–2008.

[Discharge measurements are in cubic feet per second; positive flow is towards the Gulf of Mexico and away from Morgan City; ±, plus or minus; GIWW, Gulf 

Intracoastal Waterway]

Map 

number 

(fig. 1)

Station

Instantaneous discharge

Number of 

measurements

Mean ± 

standard error Median

Range

Minimum Maximum

West of the Lower Atchafalaya River

7 GIWW at mile 99 south of Morgan City 52 13,100±1,250 13,100 -3,180 38,800

6 GIWW east of Wax Lake Outlet near Calumet 57 8,900±490 9,560 -2,920 15,500

4 GIWW at Bayou Sale Ridge near Franklin 51 8,120±710 8,650 20 20,500

3 GIWW east of Jaws Bay west of Franklin 57 8,640±538 8,640 2,000 17,000

2 GIWW west of Jaws Bay near Cypremort 57 2,940±331 3,000 -2,790 10,200

1 Charenton Drainage Canal at Baldwin 48 2,520±717 3,145 -9,760 9,540

East of the Lower Atchafalaya River

9 Avoca Island Cutoff south of Morgan City 61 9,720±2,060 11,000 -23,300 61,100

10 Bayou Penchant south of Morgan City 62 3,000±279 2,980 -2,680 8,000

12 GIWW near Bay Wallace east of Morgan City 58 6,090±488 6,210 -2,580 14,100

13 GIWW west of Houma Navigation Canal at Houma 59 5,420±458 5,545 -940 11,700

14 GIWW at Houma 62 2,190±193 2,300 -1,900 5,170

15 GIWW west of Bayou Lafourche at Larose 62 1,690±178 1,530 -1,580 5,320

16 GIWW east of Bayou Lafourche at Larose 59 1,690±194 1,520 -1,860 4,930

11 Bayou Boeuf at Railroad Bridge at Amelia 58 2,300±909 2,330 -9,810 16,400
8

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

Discharge in the GIWW between the LAR and WLO 

was almost always to the west, but some of the water entering 

the GIWW at mile 99 south of Morgan City (site 7) exited the 

GIWW and flowed south towards the Gulf of Mexico rather 

than continuing west in the GIWW towards the WLO as LAR 

stage increased. In the GIWW at mile 99 south of Morgan 

City (site 7), discharge to the west increased rapidly as stage 

of the LAR increased, but the discharge exiting the GIWW 

east of WLO near Calumet (site 6) only increased gradually, 

or even decreased, as river stage increased (table 4). At stages 

above 5.5 ft, 31,500 ft3/s on average entered the GIWW; of 

this amount, 11,400 ft3/s on average reached the WLO. The 

balance flowed south towards the Gulf of Mexico.

West of the WLO, the discharge also was to the west 

and also increased with increasing stage. Much of the water 

entering the GIWW reached Jaws Bay rather than dispersing 

into the adjacent wetlands. Above 5.5 ft, 17,900 ft3/s flowed 

in the GIWW at Bayou Sale Ridge near Franklin (site 4) 

and about 14,500 ft3/s, or more than 75 percent of this flow, 

reached the GIWW east of Jaws Bay west of Franklin (site 3) 

Table 4. Discharge at selected locations in the Gulf Intracoastal Waterway and adjacent waterways averaged for indicated stage 

increments of the Lower Atchafalaya River at Morgan City, 1997–2008.

[Positive flow is towards the Gulf of Mexico and away from Morgan City; NAVD 88, North American Vertical Datum of 1988; ±, plus or minus; GIWW, Gulf 

Intracoastal Waterway; <, less than; >, greater than]

Stage of 

the Lower 

Atchafalaya River 

at Morgan City 

(NAVD 88)

Mean discharge (±1 standard error)

(cubic feet per second)

Stations west of the Lower Atchafalaya River

Map number 7 6 4 3 2 1

GIWW at 

mile 99 south 

of Morgan City

GIWW east of 

Wax Lake Outlet 

near Calumet

GIWW at Bayou 

Sale Ridge 

near Franklin

GIWW east of 

Jaws Bay west 

of Franklin

GIWW west of 

Jaws Bay near 

Cypremort

Charenton 

Drainage Canal 

at Baldwin

<2.99 5,510±925 6,350±695 3,560±491 6,640±385 2,560±491 3,230±915

3.0–3.99 13,600±805 9,380±717 9,500±348 9,520±539 3,980±468 3,240±1,320

4.0–5.49 19,700±1,060 12,100±506 13,600±537 12,200±628 3,660±736 2,370±1,660

>5.5 31,500±2,690 11,400±979 17,900±1,780 14,500±562 4,000±1,150 -5,620±2,740

Stations east of the Lower Atchafalaya River

Map number 9 10 12 13 11

Avoca Island 

Cutoff south of 

Morgan City

Bayou Penchant 

south of Morgan 

City

GIWW near Bay 

Wallace east of 

Morgan City

GIWW west 

of Houma 

Navigation Canal 

at Houma

Bayou Boeuf at 

Railroad Bridge 

at Amelia

<2.99 2,670±2,900 1,430±276 3,330±584 2,510±423 1,010±1,440

3.0–3.99 10,700±3,270 4,170±475 6,480±626 6,180±561 871±1,540

4.0–5.49 17,300±2,970 4,710±456 8,930±391 8,480±625 6,320±1,400

>5.5 22,700±9,630 4,720±650 11,100±573 9,990±845 2,850±2,330

Map number 14 15 16

GIWW at 

Houma

GIWW west of 

Bayou Lafourche 

at Larose

GIWW east of 

Bayou Lafourche 

at Larose

<2.99 1,550±272 905±221 873±256

3.0–3.99 2,210±389 1,740±304 1,830±364

4.0–5.49 2,990±317 2,770±416 2,650±414

>5.5 3,630±405 2,730±366 2,690±386
Discharge, Suspended Sediment, and Salinity

9

(table 4). At low river stage, there appeared to be inflow 

into the GIWW between Bayou Sale Ridge and Jaws Bay 

that did not originate from the WLO. Much of the water 

entering the GIWW at the WLO discharged into Jaws Bay. 

Discharge in the GIWW west of Jaws Bay near Cypremort 

(site 2) increased when stage of the LAR reached 3 ft but then 

remained relatively constant at about 4,000 ft3/s, with further 

increases in LAR stage. The Charenton Drainage Canal at 

Baldwin (site 1) contributed around 2,400 to 3,300 ft3/s to the 

opening to Jaws Bay at most stages of the LAR. At the highest 

LAR stage, net discharge in the Charenton Drainage Canal at 

Baldwin (site 1) was found to be in an inland direction.

East of Morgan City, water entered the GIWW from the 

LAR primarily through the Avoca Island Cutoff. Additional 

water entered the GIWW through Bayou Boeuf at Railroad 

Bridge at Amelia (site 11). Stage of the LAR at Morgan City 

(site 8) influenced flows as far away as Bayou Lafourche at 

Larose. Bayou Boeuf at Railroad Bridge at Amelia (site 11) 

usually contributed water to the GIWW regardless of stage of 

the LAR at Morgan City (site 8) (table 4). At high river stage, 

the Avoca Island Cutoff (site 9) supplied the majority of water 

to the GIWW (table 4). Discharge here increased as stage of 

the LAR at Morgan City (site 8) increased. Discharge in the 

Avoca Island Cutoff south of Morgan City (site 9) averaged 

22,700 ft3/s when stage of the LAR at Morgan City (site 8) 

exceeded 5.5 ft. Some of the water flowed from the Avoca 

Island Cutoff into Bayou Penchant. Discharge in the GIWW 

near Bay Wallace east of Morgan City (site 12) increased 

as stage of the LAR at Morgan City increased and averaged 

11,100 ft3/s towards Houma when stage was above 5.5 ft. 

Discharge in the GIWW west of the HNC at Houma (site 13) 

averaged about 10,000 ft3/s under the same conditions.

The majority of the water in the GIWW reaching the 

HNC from the west flowed south into the HNC; discharge in 

the GIWW at Houma (site 14) was considerably lower than 

west of the HNC at Houma (site 13); for example, at stages of 

the LAR at Morgan City (site 8) greater than 5.5 ft, discharge 

continuing east across the HNC and towards Bayou Lafourche 

averaged about 3,600 ft3/s, or just over one-third of the amount 

reaching the HNC from the west. Discharge in the GIWW at 

Houma (site 14) increased with increasing stage of the LAR 

at Morgan City (site 8). Farther east, discharge on both sides 

of Bayou Lafourche was similar for all four classes of LAR 

stage. Discharge in the GIWW west (site 15) and east (site 16) 

of Bayou Lafourche at Larose increased with increasing stage 

of the LAR at Morgan City up to 4 ft but remained relatively 

constant at about 2,700 ft3

/s when stage of the LAR exceeded 

this threshold. A small amount of water enters the GIWW 

from Bayou Lafourche on the northwest side, and a similar 

amount flows from the GIWW into Bayou Lafourche along 

the southeast side (Swarzenski, 2003).

Discharge at most locations on the GIWW increased 

as stage of the LAR at Morgan City (site 8) increased. This 

was demonstrated in the previous section by averaging 

instantaneous measurements made for four stage increments 

of the LAR at Morgan City (site 8) (table 4). To what degree 

source waters controlled discharge at various locations along 

the GIWW also can be assessed by comparing monthly 

averaged discharge from continuously operating gages in 

the GIWW to that in the WLO and LAR (figs. 3 and 4). 

Significant (p<0.001) high correlation coefficients in what at 

least east of Morgan City are fairly open hydrologic systems 

demonstrate how tightly flow along the GIWW is controlled 

by flows at the sources. 

Discharge of source waters correlated significantly with 

discharge in the GIWW on both sides of Morgan City. To the 

west of the WLO, monthly average discharge in the GIWW 

at Bayou Sale Ridge near Franklin was highly correlated 

with monthly average discharge of the WLO at Calumet (site 

5; r2

=0.95, p< 0.001) (fig. 3). Monthly average discharge in 

the GIWW near Bay Wallace east of Morgan City (site 12) 

increased linearly with increasing monthly average discharge 

of the LAR at Morgan City (site 8; r2=0.92, p<0.001) and 

when river stage was 3 ft or less above with monthly average 

discharge in Bayou Boeuf at Railroad Bridge at Amelia (site 

11; r2=0.58, p<0.001) (fig. 4). When stage of the LAR at 

Morgan City (site 8) was below 3 ft, on average more than 

one-third of the discharge in the GIWW near Bay Wallace 

east of Morgan City (site 12) came from Bayou Boeuf at 

Railroad Bridge at Amelia (site 11). When flow of the LAR 

was low, the Verret subbasin became a major source of water, 

maintaining the eastward flow in the GIWW. Monthly average 

discharge in the GIWW at Houma (site 14) increased steadily 

with increases in monthly average discharge of the GIWW 

near Bay Wallace east of Morgan City (site 12; r2=0.86, 

p<0.001) (fig. 4). Farther east, discharge in the GIWW west of 

Bayou Lafourche at Larose (site 15) was more weakly related 

to discharge in the GIWW near Bay Wallace east of Morgan 

City (site 12) (r2=0.62, p<0.001). 

Monthly average discharge at Wax Lake Outlet at Calumet,

 in cubic feet per second

0 50,000 100,000 150,000 200,000

0

5,000

10,000

15,000

20,000

r 2=0.95, p<0.001

Monthly average discharge in the GIWW at 

Bayou Sale Ridge near Frankllin, in cubic feet per second

Figure 3. Relation between monthly average discharge in the 

Gulf Intracoastal Waterway (GIWW) at Bayou Sale Ridge near 

Franklin and monthly average discharge of the Wax Lake Outlet at 

Calumet. 
10

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

Interannual Variability

The amount of river water flowing along the GIWW 

depends on stages of the WLO and the LAR and will vary 

considerably from year to year depending on the hydrograph 

of those source waters. On average, hydraulic head differences 

between source waters and the GIWW are most pronounced 

during spring floods of the Atchafalaya River. This is the 

period when flows in the GIWW should be high. Year-to-year 

variations in Atchafalaya River flow, caused by droughts or 

large amounts of precipitation in the entire drainage basin, for 

example, also will influence flow in the GIWW. 

The annual discharge of water flowing in the GIWW was 

greatest during the year with sustained high water of source 

waters (1993) (tables 5 and 6). This was the year when the 

most freshwater was delivered to coastal Louisiana through 

the GIWW. The steady above average discharge during the 

second half of the year, when stage of the LAR at Morgan 

City (site 8) remained anomalously high, was the primary 

difference; however, in just the first half of the year, the amount 

of water flowing in 1993 in the GIWW surpassed the amount 

of water under peak flood conditions of source waters in 2008. 

For example, the average discharge from January to June in 

1993 was 15,500 ft3

/s in the GIWW at Bayou Sale Ridge near 

Franklin (site 4); during the peak flood year in 2008, discharge 

over the same time period was about 2,000 ft3/s less (table 5). 

For the second half of the year, discharge at Bayou Sale Ridge 

near Franklin (site 4) was 12,100 ft3/s in 1993 but decreased to 

about 7,000 ft3/s in 2008. The pattern was the same to the east 

of Morgan City. Discharge in the GIWW near Bay Wallace 

east of Morgan City (site 12) during the first 6 months of 1993 

averaged about 9,900 ft3/s, compared with 8,740 ft3/s during 

the first 6 months of the peak flow year (2008) (table 6). In the 

second half of the year, discharge averaged 8,030 ft3/s in 1993 

compared with 5,220 ft3/s in 2008. The same trends also held 

at greater distances from the source waters, but the differences 

were less pronounced. To the west of Morgan City, discharge in 

the GIWW west of Jaws Bay near Cypremort (site 2) averaged 

Monthly average discharge, in cubic feet per second

Monthly average discharge, in cubic feet per second

0

0

4,000

8,000

12,000

0 4,000

100,000 200,000 300,000

8,000 12,000

-2,000

0

2,000

4,000

Lower Atchafalaya River at Morgan City Bayou Boeuf at Railroad Bridge near Amelia 

GIWW near Bay Wallace east of Morgan City GIWW near Bay Wallace east of Morgan City

0 4,000 8,000 12,000

-4,000 0 4,000 8,000 12,000

-2,000

0

2,000

4,000

0

4,000

8,000

12,000

Stage of Lower Atchafalaya River 

at Morgan City, in feet above 

North American Vertical Datum 

of 1988

4 feet or more

Between 3 and 4 feet

3 feet or less

GIWW near Bay Wallace east of Morgan CityGIWW near Bay Wallace east of Morgan City

GIWW at Houma 

GIWW west of Bayou Lafourche at Larose

EXPLANATION

r 2=0.86, p<0.001 r 2=0.62, p<0.001

r 2

=0.92, p<0.001 r 2

=0.58, p<0.001 GIWW Gulf Intracoastal Waterway 

A B

C D

Figure 4. Relation between monthly average discharge in the Gulf Intracoastal Waterway (GIWW) near Bay Wallace east of Morgan 

City and monthly average discharge of the Lower Atchafalaya River at Morgan City or Bayou Boeuf at Railroad Bridge near Amelia 

(panels A and B); graphs showing relation between monthly average discharge in the GIWW at Houma or west of Bayou Lafourche at 

Larose and monthly average discharge of the GIWW near Bay Wallace east of Morgan City (panels C and D). Correlation in panel B is 

only for a Lower Atchafalaya River stage of 3 feet or less North American Vertical Datum of 1988 (NAVD 88).
Discharge, Suspended Sediment, and Salinity

11

Table 5. Estimated discharge in the Gulf Intracoastal Waterway at stations west of the Lower Atchafalaya River at Morgan City; data 

presented are for averages from 1997 to 2008, for a sustained high water year (1993), for a low water year (2006), and for a peak flood 

year (2008).

[Values are estimated by using average instantaneous discharge values shown in table 4 and the percent time the Lower Atchafalaya River at Morgan City was at 

the corresponding stage intervals for the indicated time period]

Map 

number 

(fig. 1)

Station name

Average discharge

(cubic feet per second)

Average

(1997–2008)

Sustained high water 

(1993)

Low water

(2006)

Peak flood

(2008)

Jan.–June July–Dec. Jan.–June July–Dec. Jan.–June July–Dec. Jan.–June July–Dec.

7 GIWW at mile 99 south 

of Morgan City

17,300 6,560 25,100 17,500 7,220 6,160 22,000 10,300

6 GIWW east of Wax 

Lake Outlet near 

Calumet

11,000 6,740 11,700 11,100 6,990 6,590 10,600 8,230

4 GIWW at Bayou Sale 

Ridge near Franklin

12,000 4,330 15,500 12,100 4,810 4,030 13,500 7,030

3 GIWW east of Jaws 

Bay west of Franklin

11,200 7,010 13,200 11,300 7,250 6,870 12,000 8,440

2 GIWW west of Jaws 

Bay near Cypremort

3,750 2,740 3,820 3,670 2,860 2,670 3,680 3,170

1 Charenton Drainage 

Canal at Baldwin1

2,820 2,190 No data No data 1,760 2,270 2,740 2,600

1

Values calculated from continuous discharge gage, starting January 2002. 

Table 6. Estimated discharge in the Gulf Intracoastal Waterway at stations east of the Lower Atchafalaya River at Morgan City; data 

presented are for averages from 1997 to 2008, for a sustained high water year (1993), for a low water year (2006), and for a peak flood 

year (2008).

[Values are estimated by using average instantaneous discharge values shown in table 4 and the percent time the Lower Atchafalaya River at Morgan City was at 

the corresponding stage intervals for the indicated time period; GIWW, Gulf Intracoastal Waterway] 

Map 

number

(fig. 1)

Station name

Average discharge

(cubic feet per second)

Average

(1997–2008)

Sustained high water 

(1993)

Low water

(2006)

Peak flood

(2008)

Jan.–June July–Dec. Jan.–June July–Dec. Jan.–June July–Dec. Jan.–June July–Dec.

9 Avoca Island Cutoff Channel 

south of Morgan City

14,700 3,970 19,700 15,400 4,370 3,450 16,700 8,150

10 Bayou Penchant south of 

Morgan City

4,450 1,790 4,710 4,370 2,010 1,640 4,160 2,810

12 GIWW near Bay Wallace east 

of Morgan City

7,960 3,740 9,900 8,030 4,000 3,580 8,740 5,220

13 GIWW west of Houma 

Navigation Canal at Houma

7,560 2,990 9,150 7,580 3,280 2,800 7,970 4,620

14 GIWW at Houma 2,690 1,640 3,280 2,720 1,690 1,600 2,960 1,990

15 GIWW west of Bayou 

Lafourche at Larose

2,370 1,010 2,740 2,410 1,080 973 2,350 1,470

16 GIWW east of Bayou 

Lafourche at Larose

2,330 997 2,660 2,340 1,080 950 2,300 1,460

11 Bayou Boeuf at Railroad 

Bridge at Amelia1

2,530 1,820 No data No data 953 1,290 1,840 2,300

1

Values calculated from continuous discharge gage; record begins in February 2003. 
12

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

3,820 ft3/s during the first 6 months of 1993 and 3,680 ft3/s 

during the same time period in 2008 (table 5). In the GIWW 

at Houma (site 14), the discharge during the second half of the 

year was 2,720 ft3/s in 1993 and 1,990 ft3/s in 2008 (table 6). 

The key to large inflows of freshwater to coastal wetlands 

through the GIWW is sustained high water of the source 

waters throughout an entire year. Peak floods can deliver large 

quantities of freshwater through the GIWW but only for short 

periods of time.

Conversely, annual discharge in the GIWW was 

substantially below average for the first 6 months of the year 

when stage of the LAR (site 8) remained low for most of the 

year, as occurred in 2006, but was not substantially different 

from average discharges during the second half of the year. 

Discharge in the GIWW at Bayou Sale Ridge near Franklin 

(site 4) averaged 12,000 ft3/s for the first 6 months from 1997 

to 2008 but only 4,810 ft3/s during the same time period 

in 2006 (table 5). From July through December, discharge 

here averaged 4,330 ft3/s from 1997 to 2008 and 4,030 ft3/s 

in 2006. Discharge in the GIWW near Bay Wallace east of 

Morgan City (site 12) averaged 7,960 ft3/s during the first 

6 months of the year from 1997 to 2008 but only 4,000 ft3/s 

during the same time period in 2006 (table 6). Discharge at 

site 12 in the second half of the year was 3,740 ft3/s from 1997 

to 2008 and 3,580 ft3/s in 2006. Farther from the sources of 

the water flowing into the GIWW, discharge in the GIWW 

both west and east of Bayou Lafourche at Larose (sites 15 

and 16, respectively) averaged about 1,000 ft3/s from July 

to December in 2006, very similar to the long-term average 

discharge from 1997 to 2008 during the same time period 

(table 6). 

The previous values showing interannual variability in 

flow along the GIWW were generated by indirect methods, 

by using averages of all discrete measurements made between 

1997 and 2008 for four stage classes of the LAR at Morgan 

City (site 8); thus, the values shown in tables 5 and 6 are 

estimated values. For the GIWW at the Bayou Sale Ridge 

near Franklin (site 4), the GIWW at Houma (site 14), and 

the GIWW west of Bayou Lafourche at Larose (site 15), 

direct tabulations of interannual variability in discharge were 

possible by using computed discharge records, although the 

period of record when continuous gaging stations were in 

operation was shorter. Comparisons between the two methods 

allow a qualitative assessment of the indirect method to 

estimate discharge on the GIWW. This is valuable because 

the majority of stations on the GIWW are ungaged and only 

instantaneous measurements are available. 

Continuous discharge data for the same time periods also 

are shown for two controlled diversions of Mississippi River 

water. At these sites, river water is actively moved over and 

under flood control levees through control structures and into 

the adjacent estuarine waters and marshes to mitigate wetland 

loss. Even small amounts of river water are presumed to have 

beneficial effects on marshes and enhance restoration (Nyman, 

2014; Coastal Protection and Restoration Authority, 2015). 

The comparison of controlled diversion discharge with the 

discharge in the GIWW is one measure of the potential of the 

GIWW for wetland restoration.

Methods used to estimate discharge indirectly (tables 5 and 

6) both overestimated and underestimated computed discharge 

at these same sites by as much as 20–35 percent (tables 5–7). In 

general, the indirect method tended to overestimate discharge, 

but in the GIWW at Bayou Sale Ridge near Franklin (site 4), 

the direct calculations were higher for the first 6 months for the 

low water year 2006. As a first approximation, indirect methods 

of estimating discharge in the GIWW are useful, but direct 

measurements are preferable and needed to better quantify 

the amount of water being conveyed by the GIWW under 

hydrologically varying conditions. 

Discharge in the GIWW was comparable to the controlled 

discharges at the diversion sites, and in at least one site (for 

example, at Bayou Sale Ridge at Franklin [site 4]), discharge 

was almost always much greater under all hydrological 

conditions. During the period 2003–8, average discharge in 

the GIWW at Houma (site 14) and west of Bayou Lafourche at 

Larose (site 15) was only 400–800 ft3/s lower than at the two 

controlled diversions for the January–June period. This part of 

coastal Louisiana is too far away from the Mississippi River 

to benefit from controlled diversions on the river. The GIWW 

is the only means of getting river water to the area marshes; 

however, controlled diversions were better able to effectively 

discharge river water into coastal basins during years of below 

average flow, such as 2006 (table 7).

Trends

In addition to the short-term interannual variability in 

discharge along the GIWW that depends on the characteristics 

of a particular water year, long-term trends in hydrography 

of the LAR will also affect the magnitude and duration of 

discharge along the GIWW. These include changes in the length 

of time the LAR at Morgan City (site 8) remains above stages 

that induce predictable flow in the GIWW. This would change 

the duration in any given year that the GIWW effectively 

functions as a distributary of floodwaters of the Atchafalaya 

River. In the absence of direct discharge measurements reaching 

back in time, analysis of long-term trends of the stage of the 

LAR at Morgan City (site 8) is the only way to understand longterm trends in flow along the GIWW. Also of interest is whether 

relations between a given stage of the LAR at Morgan City 

(site 8) and the observed discharge at a selected point along the 

GIWW are changing over time. 

The duration a given stage of the LAR at Morgan City 

(site 8) exceeded a given elevation increased from the 1940s 

to 2008, most dramatically during the 1970s (fig. 5). During 

the early 1970s, 3 consecutive years of high water occurred 

that likely contributed to the increase, but the shift persisted 

in the following decades, even late each year in November 

and October. Infilling of the river bed and the initial subaerial 

growth of the deltas at the mouth of the LAR and WLO, which 

would increase backwater effects, probably contributed to the 

shift. 
Table 7. Monthly average continuous discharge at stations with continuous index velocity measurements along the Gulf Intracoastal Waterway and Caernarvon freshwater diversions; data presented are for averages from 2003 to 2008, for a low water year (2006), and for a peak flood year (2008).[Discharge is in cubic feet per second; some months have only partial data; GIWW, Gulf Intracoastal Waterway]

Average 2003–8 Low water (2006) Peak fl

GIWW at 

Bayou 

Sale 

Ridge 

near 

Franklin

GIWW 

at 

Houma 

GIWW 

west of 

Bayou 

Lafourche 

at 

Larose

Davis 

Pond 

Freshwater 

Diversion 

near 

Boutte

Caernarvon 

Outfall 

Channel 

at 

Caernarvon

GIWW at 

Bayou 

Sale 

Ridge 

near 

Franklin

GIWW 

at 

Houma

GIWW 

west of 

Bayou 

Lafourche 

at 

Larose

Davis 

Pond 

Freshwater 

Diversion 

near 

Boutte

Caernarvon 

Outfall 

Channel 

at 

Caernarvon

GIWW at 

Bayou 

Sale 

Ridge 

near 

Franklin

GIWW 

at 

Houma

GIWW 

west 

Bayo

Lafourc

at 

Laros

Jan. 9,370 1,770 1,970 1,700 3,510 5,080 1,210 1,070 1,730 2,670 8,550 507 No da

Feb. 10,000 1,780 2,430 2,930 4,070 9,280 No data 1,410 3,630 3,100 9,940 1,040 1,89

Mar. 11,000 2,180 1,690 2,090 4,650 7,400 No data 349 3,500 3,630 14,490 2,510 2,35

Apr. 10,500 2,210 1,680 3,090 2,080 7,550 No data 662 3,990 3,380 18,900 3,400 2,30

May 10,600 1,870 1,370 1,970 717 8,740 No data 1,090 2,910 1,610 18,000 4,470 3,03

June 9,370 1,840 1,820 1,460 707 5,550 1,250 505 2,500 477 14,600 3,580 2,84

July 8,110 1,600 1,800 1,990 614 3,400 838 563 No data 395 13,400 3,500 2,22

Aug. 4,530 1,150 863 1,500 753 3,270 1,010 924 No data 834 8,000 2,880 63

Sept. 4,330 427 97 1,150 738 3,520 1,020 846 No data 1,610 6,510 -114 -1,55

Oct. 4,590 335 338 758 1,520 4,590 1,810 649 1,370 2,430 6,210 293 68

Nov. 4,640 890 882 1,210 1,250 5,820 2,060 1,160 3,400 993 3,590 No data 43

Dec. 6,300 1,180 1,270 1,720 2,450 5,710 895 1,480 3,350 2,770 3,950 932 41

Jan.–June1 10,140 1,940 1,830 2,200 2,620 7,270 No data 848 3,040 2,480 14,100 2,580 2,48

July–Dec. 5,420 931 876 1,390 1,220 4,480 1,270 936 2,700 1,500 6,943 1,500 47

1Six-month discharge averages.
14

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

The exceedance curve of the 1990s was anomalous 

compared with all other decades. Rather than decreasing 

rapidly at stages greater than about 2–3 ft, the exceedance 

times decreased gradually. Reasons for the anomalous 

exceedance may be due to several years of high sustained 

flows during the decade; for example, as in 1993 when the 

LAR stage stayed above 3 ft for much of the year (fig. 2). A 

weir across the WLO installed in 1988 was removed in 1994 

and likely influenced patterns of discharge from the LAR and 

WLO into the GIWW as well.

The shift has increased the amount of time each year 

that the LAR at Morgan City (site 8) remains between 3 

and 4 ft or higher and, consequently, the annual discharge 

in the GIWW. If the long-term trend towards longer periods 

of higher stages of the LAR at Morgan City continues, the 

duration and amounts of flow in the GIWW could be expected 

to increase and with it the amount of river water reaching 

coastal Louisiana wetlands. The subaerial expansion of the 

deltas at the mouths of the LAR and WLO should slow down 

the travel times of the seasonal spring flood of the LAR and 

promote the lateral diversion of river water into the GIWW. 

Counter balancing this trend is the maintenance dredging in 

the LAR which reduces the infilling of the riverbed over time 

and increases the cross-channel volume at the mouth. 

Discharge in relation to stage of the WLO at Calumet 

changed in the GIWW west of Jaws Bay near Cypremort 

between 1997 and 1998 and 2007 and 2008 (fig. 6). During the 

period 2007–8, there was little westward flow past Jaws Bay, 

in contrast to the earlier period. More recent measurements are 

needed to determine whether this change is permanent. At all 

other sites, discharge measured during the two periods did not 

show clear differences. 

Suspended Sediment

Suspended-sediment concentrations west of the 

LAR were generally higher than concentrations east of 

the LAR. West of the LAR, median suspended-sediment 

concentrations were about 160 mg/L at all stations except 

for the Charenton Drainage Canal at Baldwin (site 1), which 

had lower concentrations (fig. 7). Values outside the 90th 

percentile were between 400 and 500 mg/L. To the east, 

median concentrations at the Avoca Island Cutoff and Bayou 

Penchant south of Morgan City (sites 9 and 10, respectively) 

ranged between 120 and 140 mg/L, higher than GIWW near 

Bay Wallace east of Morgan City (site 12) and GIWW west 

of HNC at Houma (site 13), where the median concentrations 

were about 100 mg/L and similar to median concentrations 

at Bayou Boeuf at the Railroad Bridge at Amelia (site 11) 

(fig. 7). The lower suspended-sediment concentrations along 

this stretch of the GIWW support the findings using discharge 

measurements that water from the Verret subbasin exits 

through Bayou Boeuf and mixes with LAR water coming from 

the Avoca Island Cutoff, diluting concentrations. Farther east, 

median suspended-sediment concentrations in the GIWW 

increased to about 120 mg/L at Houma (site 14) and to about 

160 mg/L west of Bayou Lafourche at Larose (site 15). 

Suspended sediments are added to the water column along this 

stretch of the GIWW from local sources or through continual 

resuspension. Values outside the 90th percentile only reached 

between 260 and 340 mg/L. 

Compared to the GIWW between Bayou Boeuf and 

the HNC, the higher discharge in the GIWW between the 

LAR and Jaws Bay, combined with the 25-percent higher 

average suspended-sediment concentrations, make this 

stretch potentially more useful for delivery of suspended 

sediments for wetland restoration. The large drop in discharge 

in the GIWW across Jaws Bay also makes this shallow bay 

a potential area for suspended-sediment delivery and marsh 

creation.

Salinity

The variability of freshwater inflow to the Louisiana 

coast from the Mississippi and Atchafalaya Rivers, depending 

on the stage hydrographs of a particular water year, caused 

large year-to-year fluctuations in local estuarine salinity 

regimes in many parts of south-central coastal Louisiana. The 

fluctuations were caused indirectly by transport of river water 

into basin interiors and directly by freshwater inflow from the 

Stage, in feet above North American Vertical Datum of 1988 

-1 0 1 2 3 4 5 6 7 8

0

20

40

60

80

100

1940–49

1950–59

1960–69

1970–79

1980–89

1990–99

2000–2008

EXPLANATION

Percent exceedance

Figure 5. Percent time the Lower Atchafalaya River at Morgan 

City exceeded indicated stage. Stage was averaged in 10-year 

increments starting in January 1940 and extending through 

December 2008.
Discharge, Suspended Sediment, and Salinity

15

GIWW. The salinity fluctuations in the basins demonstrate 

that river water can penetrate large parts of coastal Louisiana. 

The GIWW plays an important role in moderating salinity in 

intradistributary basins. 

River discharge heavily influences salinity in the 

Vermilion-Teche Basin from January through June (fig. 8). At 

TV04–04R (fig. 1, site 20), closer to the source of river water, 

salinity was similar and less than 1 part per thousand (ppt) 

during the early part of the year even for low and average flow 

years. Salinity at TV04–04R (site 20) was higher in the late 

summer and fall months (August–November) during the low 

water year compared with average conditions. Farther west at 

Vermilion Bay (fig. 1, site 19), a similar trend was observed, 

but average salinity was higher than that at TV04–04R 

(site 20) throughout the year (fig. 8). Sustained high water 

and peak flood conditions decreased salinity below long-term 

averages for most of the year, including in the spring and early 

summer, to levels that were similar to those at TV04–04R 

(site 20). During the low water year, the reduced volumes 

of river water allowed salinity to increase over average 

conditions by 2–5 ppt from January through June and then by 

more than 10 ppt in late summer. Freshwater likely reached 

this area through westward drift of water towards Cypremort 

Point from the mouths of the LAR and WLO and from the 

GIWW through Jaws Bay (fig. 8).

Terrebonne Basin surface waters were sensitive to 

freshwater inflows through the GIWW. Salinity in the spring 

and summer months was similar and averaged below 1 ppt 

during low water and peak flood conditions at TE28–07R 

(fig. 1, site 21) (fig. 8). Salinity during the low water year 

was not markedly higher than during average conditions, an 

indication that freshwater inflow from Bayou Boeuf continued 

to moderate salinity even under low river flow. Farther south 

at Caillou Lake (site 22), sustained high water suppressed 

salinity below average conditions for the entire year, at times 

by 7 ppt or more. During the peak flood year, salinity was 

below long-term average conditions only from early summer 

through July. During the low water year (2000), salinity spiked 

0

5,000

10,000

15,000

Stage, Lower Atchafalaya River at Morgan City, 

in feet above North American Vertical Datum of 1988

2007–8

1997–98

0 2 4 6 8 10

0

5,000

10,000

15,000

20,000

25,000

GIWW east of Jaws Bay 

west of Franklin

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

0

5,000

10,000

15,000

GIWW near Bay Wallace

east of Morgan City

GIWW west of Jaws Bay

near Cypremort

0

2,000

4,000

6,000

0

5,000

10,000

15,000

GIWW west of Houma

Navigation Canal

at Houma

GIWW at Bayou Sale Ridge

near Franklin

0

2,000

4,000

6,000

GIWW at Houma

EXPLANATION

Instantaneous discharge, in cubic feet per second

GIWW Gulf Intracoastal Waterway 

Figure 6. Comparison of instantaneous discharge measurements made in 1997 and 1998 and measurements made in 2007 and 2008 in 

relation to stage of the Lower Atchafalaya River at Morgan City.
16

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

in May and June and again during October and November, 

with values as much as 7 ppt higher than average conditions 

(fig. 8). The spikes mimic the average water conditions in 

timing but were extreme in magnitude because of the regional 

drought occurring in south-central Louisiana at the time. The 

majority of the freshwater controlling salinity at both of these 

areas most likely came from the GIWW, rather than from 

Mississippi River water drifting westwards in the Gulf of 

Mexico. At Caillou Bay (fig. 1, site 23), salinity during peak 

flood conditions only decreased to about 15 ppt (fig. 8). During 

the low water year (2000), salinity remained between 25 and 

30 ppt, about 4–5 ppt higher than salinity during average 

conditions. 

Not surprisingly, surface waters in Barataria Basin, the 

basin farthest from Morgan City in this study, were the least 

affected by freshwater inputs from the GIWW. In Barataria 

Basin, the site at Lake Salvador (fig. 1, site 24), the farthest 

inland site, was sensitive to the lack of freshwater inflow in 

2000, when the Davis Pond Freshwater Diversion Structure 

was not yet operational (fig. 8). Average monthly salinity 

increased by 2–6 ppt throughout 2000 compared with average 

conditions. The lack of local rainfall coupled with low river 

discharge may have allowed salinity to become elevated. 

During the peak flood year, salinity dropped below average 

conditions, to values similar to other inland sites, for example 

in the Teche-Vermilion basin (TE28–07R, site 21) and in 

Terrebonne Basin (TV04–04R, site 20). Freshwater entering 

the area from the Davis Pond Freshwater Diversion Structure 

contributed to the low salinity values during the peak flood 

year. Salinity concentrations at Little Lake (fig. 1, site 25) were 

below long-term averages for both the year with sustained high 

water and the peak flood year and were between less than 5 and 

16 ppt lower than during the low water year (fig. 8). Salinity 

values spiked dramatically during May and June 2000, similar 

to Caillou Lake (site 22), and were much higher than those 

for long-term average conditions. During years when river 

discharge is high, this area receives water from the GIWW 

through Bayou Perot (Swarzenski, 2003) from local runoff 

and also from Mississippi River water drifting westwards from 

the mouth and then in an inland direction. Farther south, in 

Barataria Bay north of Grand Isle (site 26), salinity during the 

low water year remained between 20 and 25 ppt, higher than 

the long-term average and substantially higher than salinity 

during 1993 and spring and early summer in 2008 (fig. 8). At 

EXPLANATION

Outlier

90th percentile

75th percentile

25th percentile

10th percentile

50th median

Stations on the GIWW east of the Lower Atchafalaya River

0

100

200

300

400

500

Stations on the GIWW west of Lower Atchafalaya River

0

100

200

300

400

500

Suspended-sediment concentration, in milligrams per liter

GIWW at mile 99 

south of Morgan City (7)

GIWW east of Wax Lake Outlet

near Calumet (6)

GIWW at Bayou Sale Ridge

near Franklin (4)

GIWW east of Jaws Bay

west of Franklin (3)

GIWW west of Jaws Bay 

near Cypremort (2)

Charenton Drainage Canal 

at Baldwin (1)

Avoca Island Cutoff 

south of Morgan City (9)

Bayou Penchant

south of Morgan City (10)

Bayou Boeuf at Railroad Bridge

at Amelia (11)

GIWW west of Houma Navigation Canal

at Houma (13)

GIWW at Houma (14)

GIWW west of Bayou Lafourche

at Larose (15)

GIWW near Bay Wallace

east of Morgan City (12)

Gulf Intracoastal 

Waterway 

GIWW

Figure 7. Statistical summary of suspended-sediment concentrations collected between 1997 and 2008 at selected locations in the 

Gulf Intracoastal Waterway (GIWW) and adjacent surface-water bodies. Site numbers in parentheses.
Discharge, Suspended Sediment, and Salinity

17

0

2

4

6

8

TE28-07R (21)

0

2

4

6

8

Caillou Bay (23)

0

5

10

15

20

25

30

35

Caillou Lake (22)

0

5

10

15

20

25

30

0

2

4

6

8

Barataria Bay (26)

0

5

10

15

20

25

30

0

5

10

15

20

Barataria Pass (27)

0

5

10

15

20

25

30

35

TV04-04R (20)

VERMILION-TECHE BASIN TERREBONNE BASIN BARATARIA BASIN

0

5

10

15

20

Average (1997–2008) 

Sustained high water (1993) 

Low water (2000)

Peak flood (2008)

Vermilion Bay (19)

EXPLANATION

Little Lake (25)

Salinity, in parts per thousand

Lake Salvador (24)

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

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

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

Month

Month

Month Month

Figure 8. Monthly average salinity at selected stations in south-central Louisiana under average, sustained high water, low water, and 

peak flood conditions of the Atchafalaya River. Not all stations were operational in 1993; months with more than 50 percent missing data 

are not shown.
18

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

the boundary between Barataria Basin and the Gulf of Mexico, 

at Barataria Pass (fig. 1, site 27), salinity remained around 30 

ppt throughout the low water year 2000, 5 ppt or more above 

the long-term average for all months except for November and 

December. During sustained high water and peak flood years, 

salinity decreased below long-term averages. Salinity in the 

fall was higher during 2008 than during the year with sustained 

high water (fig. 8).

Barataria Basin was the only intradistributary basin in 

which salinity became substantially elevated in the upper 

reaches during the low water year (2000) (fig. 9). This occurred 

before the Davis Pond Freshwater Diversion Structure was 

operational. The capability of introducing river water into 

Barataria Basin through the Davis Pond Freshwater Diversion 

Structure, which commenced operations in 2004, has reduced 

the vulnerability of the low-salinity marshes here to elevated 

salinity during low water years. In 2006, a year comparable 

to 2000 with similar low water conditions and precipitation 

for much of the year (table 2), the Davis Pond Freshwater 

Diversion Structure was operating, and the influence of 

this added source of freshwater in moderating salinity was 

readily apparent. The diversion inflow in 2006 substantially 

lowered ambient salinity in the upper reaches near Lake 

Salvador compared with 2000 when the diversion was not yet 

operational (fig. 9).

Implications for Coastal Louisiana

The salinity gradient dominates the biology and ecology of 

estuaries, which are typically highly productive environments 

(Odum, 1988). Differential salt tolerance arranges plant 

communities and controls the occurrence and distribution of 

organisms, including fish, at all levels (Telesh and Khlebovich, 

2010; Whitfield and others, 2012). Biogeochemical processing 

of detrital material and the structure and dynamics of food 

webs also are controlled by the salinity gradient (Odum, 

1988). By moving marine saltwater and fresh river water 

around basins in artificial ways, the GIWW influences natural 

processes, potentially displaces organisms, and adds yet more 

complexity to an already fragile and complicated system. 

The altered hydrology enables the GIWW to introduce 

fresh river water to parts of the Mississippi River Delta that 

cannot be reached any other way. Freshwater moderates 

salinity; this has intrinsic value in low-salinity reaches of 

estuaries, provided that the water can be delivered when it 

is most needed, for example during drought conditions or 

during the fall when salinity tends to peak in coastal waters. 

River water also is highly valued for potential nourishment 

and restoration of degraded marshes (Nyman, 2014). Some 

projects have been implemented and additional ones are being 

proposed to take advantage of the discharge in the GIWW; 

the apparent nourishing property of river water underlies the 

use of controlled river diversions off the Mississippi River in 

the Coastal Master Plan for restoration of the delta wetlands 

(Coastal Protection and Restoration Authority, 2015). 

Figure 9. Monthly average salinity in parts per thousand at 

selected stations in Barataria Basin under average and low water 

conditions of the Atchafalaya River. The Davis Pond Freshwater 

Diversion Structure was not operational in 2000.

Low water prior to Davis Pond Freshwater Diversion Structure (2000) 

Low water with Davis Pond Freshwater Diversion Structure (2006)

Average (1997–2008) 

EXPLANATION

0

2

4

6

8

Lake Salvador (24)

0

5

10

15

20

25

30

Barataria Bay (26)

0

5

10

15

20

Little Lake (25)

0

5

10

15

20

25

30

35

Barataria Pass (27)

Salinity, in parts per thousand

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

Month
Summary

19

The introduction of river water into marshes is not 

without controversy, however, and the processes by which 

river water is assumed to benefit marshes are not well 

understood (Turner, 2009). Since the early 1950s, the 

Mississippi River carries high nutrient and herbicide loads 

from midcontinent agricultural runoff during the spring 

flood (Turner and Rabalais, 2003). Recent research has 

shown that coastal eutrophication and chronic exposure to 

elevated nitrogen may contribute to degradation of salt and 

fresh marsh soils (Swarzenski and others, 2008; Turner, 

2011; Deegan and others, 2012; Valiela, 2015). These studies 

contrast with other efforts, many of which are summarized 

in Morris and others (2013) and suggest that nutrients are 

not harmful to marshes. The issue is difficult to research 

in controlled experimental settings because both beneficial 

and adverse marsh responses might take years to manifest 

themselves. In this instance too, the river water moving 

in the GIWW can serve a valuable purpose. The GIWW 

has been discharging river water into wetlands for decades 

and provides a quasi-natural setting in which to evaluate 

wetland responses to long-term influx of nutrient-rich waters 

(Swarzenski and others, 2008). The newly built diversions 

have only been operating for short periods, and outcomes 

may not be understood for many years. Careful study of 

the surface-water hydrology of the GIWW and the nearby 

marshes receiving long-term inflow of this river water 

could yield helpful clues about the use of river water and 

controlled diversions for coastal restoration not attainable 

through other ways. 

Understanding flows in the GIWW and knowing 

when the GIWW acts as a source of water to surrounding 

surface waters and wetlands or as a sink receiving water 

from adjacent wetlands help in assessing estuarine 

structure and function. This understanding also is important 

to guide wetland restoration activities. Computed discharge 

data at both ends of discrete segments of the GIWW are 

needed to achieve this type of understanding. How this 

might work is illustrated for the segment between the HNC 

and Bayou Lafourche, the only segment along the GIWW 

that has gages continuously computing discharge at both 

ends. In March, April, and May, about 500 ft3

/s discharged 

from the GIWW into the surrounding wetlands under longterm average conditions (table 7). During the peak flood 

year, this amount exceeded 1,100 ft3/s both in April and in 

May. Under long-term average conditions in January and 

February, water flowed from the surrounding wetlands 

into the GIWW along this segment (table 7). It was this 

inflow that maintained the prevailing eastward hydraulic 

gradient.

Similar analyses are possible elsewhere along the 

GIWW if additional gages measuring continuous velocity 

and stage are installed at the beginning and ends of discrete 

segments. Such information is required to make optimal use 

of freshwater flowing in the GIWW for wetland restoration 

projects. 

Summary

Analysis of over 10 years of synoptic and continuous 

discharge measurements showed that the Gulf Intracoastal 

Waterway (GIWW) distributes river water and suspended 

sediments to intradistributary basins and associated coastal 

wetlands in south-central Louisiana at least 30 and 50 miles 

west and east, respectively, of Morgan City. The flow is 

passive and follows natural hydraulic gradients between water 

levels of the source waters, the Lower Atchafalaya River 

(LAR) and Wax Lake Outlet (WLO), and water levels in the 

GIWW and surrounding surface waters. The magnitude of the 

discharge in the GIWW increased as stages of the LAR and 

WLO increased. Discharge decreased with increasing distance 

from the LAR and WLO. In general, discharge increases 

steadily as stage of source waters increases; discharge at many 

locations in the GIWW becomes predictable when stage of 

the LAR at Morgan City (site 8) is between 3 and 4 feet (ft) 

referenced to the North American Vertical Datum of 1988 or 

higher. For some parts of coastal Louisiana too far away from 

the Mississippi River, the GIWW is the only means of getting 

river water to adjacent surface waters.

When stage of the LAR at Morgan City (site 8) was 

above 5.5 ft, an average of 31,500 cubic feet per second (ft3

/s) 

entered the GIWW at mile 99 south of Morgan City (site 7). 

Of this inflow, on average 11,400 ft3/s reached the WLO; the 

balance flowed south towards the Gulf of Mexico. At the same 

stage, discharge averaged about 18,000 ft3/s in the GIWW at 

the Bayou Sale Ridge near Franklin (site 4) and 14,500 ft3/s in 

the GIWW east of Jaws Bay west of Franklin (site 3). Much 

of this water exited into Jaws Bay, a shallow bay. Only about 

4,000 ft3/s continued west in the GIWW west of Jaws Bay 

near Cypremort (site 2) when stage of the LAR at Morgan 

City (site 8) was above 5.5 ft. East of Morgan City, discharge 

averaged 23,000 ft3/s at the Avoca Island Cutoff south of 

Morgan City (site 9) when stage of the LAR at Morgan City 

(site 8) was greater than 5.5 ft. Under these conditions, about 

10,000 ft3/s of water flowed in the GIWW west of Houma 

Navigation Canal (HNC) at Houma (site 13), and only about 

3,600 ft3

/s continued in the GIWW towards Bayou Lafourche. 

When the LAR stage was high, the HNC captured substantial 

amounts of water flowing east in the GIWW. On average, 

about 2,700 ft3/s flowed east in the GIWW east of Bayou 

Lafourche at Larose (site 16) and into Barataria Basin when 

stage of the LAR at Morgan City exceeded 4 ft. When stage 

of the LAR at Morgan City (site 8) was below 3 ft, on average 

more than one-third of the discharge in the GIWW near Bay 

Wallace east of Morgan City (site 12) came from Bayou 

Boeuf. During years when LAR flow was low, Bayou Boeuf, 

draining the Verret subbasin, became a major source of water 

maintaining eastward flow in the GIWW. 

The amount of river water flowing along the GIWW was 

shown to vary considerably from year to year depending on 

the hydrograph of the LAR. The annual discharge of water 

in the GIWW was greatest during a year with sustained 
20

Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters

high water (1993). The above average discharge during the 

second half of the year, when stage of the LAR at Morgan 

City (site 8) remained anomalously high, provided the main 

difference, but from January to June overall flow was greater 

in 1993 than flow during the peak flood of 2008; for example, 

the average discharge at the GIWW at Bayou Sale Ridge near 

Franklin (site 4) from January to June 1993 was 15,500 ft3/s, 

about 2,000 ft3/s more than for the peak flood year of 2008 and 

over 10,000 ft3/s more than during the drought year of 2006 

for the same time interval. For the second half of 1993, the 

average discharge at this same site was 12,100 ft3/s, compared 

with about 7,000 ft3/s in 2008 and 4,000 ft3/s for the same 

time period in 2006. Conversely, discharge in the GIWW 

was noticeably below average for the first 6 months of a year 

when flow of the LAR remained low for most of the year, as 

occurred in 2006, but was not that different during the second 

half of the year.

The length of time stage of the LAR at Morgan City 

(site 8) exceeding a given elevation increased from the 

1940s to 2008, most dramatically during the 1970s. This 

shift increases the amount of time the GIWW functions as a 

predictable distributary of river water. Similar shifts in the 

future could be expected to increase the duration and amounts 

of river water reaching coastal Louisiana wetlands through the 

GIWW. 

Median suspended-sediment concentrations in the GIWW 

to the west of Morgan City were about 160 milligrams per 

liter (mg/L). In the GIWW east of Morgan City, median 

concentrations were 120–160 mg/L, except in Bayou Boeuf 

at Railroad Bridge in Amelia and the parts of the GIWW 

between Bayou Boeuf and the HNC; median concentrations 

here were around 100 mg/L. Compared to the GIWW between 

Bayou Boeuf and the HNC, the higher discharge in the GIWW 

between the LAR and Jaws Bay, combined with the 25-percent 

higher average suspended-sediment concentrations, make 

this stretch potentially more useful for delivery of suspended 

sediments for wetland restoration.

The variability of freshwater inflow to the Louisiana 

coast from the Mississippi and Atchafalaya Rivers caused 

large year-to-year fluctuations in local estuarine salinity 

regimes in many parts of south-central coastal Louisiana. 

These fluctuations demonstrate that river water penetrates 

much of the Louisiana coast. The contributing role of 

freshwater inflow through the GIWW and controlled river 

diversions was evident in the intradistributary basins west and 

east of Morgan City; for example, salinity in surface waters 

just south of the GIWW between Bayou Boeuf and the HNC 

remained low even during a year with prolonged low water 

(2000). This was not the case farther east in Barataria Basin, 

where only controlled inflow from the Davis Pond Freshwater 

Diversion Structure could moderate salinity in the upper 

reaches of the basin during a low water year. The GIWW plays 

an important role in moderating salinity in intradistributary 

basins.

References Cited

Coastal Protection and Restoration Authority, 2014, 

Miscellaneous hydrologic data: Accessed December 

20, 2014, at http://cims.coastal.la.gov/DataDownload/

DataDownload.aspx?type=hydro_hourly.

Coastal Protection and Restoration Authority, 2015, 2012 

Coastal master plan: Accessed January 20, 2015, at 

http://coastal.la.gov/a-common-vision/2012-coastal-masterplan/.

Deegan, L.A., Johnson, D.S., Warren, R.S., Peterson, B.J., 

Fleeger, J.W., Fagherazzi, Sergio, and Wollheim, W.M., 

2012, Coastal eutrophication as a driver of salt marsh loss: 

Nature, v. 490, p. 388–392. 

Edwards, T.K., and Glysson, G.D., 1999, Field methods for 

measurement of fluvial sediment: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. C2, 89 p.

Morlock, S.E., Nguyen, N.T., and Ross, J.H., 2001, Feasibility 

of acoustic Doppler velocity meters for the production of 

discharge records from U.S. Geological Survey streamflowgaging stations: U.S. Geological Survey Water-Resources 

Investigations Report 01–4157, 56 p.

Morris, J.T., Shaffer, G.P., and Nyamn, J.A., 2013, Brinson 

review—Perspectives on the influence of nutrients on 

the sustainability of coastal wetlands: Wetlands, v. 33, 

p. 975–988.

National Oceanic and Atmospheric Administration, National 

Climatic Data Center, 2009, Climate data online: Accessed 

January 12, 2009, at http://www7.ncdc.noaa.gov/CDO/

CDODivisionalSelect.jsp. 

Nyman, R.A., 2014, Integrating successional ecology and 

the delta lobe cycle in wetland research and restoration: 

Estuarines and Coasts, v. 37, p. 1490–1505.

Odum, W.E., 1988, Comparative ecology of tidal freshwater 

and salt marshes: Annual Review of Ecology and 

Systematics, v. 19, p. 147–176.

Paille, Ronald, 1997, Lower Atchafalaya Basin Reevaluation 

Study—Planning aid report on freshwater inflows to the 

Terrebonne Basin—Report submitted to the New Orleans 

District, U.S. Army Corps of Engineers: Lafayette, 

Louisiana, U.S. Fish and Wildlife Service, Ecological 

Services, 28 p.

Simpson, M.R., and Oltmann, R.N., 1993, Dischargemeasurement system using an Acoustic Doppler Current 

Profiler with applications to large rivers and estuaries: U.S. 

Geological Survey Water-Supply Paper 2395, 32 p.
References Cited

21

Swarzenski, C.M., 2003, Surface-water hydrology of the Gulf 

Intracoastal Waterway in south-central Louisiana, 1996–99: 

U.S. Geological Survey Professional Paper 1672, 51 p. 

Swarzenski, C.M., Doyle, T.W., Fry, Brian, and Hargis, T.G., 

2008, Biogeochemical response of organic-rich freshwater 

marshes in the Louisiana delta plain to chronic river water 

influx: Biogeochemistry, v. 90, p. 49–63. 

Telesh, I.V., and Khlebovich, V.V., 2010, Principal processes 

within the estuarine salinity gradient—A review: Marine 

Pollution Bulletin 61, p. 149–155. 

Turner, R.E., 2009, Doubt and the values of an ignorancebased world view for restoration—Coastal Louisiana 

wetlands: Estuaries and Coasts, v. 32, p. 1054–1068.

Turner, R.E., 2011, Beneath the salt marsh canopy—Loss of 

soil strength with increasing nutrient loads: Estuaries and 

Coasts, v. 34, p. 1084–1093.

Turner, R.E., and Rabalais, N.N., 2003, Linking landscape and 

water quality in the Mississippi River Basin for 200 years: 

BioScience, v. 53, p. 563–572.

U.S. Army Corps of Engineers, 2009, Stage data for the 

Atchafalaya River at Simmesport and for the Lower 

Atchafalaya River at Morgan City, 1940–2008: Accessed 

January 12, 2009, at http://rivergages.mvr.usace.army.mil/

WaterControl/search2.cfm. 

Valiela, V., 2015, The Great Sippewissett salt marsh plots—

Some history, highlights, and contrails from a long-term 

study: Estuaries and Coasts, v. 38, p. 1099–1120.

Wells, F.C., and Demas, C.R., 1977, Hydrology and water 

quality of the Atchafalaya River Basin: Louisiana 

Department of Transportation and Development, Office of 

Public Works Water Resources Technical Report no. 14, 

53 p. 

Whitfield, A.K., Elliot, M., Basset, A., Blaber, S.J.M., and 

West, R.J., 2012, Paradigms in estuarine ecology—A review 

of the Remane diagram with a suggested revised model 

for estuaries: Estuarine, Coastal and Shelf Science, v. 97, 

p. 78–90.

Publishing support provided by

Lafayette Publishing Service Center
Swarzenski and Perrien—Discharge, Suspended Sediment, and Salinity in the Gulf Intracoastal Waterway and Adjacent Surface Waters—SIR 2015–5132

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155132

I SBN 978- 1- 4113-3972- 9

9 7 8 1 4 11 339729