SURFACE-WATER HYDROLOGY OF THE

GULF INTRACOASTAL WATERWAY IN

SOUTH-CENTRAL LOUISIANA, 1996-99

SURFACE-WATER HYDROLOGY OF THE

GULF INTRACOASTAL WATERWAY IN

SOUTH-CENTRAL LOUISIANA, 1996-99

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1672

Prepared in cooperation with the

U.S. ARMY CORPS OF ENGINEERS

NEW ORLEANS DISTRICT

U.S. Department of the Interior

U.S. Geological Survey
Cover photographs

Clockwise from top left: Bayou Penchant south of Morgan City showing water from the Avoca 

Island Cutoff Channel; water draining from the Gulf Intracoastal Waterway into adjacent western 

Terrebonne marshes; healthy freshwater marshes dominated by Panicum hemitomon, degraded 

western Terrebonne marshes. 

Background: Gulf Intracoastal Waterway near Bay Wallace east of Morgan City. 

(Photographs by author)
SURFACE-WATER HYDROLOGY OF THE 

GULF INTRACOASTAL WATERWAY IN 

SOUTH-CENTRAL LOUISIANA, 1996-99 

By Christopher M. Swarzenski

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1672

Prepared in cooperation with the 

U.S. ARMY CORPS OF ENGINEERS

NEW ORLEANS DISTRICT 
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this report is for descriptive purposes 

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

Reston, Virginia 2003

For sale by the

U.S. Geological Survey

Branch of Information Services

Box 25286, Federal Center

Denver, CO 80225

Telephone: 1-888-ASK-USGS

Library of Congress Cataloging-in-Publication Data

Swarzenski, Christopher M., 1958-

Surface-water hydrology of the Gulf Intracoastal Waterway in

South-Central Louisiana, 1996-99 / by Christopher M. Swarzenski ;

prepared in cooperation with the U.S. Army Corps of Engineers, New

Orleans District.

 p. cm. -- (U.S. Geological Survey professional paper ; 1672)

Includes bibliographical references (p. ).

 ISBN 0-607-90626-X

1. Hydrology--Louisiana--Atchafalaya River Region. 2. Hydrology--Gulf

Intracoastal Waterway. 3. Salinity--Gulf Intracoastal Waterway. I.

United States Army Corps of Engineers, New Orleans District II.

Geological Survey (U.S.) III. Title. IV. Series.

GB705.L8S93 2003

551.48'09763--dc22

 2003049291
 Contents iii

CONTENTS

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

Introduction ................................................................................................................................................ 2

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

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

The Gulf Intracoastal Waterway ......................................................................................... 4

Physiography of Coastal Louisiana .................................................................................... 4

Major Rivers ....................................................................................................................... 5

Approach and Methods................................................................................................................. 5

Discharge and Suspended Sediment ................................................................................... 5

Stage and Water-Level Gradients ....................................................................................... 7

Acknowledgments ....................................................................................................................... 7

Hydrologic Conditions During Study ........................................................................................................ 7

Surface-Water Hydrology .......................................................................................................................... 9

Flow Characteristics ..................................................................................................................... 9

Discharge in Relation to Stage of the Lower Atchafalaya River and Precipitation ............ 14

Between Lower Atchafalaya River and Houma Navigation Canal ............................ 14

East of Houma Navigation Canal ............................................................................... 17

Between Lower Atchafalaya River and Wax Lake Outlet ......................................... 17

West of the Wax Lake Outlet ...................................................................................... 17

Discharge in the Gulf Intracoastal Waterway from 1997-99 .............................................. 19

Flow Reversals .................................................................................................................... 23

Net Gain or Loss of Water Along Major Segments ............................................................ 25

Magnitude, Duration, and Timing of Flow ......................................................................... 29

Stage and Water-Level Gradients ................................................................................................ 32

Selected Water-Quality Constituents ........................................................................................... 36

Suspended Sediment ........................................................................................................... 36

Salinity .................................................................................................................................38

Summary and Conclusions .........................................................................................................................44

Selected References .................................................................................................................................... 46

Appendix: Regression Equations ............................................................................................................... 49

PLATE (in pocket)

1. Map showing surface-water data-collection sites, and graphs showing instantaneous discharge and daily-average salinity at selected sites in relation to daily-average stage of the 

Lower Atchafalaya River at Morgan City, south-central Louisiana, October 1996 to 

December 1999

FIGURES

1. Map of coastal Louisiana showing the Mississippi River, Atchafalaya River, and the Gulf

Intracoastal Waterway..................................................................................................................... 3

2-8. Graphs showing:

 2. Instantaneous discharge of the Lower Atchafalaya River at Morgan City and Wax Lake 

Outlet at Calumet, and daily-average stage of the Lower Atchafalaya River at Morgan 

City, Louisiana, October 1996 to December 1999 .................................................................... 8
iv Contents

 3. Monthly total precipitation and departure from normal at Morgan City and Houma,

Louisiana, and Palmer Hydrological Drought Index for the Southeast and South-Central 

Climatological Divisions in Louisiana, October 1996 to December 1999 ................................ 9

 4. Daily resultant wind speed and direction at New Orleans, Louisiana, daily-average 

stage at selected sites in coastal Louisiana, and daily total precipitation at Morgan 

City, Louisiana, from September 1 to November 30, 1997 and 1998........................................10

 5. Daily-average discharge and stage at Bayou Penchant south of Morgan City, Bayou 

Boeuf at railroad bridge at Amelia, and Gulf Intracoastal Waterway at Cypremort, 

Louisiana, October 1996 to December 1999 .............................................................................13

 6. Daily-average stage of the Lower Atchafalaya River at Morgan City and three-hour 

average velocity of Bayou L’Eau Bleu, Louisiana, May to December 1996.............................14

 7. Monthly-average stage of the Lower Atchafalaya River at Morgan City and Bayou 

Boeuf at railroad bridge at Amelia, monthly total precipitation at Morgan City, and 

monthly-average discharge at Bayou Boeuf at railroad bridge at Amelia, Louisiana, 

October 1996 to December 1999 ...............................................................................................15

 8. Monthly-average stage of the Lower Atchafalaya River at Morgan City and Bayou 

Penchant south of Morgan City, monthly total precipitation at Morgan City, 

and monthly-average discharge of Bayou Penchant south of Morgan City, Louisiana, 

October 1996 to December 1999 ...............................................................................................16

9. Map showing instantaneous discharge at sites between the Lower Atchafalaya River 

and Houma Navigation Canal, Louisiana, April, June, and July 1999...........................................18

10. Graphs showing monthly-average stage of the Lower Atchafalaya River at Morgan City 

and the Gulf Intracoastal Waterway at Cypremort, monthly total precipitation at Morgan 

City, and monthly-average discharge in the Gulf Intracoastal Waterway at Cypremort, 

Louisiana, October 1996 to December 1999 .................................................................................. 20

11. Graphs and maps showing daily-average salinity in the Gulf Intracoastal Waterway east

of Houma Navigation Canal at Houma, east of Larose, and in Little Lake near Cut Off, 

and daily-average velocity and stage of the Houma Navigation Canal at Dulac in 

September and October 1997; synoptic surveys of salinity in the Gulf Intracoastal 

Waterway and associated surface waters in Louisiana, October 9, 14, and 17, 1997 .................... 24

12-25. Graphs showing:

12. Relation between net inflow (positive) or outflow (negative) of water in three 

segments of the Gulf Intracoastal Waterway (GIWW) east of Morgan City and

instantaneous discharge in the GIWW near Bay Wallace east of Morgan City and 

the 21-day average discharge at Bayou Boeuf at railroad bridge at Amelia, 

Louisiana, 1997-99 .................................................................................................................. 26

13. Relation between net inflow (positive) or outflow (negative) of water in three 

segments of the Gulf Intracoastal Waterway (GIWW) west of the Wax Lake Outlet

and instantaneous discharge in the GIWW west of Wax Lake Outlet south of 

Calumet and the 21-day average discharge at Bayou Boeuf at railroad bridge at 

Amelia, Louisiana, 1997-99 .................................................................................................... 27

14. Daily-average stage of the Lower Atchafalaya River at Morgan City, Louisiana, 

for characteristic flows of the Lower Atchafalaya River......................................................... 29

15. Percent of time in a year discharge at selected sites is equaled or exceeded for characteristic flows of the Lower Atchafalaya River, Louisiana ........................................................... 30

16. Relation between instantaneous discharge in the Gulf Intracoastal Waterway 

west of Houma Navigation Canal at Houma and same-day average stage of 

the Lower Atchafalaya River at Morgan City, Louisiana, in 1983-84 and 1997-98 ............... 31

17. Average monthly salinity at Caillou Lake (Sister Lake) southwest of Dulac, Louisiana, 

from 1972-77 and from 1995-99 ............................................................................................. 31

18. Average monthly salinity in the Gulf Intracoastal Waterway at the intersections with 

Houma Navigation Canal and Bayou Lafourche at Larose, Louisiana, from 1957-60, 

1972-77, and 1983-88.............................................................................................................. 32
 Contents v

19. Monthly-average stage for the Lower Atchafalaya River at Morgan City, Avoca Island 

Cutoff Channel south of Morgan City, Bayou Penchant south of Morgan City, 

Bayou Boeuf at railroad bridge at Amelia, and in the Gulf Intracoastal Waterway 

(GIWW) near Bay Wallace east of Morgan City and east of Houma Navigation Canal at 

Houma, Louisiana, October 1996 to December 1999............................................................. 33

20. Relation between monthly-average stage at selected sites east of Morgan City and 

monthly-average stage of the Lower Atchafalaya River at Morgan City and Houma 

Navigation Canal at Dulac; and between monthly-average stage at selected sites on 

the Gulf Intracoastal Waterway west of Wax Lake Outlet and monthly-average stage 

of Wax Lake Outlet at Calumet and Vermilion Bay near Cypremort Point, Louisiana, 

October 1996 to December 1999............................................................................................. 34

21. Monthly-average stage for the Lower Atchafalaya River at Morgan City, the 

Houma Navigation Canal at Dulac, Company Canal at Highway 1 at Lockport, 

and in the Gulf Intracoastal Waterway (GIWW) east of Houma Navigation Canal 

at Houma, east of Larose, and at the intersection with Bayou Perot, Louisiana, 

October 1996 to December 1999............................................................................................. 35

22. Monthly-average stage for the Wax Lake Outlet at Calumet, Vermilion Bay near 

Cypremort Point, and in the Gulf Intracoastal Waterway (GIWW) west of Wax 

Lake Outlet south of Calumet, east of Jaws Bay west of Franklin, and at 

Cypremort, Louisiana, October 1996 to December 1999 ....................................................... 35

23. Relation between monthly-average salinity at selected sites in south-central 

Louisiana and monthly total precipitation at Morgan City and monthly-average 

stage of Lower Atchafalaya River at Morgan City, October 1996 to December 1999 ........... 41

24. Cumulative monthly precipitation by year (1996-99) and average (1961-90) for 

the South-Central Climatological Division in Louisiana ........................................................ 42

25. Relation between monthly-average salinity at eight sites in coastal Louisiana and 

monthly-average stage of Lower Atchafalaya River at Morgan City and Houma 

Navigation Canal at Dulac, October 1996 to December 1999................................................ 43

TABLES

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

(GIWW) and adjacent surface-water bodies, south-central Louisiana............................................ 6

2. Summary of instantaneous discharge measurements made at selected sites along or near 

the Gulf Intracoastal Waterway (GIWW) in south-central Louisiana, 1997-99.............................. 11

3. Supplemental instantaneous discharge measurements made at the Gulf Intracoastal 

Waterway (GIWW) west of Bayou Perot, in Bayou Perot south of the Gulf Intracoastal 

Waterway, and in the Houma Navigation Canal at Dulac, south-central Louisiana, 1997-99......... 12

4. Monthly-average discharge calculated from regression equations and from stage-discharge 

relations for sites along or near the Gulf Intracoastal Waterway (GIWW) east of the Lower 

Atchafalaya River, 1997-99............................................................................................................. 21

5. Monthly-average discharge calculated from regression equations and from stage-discharge 

relations for sites along the Gulf Intracoastal Waterway (GIWW) west of the Lower 

Atchafalaya River, 1997-99............................................................................................................. 22

6. Suspended-sediment concentrations and discharge at selected sites along or near the Gulf 

Intracoastal Waterway (GIWW) in south-central Louisiana, 1997-99............................................ 37

7. Correlation matrix for monthly-average salinity at selected sites, monthly-average stage of the 

Lower Atchafalaya River at Morgan City, Louisiana, monthly total precipitation at 

Morgan City, and Palmer Hydrological Drought Index................................................................... 40
vi Contents

CONVERSION FACTORS, DATUMS, AND ABBREVIATED 

WATER-QUALITY UNITS

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

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

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

tons per day (tons/d) 0.9072 megagram per day (Mg/d)

Horizontal coordinate information in this report is referenced to the North American Datum of 1927. 

Vertical coordinate information in this report is referenced to the North American Vertical Datum of 1988 (NAVD88) and the 

National Geodetic Vertical Datum of 1929 (NGVD29)--a geodetic datum derived from a general adjustment of the first-order level 

nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Abbreviated water-quality units:

milligrams per liter (mg/L)

parts per thousand (ppt)
ABSTRACT 1

ABSTRACT

The flow of freshwater and suspended sediment from the Lower Atchafalaya River (LAR) 

and Wax Lake Outlet (WLO) into and along the 

Gulf Intracoastal Waterway (GIWW) and selected 

adjacent surface-water bodies between Cypremort 

and Larose in south-central Louisiana, from October 1996 to December 1999, was characterized 

using instantaneous and computed continuous discharge measurements and measurements of suspended-sediment concentrations. The GIWW 

parallels the entire Louisiana coast near the wetland/upland interface. Following natural hydraulic 

gradients, the GIWW captures water and sediment 

from the southward flowing LAR and the WLO 

where it crosses those waterways, and distributes 

this freshwater and sediment to points east and 

west.

East of Morgan City, La., an average of 

12,200 ft3

/s (cubic feet per second) of water flowed 

from the LAR into the Avoca Island Cutoff Channel. The LAR was the primary source of water to 

the GIWW east of Morgan City. Drainage from the 

Verret Subbasin through Bayou Boeuf contributed 

an average of 1,000 ft3/s to the eastward flow in the 

GIWW. Eastward flow in the GIWW near Bay 

Wallace east of Morgan City and to the west of the 

Houma Navigation Canal (HNC) at Houma, La., 

averaged about 5,700 ft3

/s. Average flow in the 

GIWW east of the HNC at Houma was 2,610 ft3

/s 

to the east, and 2,200 ft3

/s east of Bayou Lafourche 

at Larose, also to the east.

Measured discharge in the GIWW was 

always to the west between the LAR and WLO. 

Water entered this stretch of the GIWW from the 

LAR. The WLO was the primary source of water 

to the GIWW west of WLO. Discharge in the 

GIWW averaged 9,460 ft3

/s west of WLO south of 

Calumet and 8,230 ft3

/s east of Jaws Bay west of 

Franklin. Average discharge in the GIWW west of 

Jaws Bay near Cypremort was 3,310 ft3/s and at 

Cypremort was 1,350 ft3

/s. Average discharge was 

to the west at all four locations, but discharge as 

high as 2,830 ft3/s was measured flowing eastward 

toward Jaws Bay in the GIWW at Cypremort.

In bayous and canals in most of coastal Louisiana, including the GIWW, stage narrowly fluctuates around the Gulf of Mexico level. Where the 

GIWW crosses the LAR and WLO, stage can reach 

3 ft (feet) or more above the North American Vertical Datum of 1988 (NAVD88). Flow in the GIWW 

results from these differences in stage. When the 

LAR at Morgan City reached 3 to 4 ft above 

NAVD88, flow in the GIWW became more predictable. Discharge at most sites between the HNC 

and Jaws Bay increased in varying amounts as 

stage of the LAR at Morgan City increased beyond 

3 ft above NAVD88. At sites in the GIWW east of 

HNC, discharge did not increase predictably. For 

all measurements made when the LAR at Morgan 

City was 3 ft or more above NAVD88, average discharge was about 3,100 ft3/s in the GIWW east of 

HNC at Houma and 2,880 ft3/s east of Bayou 

Lafourche at Larose. The LAR at Morgan City is 

3 ft or more above NAVD88 for about 7 months in 

a normal year. 

When the LAR at Morgan City was less than 

3 ft above NAVD88, water in the GIWW flowed 

along the prevailing water-level gradients, to the 

east between Bay Wallace and the HNC and to the 

west between WLO and Jaws Bay. However, local 

runoff and drainage from areas adjacent to the 

GIWW became more significant in maintaining 

flow at low LAR stage. Discharge was consistently higher in the GIWW west of the HNC at 

Houma than farther west near Bay Wallace east of 

Morgan City, when the LAR at Morgan City was 

SURFACE-WATER HYDROLOGY OF THE 

GULF INTRACOASTAL WATERWAY IN 

SOUTH-CENTRAL LOUISIANA, 1996-99

By Christopher M. Swarzenski
2 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

less than 3 ft above NAVD88. Westward flow in 

the GIWW between Houma and Morgan City was 

observed near Bay Wallace east of Morgan City but 

was never observed west of the HNC at Houma. 

Discharge in the GIWW east of Jaws Bay west of 

Franklin, La., frequently was higher than in the 

GIWW west of WLO south of Calumet, La., at low 

LAR stage.

East of the LAR, suspended-sediment concentrations averaged about 162 mg/L (milligrams 

per liter) at the two sites closest to the LAR, Avoca 

Island Cutoff Channel and Bayou Penchant south 

of Morgan City. Suspended-sediment concentrations averaged 91 mg/L in the water entering the 

GIWW through Bayou Boeuf at railroad bridge at 

Amelia. The water in the GIWW near Bay Wallace 

east of Morgan City to the HNC was intermediate 

in suspended-sediment concentrations (108-

114 mg/L), reflecting the mixture of the two water 

types. Suspended-sediment concentrations in the 

GIWW near Bay Wallace east of Morgan City and 

east and west of the HNC were comparable, but in 

the GIWW east of Larose, they increased by about 

45 percent. To the west of WLO, suspended-sediment concentrations averaged between 167 and 

191 mg/L. 

Freshwater flow directly affected salinity 

patterns in the GIWW and adjacent surface-water 

bodies. Monthly-average salinity at many sites, 

both in and adjacent to the GIWW, was related to 

monthly-average stage of the LAR at Morgan City. 

When the LAR at Morgan City is 3 or more 

ft above NAVD88, the GIWW effectively distributes freshwater and sediment from the LAR and 

WLO to points in coastal Louisiana 30 to 50 miles 

east and west of Morgan City. The freshwater and 

sediment, some of which originate indirectly from 

the Mississippi River, are valuable resources for 

building wetlands and could be incorporated into 

ongoing efforts to restore coastal Louisiana.

INTRODUCTION

Regional and local modifications to surfacewater hydrology in coastal Louisiana have disrupted the natural inflow of freshwater and sediment to wetlands for many years (Mendelssohn 

and others, 1984; Turner and Cahoon, 1988). The 

modifications have played a major role in the rapid 

conversion of land to open water that has characterized much of coastal Louisiana for the past 50 or 

more years (Gagliano and others, 1981; Turner and 

Cahoon, 1988; Boesch and others, 1994; Reed, 

1995). The two largest modifications to the flow of 

freshwater and sediment into and through coastal 

Louisiana wetlands have been the leveeing of the 

Mississippi River and the construction of the Gulf 

Intracoastal Waterway (GIWW) in the early part of 

the 20th century. The confinement of the Mississippi River between levees in much of southeastern 

Louisiana has reduced and in places eliminated the 

frequent over-bank flooding that historically 

brought freshwater and sediment to coastal wetlands. The GIWW has modified surface-water 

hydrology in coastal Louisiana in ways that presently are not well understood.

The GIWW is the largest continuous ship 

channel in coastal Louisiana, traversing the entire 

Louisiana coast along the wetland/upland interface 

(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 to parts of Louisiana's coastal wetlands during the annual spring flood (Paille, 1997). 

This flow potentially could support coastal restoration efforts with a minimum of cost (R.F. Paille, 

U.S. Fish and Wildlife Service, oral commun., 

1999). Distinct from its potential for ecological 

benefits, the GIWW also provides an exit point for 

flood waters carried through the Atchafalaya River 

Basin. However, the flow of freshwater and transport of suspended sediment in the GIWW and the 

relation of these activities to stage of the LAR are 

not well understood.

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

began to operate a network of data-collection sites 

that monitor velocity, stage, and salinity in the 

GIWW and several connecting waterways. This 

effort was initiated in cooperation with Federal 

agencies concerned with flood control and environmental quality, principally the U.S. Army Corps of 

Engineers (USACE), but also including the U.S. 

Fish and Wildlife Service (USFWS), Natural 

Resources Conservation Service, the U.S. Environmental Protection Agency, and the Coastal Wetlands Protection, Planning, and Restoration Act 

task force. From 1997-99 the primary data-collec-
 INTRODUCTION 3

Figure 1. Coastal Louisiana showing the Mississippi River, Atchafalaya River, and the Gulf 

Intracoastal Waterway.

tion effort focused on the GIWW between Cypremort, La., to the west, and Larose, La., to the east 

(pl. 1). Occasionally, measurements east of Larose 

were made in Bayou Perot and in the GIWW just 

west of Bayou Perot. In 1998, the USGS, in cooperation with the USACE, began a study to evaluate 

the collected data and describe hydrologic conditions in the GIWW in south-central Louisiana. 

Purpose and Scope

This report characterizes the flow of water 

and suspended sediment from the LAR and WLO 

into and along the GIWW and in selected adjacent 

surface-water bodies between Cypremort and 

Larose from October 1996 to December 1999 

(pl. 1). The relative importance of the two primary sources of water sustaining flow along the 

GIWW, the Atchafalaya River (LAR and WLO) 

and local precipitation and runoff, are evaluated. 

The distribution of stage and water-level gradients 

in the study area are discussed to evaluate flow 

characteristics. Suspended-sediment concentrations and discharge are discussed. The present-day 

salinity patterns are discussed in relation to flow 

and local runoff as are changes to historical salinity 

patterns affected by flow in selected areas along 

the coast. 

Quantifying the amount of water flowing in 

the GIWW and how this relates to sediment transport and salinity conditions in coastal Louisiana 

may help State and Federal agencies in designing 

coastal wetland restoration projects and implementing flood-control measures.

Description of Study Area

The GIWW is part of the Intracoastal Waterway, a continuous system of navigation channels 

connecting the Atlantic and Gulf Coast States. The 

channels permit extensive ship-borne commerce 

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Lake

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4 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

between the states. The USACE maintains a channel depth of at least 20 ft and a width of at least 

200 ft for the GIWW in the study area. The part of 

the GIWW in the study area is in south-central 

Louisiana and extends from Cypremort on the west 

to the intersection of the GIWW and Bayou Perot 

on the east. The Verret Subbasin of Terrebonne 

Basin (hereain referred to as the Verret Subbasin), 

to the north and east of Morgan City, La., drains 

into the GIWW and is included in the study area 

(pl. 1). Plate 1 also shows the Teche-Vermilion, 

Terrebonne, Timbalier, and Barataria interdistributary Basins, which are linked by the GIWW in the 

study area. Houma, La., and Morgan City are 

regional population centers.

The Gulf Intracoastal Waterway

East of Morgan City, surface flow in the 

LAR is hydraulically disconnected from the 

GIWW by the Bayou Boeuf locks at Morgan City 

and the Avoca Island levee. The levee was completed around 1952 (N.J. Powell, USACE, oral 

commun., 2001). Water in the LAR is prevented 

from flowing eastward between the Bayou Boeuf 

locks and the levee’s termination at the Avoca 

Island Cutoff Channel, about 10 mi south of Morgan City (pl. 1). The Avoca Island Cutoff Channel 

connects the LAR to the GIWW. Because of prevailing hydraulic gradients, the Avoca Island Cutoff Channel conveys a portion of LAR water in a 

northeasterly direction toward the GIWW. The 

Avoca Island Cutoff Channel becomes Bayou 

Chene where it joins with Bayou Penchant, prior to 

reaching the GIWW. Bayou Boeuf connects to the 

GIWW between the Bayou Boeuf locks at Morgan 

City and the confluence of the GIWW and Bayou 

Chene. Bayou Boeuf is the primary hydrologic 

connection between the GIWW and the Verret Subbasin. The GIWW extends eastward to Houma, 

across the Houma Navigation Canal (HNC) and 

Bayou Lafourche toward Bayou Perot and New 

Orleans, Louisiana. West of Morgan City, the 

GIWW connects to the LAR just below the city 

and extends westward past Jaws Bay and Cypremort to Weeks Bay.

Much of the GIWW between Cypremort and 

Bayou Perot can be categorized as one of two 

hydrologically distinct types of segments. There 

are longer segments (1) where lateral flow, that is 

flow parallel to the coast, predominates. The 

GIWW might lose or gain water along such segments. A different type of segment (2) occurs 

where the GIWW crosses a waterway, such as a 

canal or bayou. Substantial amounts of water can 

enter or leave the GIWW in a very short distance. 

Jaws Bay, on the west side of Morgan City, and the 

HNC are major examples of the second type of 

segment along the GIWW in the study area. West 

of WLO, Jaws Bay separates two segments that 

convey water. This arrangement of conveyance, 

opening, and conveyance is mirrored on the east 

side of the LAR starting east of the confluence of 

Bayou Chene and the GIWW. The lateral flow of 

water in the GIWW between Bay Wallace and 

Bayou Lafourche is split into two segments at 

Houma by the HNC. 

Physiography of Coastal Louisiana

The study area lies mostly within the coastal 

zone. The Louisiana Gulf Coast contains about 

5,800 mi2 of wetlands (Chabreck, 1972), most of 

which occur in the Mississippi River Delta Plain 

(fig. 1). The wetlands form a continuum from 

inland swamps and fresh marsh to salt marsh, 

based on tolerances of the emergent vegetation to 

flooding and the local open-water salinity (Gosselink, 1984). Local salinity depends on the mixing of marine waters from the Gulf of Mexico and 

freshwater runoff from river flow and precipitation. 

The coastal wetlands were formed by active deposition of mineral sediment carried by the Mississippi River during the last 6,000 to 8,000 years, 

after sea level stabilized to approximate current 

levels (Kolb and Van Lopik, 1958; Frazier, 1967), 

and more recently in freshwater areas, through 

accumulations of organic matter almost exclusively derived locally (Swarzenski and others, 

1991). The Mississippi River Delta Plain is comprised of six to seven overlapping lobate deltas 

(Frazier, 1967). The delta lobes formed as the Mississippi River switched channels to follow a more 

hydraulically efficient path to the Gulf of Mexico. 

The switching of channels and loci of delta building occurred about every 1,000 years, and has created a physiography of large areas of low-elevation 

wetlands, termed inter-distributary basins, separated by natural levees which decrease in elevation 

toward the Gulf of Mexico (pl. 1). The natural 

levees act as hydrologic barriers to the cross-basin 

flow of water. Cross-basin flow is generally 
 INTRODUCTION 5

restricted to man-made canals that traverse these 

levees, such as the GIWW. In the study area, the 

GIWW hydrologically links the Teche-Vermilion, 

Terrebonne, Timbalier, and Barataria inter-distributary Basins (pl. 1).

Major Rivers

The Mississippi River and the Atchafalaya 

River are major sources of freshwater to the Louisiana coast in general, and in varying amounts to 

the study area. Because of the extensive levee system, the Mississippi River mostly flows into the 

Gulf of Mexico, and enters coastal Louisiana wetlands only indirectly as Gulf of Mexico waters 

work their way into the estuaries of the various 

bays. The Atchafalaya River has a higher hydraulic gradient and shorter path to the Gulf of Mexico 

than the Mississippi River but is prevented from 

capturing the entire flow of the Mississippi River 

by the Old River Control Structure (fig. 1). Since 

completion of the structure in the 1950's, about 30 

percent of Mississippi River volume is diverted 

into the Atchafalaya River (Wells and Demas, 

1977, p. 6). This water comprises most of the flow 

of the Atchafalaya River, and flow from the Red 

River provides some additional water (fig. 1). The 

water flows through the Atchafalaya Basin and 

enters the Gulf of Mexico through WLO and the 

LAR.

Approach and Methods

Synoptic measurements of discharge and 

suspended sediment were made along the GIWW 

in the study area during different times of the year. 

The synoptic data were supplemented by continuous data collected at three sites from which hourly 

discharge was computed. Water-level gradients in 

the study area were established by comparing stage 

at sites along and adjacent to the GIWW. Salinity 

fluctuations at coastal sites were used to track 

freshwater inflow from the GIWW. Table 1 lists all 

sites used for this report and the type of data presented. Site locations are shown on plate 1.

Discharge and Suspended Sediment

Instantaneous discharge measurements and 

computed continuous discharge, derived from relations between instantaneous discharge and hourly 

recording point-velocity and stage measurements 

at the same location, were used to describe flow at 

selected sites along the GIWW (table 1, pl. 1). 

Between April 1997 and December 1999, 13 to 19 

measurements were made at these sites at varying 

stages of the LAR. Measurements were made 

along the entire length of the GIWW from Cypremort to Bayou Perot on synoptic trips of 2 to 4 

days. The measurements were made with a 600- 

kilohertz boat-mounted acoustic Doppler current 

profiler (ADCP) (Simpson and Oltmann, 1993). In 

the GIWW at Cypremort, at Bayou Penchant south 

of Morgan City, and at Bayou Boeuf at railroad 

bridge at Amelia, La., stage and point-velocity 

were recorded hourly (continuously) by onsite 

data-recording equipment. A relation between 

these variables and the instantaneous discharge 

measurements across the entire channel cross-section was developed to compute continuous discharge (Morlock, 2001). The data from these sites 

were tabulated as hourly discharge values. While 

instantaneous discharge measurements were being 

made in the GIWW, suspended sediment usually 

was sampled at points in the center, left, and right 

of the channel cross section with a depth-integrating sampler (Edwards, 1999). There are not corresponding suspended-sediment measurements for 

every discharge measurement. 

Flow direction can vary at and between sites 

in the low-gradient coastal area so a sign convention was used to label flow direction at the sites. 

Flow away from Morgan City and toward the Gulf 

of Mexico was labeled positive. To the east of 

Morgan City, positive flow is to the east and south 

with the exception of the Avoca Island Cutoff 

Channel. Here, positive flow is to the northeast, 

away from the LAR (pl. 1). West of Morgan City, 

positive flow is to the west; negative flow indicates 

flow toward Morgan City. 

Monthly-average discharge for the study 

period was determined at four synoptic sites along 

the GIWW--west of HNC at Houma, near Bay Wallace east of Morgan City, west of WLO south of 

Calumet, and east of Jaws Bay west of Franklin--

for the difference in discharge between the GIWW 

east and west of Jaws Bay (across Jaws Bay) and 

for the three recording sites at Bayou Boeuf at railroad bridge at Amelia, Bayou Penchant south of 

Morgan City, and the GIWW at Cypremort. At the 

continuously recording sites, discharge data were 

averaged by month. At the other site, relations 
6 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Table 1. Data-collection sites and type of data collected along the Gulf Intracoastal Waterway (GIWW) and adjacent surface-water bodies, south-central Louisiana [USGS, U.S. Geological Survey; DWF, Louisiana Department of Wildlife and Fisheries; NAVD88, North American Vertical Datum of 1988; GPS, Global Positioning System; USACE, U.S. Army Corps of Engineers; DNR, Louisiana Department of Natural Resources; DEQ, Louisiana Department of Environmental Quality; USFWS, U.S. Fish and Wildlife Service; --, not applicable; NGVD29, National Geodetic Vertical Datum of 1929] 

1Elevation surveys were accomplished using GPS and conventional leveling; USGS survey was done as part of this study, using a single first order benchmark at Patterson, 

 Louisiana.

Map 

number 

(pl. 1)

Site name Operator/

owner

Continuous data Instantaneous data

Datum: survey agency 

(method)1 Discharge Velocity Stage Salinity Discharge Suspended sediment

1 Vermilion Bay near Cypremort Point USGS/DWF -- -- X X -- -- NAVD88: USGS (GPS)

2 GIWW at Cypremort USGS/USACE X X X -- X X NAVD88: USGS (GPS)

3 GIWW west of Jaws Bay near Cypremort USGS/USACE -- -- -- -- X X --

4 GIWW east of Jaws Bay west of Franklin USGS/USACE -- -- X -- X X NAVD88: USGS (GPS)

5 GIWW west of Wax Lake Outlet south of Calumet USGS/USACE -- -- X -- X X NAVD88: USACE

6 Wax Lake Outlet at Calumet USGS/USACE -- -- X -- X -- NAVD88: USGS (GPS)

7 GIWW east of Wax Lake Outlet near Calumet USGS/USACE -- -- -- -- X X --

8 GIWW at Lower Atchafalaya River south of Morgan City USGS/USACE -- -- -- -- X X --

9 Lower Atchafalaya River at Morgan City USGS/USACE -- -- X -- X -- NAVD88: USGS (GPS)

10 Avoca Island Cutoff Channel south of Morgan City USGS/USACE -- -- X -- X X NAVD88: USGS (GPS)

11 Bayou Penchant south of Morgan City USGS/USACE X X X -- X X NAVD88: USGS (GPS)

12 Bayou Boeuf at railroad bridge at Amelia USGS/USACE X X X -- X X NAVD88: USGS (GPS)

13 GIWW near Bay Wallace east of Morgan City USGS/USACE -- -- X -- X X NAVD88: USGS (GPS)

14 Bayou Penchant at Brady Canal DNR -- -- -- X -- -- --

15 Bayou du Large at Theriot USGS/DEQ -- -- -- X -- -- --

16 GIWW west of Houma Navigation Canal at Houma USGS/USACE -- -- -- -- X X --

17 GIWW east of Houma Navigation Canal at Houma USGS/USACE -- -- X X X X NAVD88: USGS (GPS)

18 Company Canal at Highway 1 at Lockport USGS/USACE -- -- X -- -- -- NAVD88: USGS (GPS)

19 Bayou L’Eau Bleu USGS/USFWS -- X -- X -- -- --

20 GIWW west of Bayou Lafourche at Larose USGS/USACE -- -- -- -- X X --

21 GIWW east of Bayou Lafourche at Larose USGS/USACE -- -- -- -- X X --

22 GIWW east of Larose DNR -- -- X X -- -- NAVD88: DNR (GPS)

23 GIWW west of Bayou Perot USGS -- -- -- -- X -- --

24 Intersection of GIWW and Bayou Perot DNR -- -- X X -- -- NAVD88: DNR (GPS)

25 Bayou Perot south of the GIWW USGS -- -- -- -- X -- --

26 Grand Bayou tributary west of Galliano USGS/DNR -- -- -- X -- -- --

27 Bayou Jean Lacroix near Montegut USGS/DNR -- -- -- X -- -- --

28 Bayou Terrebonne at Bush Canal, south of Chauvin USGS/DNR -- -- -- X -- -- --

29 Houma Navigation Canal at Dulac USGS/USACE -- X X X X -- NGVD29 (conventional levels)

30 Caillou Lake (Sister Lake) southwest of Dulac USGS/DWF -- -- -- X -- -- --

31 Côte Blanche 4 DNR -- -- -- X -- -- NAVD88: DNR (GPS)

32 Little Lake near Cut Off USGS/DNR -- -- -- X -- -- --
 HYDROLOGIC CONDITIONS DURING STUDY 7

between instantaneous discharge measurements 

and same-day average stage of the LAR at Morgan 

City were used to develop regression equations. 

The difference in same-day discharge across Jaws 

Bay was used in the regression from this location. 

Only measurements made when the LAR at Morgan City was 2 ft or more above NAVD88 were 

used in developing the regression equations. Discharge measurements in the GIWW after the high 

rainfall event in 1998 also were eliminated from 

the regressions because of the explainable departure from expected values. Monthly-average discharge was computed for those months when the 

LAR at Morgan City averaged 3 ft or more above 

NAVD88. The appendix contains the equations 

used at the respective stations. Stage of the LAR at 

Morgan City was 3 ft or more above NAVD88 for 

the first 7 months of 1997, 1998, and 1999. 

Data from the individual synoptic trips were 

used to determine if and under what conditions 

wetlands or waterways adjacent to a specific segment of the GIWW were storing water from or 

releasing water to that particular segment. Discharge measured at the downstream end of a particular segment was subtracted from that entering the 

same segment for each synoptic trip. This difference was plotted against either the discharge entering the GIWW from the respective river source or 

the prior 21-day average discharge at Bayou Boeuf 

at railroad bridge at Amelia. The 21-day discharge 

at Bayou Boeuf at railroad bridge at Amelia was 

used to approximate runoff generated from local 

rainfall entering the GIWW and coastal Louisiana. Negative values indicate water from the surrounding wetlands is draining into the GIWW and 

leaving the segment. Positive values indicate less 

water is flowing out of the segment than entering; 

water is being stored in the segment.

Stage and Water-Level Gradients

Gages measuring stage are distributed at 

sites throughout the study area (table 1). Data from 

these sites were used to compare stage and establish water-level gradients in the GIWW and 

selected adjacent bayous and canals. All gages 

used in this study were surveyed to NAVD88, with 

a Global Positioning System (GPS) using a first 

order benchmark at Patterson, La., as part of this 

study except where noted (table 1). The gage in 

the HNC at Dulac was not part of the USGS GPS 

survey, but was surveyed by the USGS from a 

nearby benchmark to the NGVD29. Two sites 

were surveyed by Louisiana Department of Natural 

Resources (DNR) to NAVD88 datum. The importance of reliable elevation data in establishing gradients increases with distance from the LAR, as the 

gradient becomes smaller. Small differences in 

gradient may be attributed to differences in surveys 

rather than actual differences in measured stage.

Monthly-average values for stage were used 

in this comparison to determine water-level gradients across the study area. To the east of Morgan 

City, stage data from LAR at Morgan City and 

HNC at Dulac were used as the upstream and 

downstream points. To the west of WLO, stage 

data from WLO at Calumet and Vermilion Bay 

near Cypremort Point were used as the upstream 

and downstream points. 

Acknowledgments

The author extends his thanks to Troy Constance and Nancy J. Powell of the U.S. Army 

Corps of Engineers and Ronald F. Paille of the 

U.S. Fish and Wildlife Service for their support 

with this project. Paille was among the first to recognize the potential value of the flow of freshwater and sediments in the GIWW to restoration of 

coastal Louisiana wetlands. The field work and 

data collection were carried out by C. Kevin Labbe 

and Andrew Tarver of the USGS office in Baton 

Rouge. 

HYDROLOGIC CONDITIONS

DURING STUDY

Stage and discharge measured in LAR at 

Morgan City and discharge measured at WLO at 

Calumet from October 1996 to December 1999 

have similar hydrographs (fig. 2). Stage of LAR at 

Morgan City reached almost 8 ft above NAVD88 

for a brief time in April. The spring rise started in 

late February 1997 and continued through midJuly. After July, stage and discharge remained low 

until early January 1998. The spring flood in 1998 

started earlier and, although not reaching the levels 

of 1997, remained high until late July. Stage during the low water season (August through December) in 1998 was slightly higher than in 1997. In 
8 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

1999, the spring flood also developed later than in 

1997 and had a smaller volume. During the low 

water season, the stage of LAR at Morgan City 

was at least 3 ft above NAVD88 only in 1996. In 

November and December 1999, stage of LAR at 

Morgan City frequently was less than 1.5 ft above 

NAVD88. 

Peak measured instantaneous discharge for 

the period of study was about 380,000 ft3/s for 

LAR at Morgan City and 220,000 ft3

/s for WLO at 

Calumet for a combined discharge of about 

600,000 ft3/s. The LAR at Morgan City received 

from 43 to 75 percent of the combined discharge, 

with most of the variability occurring when LAR at 

Morgan City was 3 ft or less above NAVD88. For 

all measurements, an average of 59 percent ± 5 

percent of the combined discharge entered the 

LAR at Morgan City. The average percentage of 

Atchafalaya River flow entering LAR at Morgan 

City determined by measurements during this 

study is similar to the division of discharge determined by measurements made between 1980 and 

1982 (Arcement, 1988).

Precipitation also affects flow in the GIWW. 

Monthly precipitation data are shown for weather 

stations at Morgan City and Houma (fig. 3). Data 

for the Palmer Hydrological Drought Index (PHDI) 

for the South-Central Division, which extends west 

from Morgan City halfway to the Texas border, and 

for the Southeast Division, which extends east of 

Morgan City across the Mississippi River to the 

border with Mississippi, also are presented 

(National Oceanic and Atmospheric Administration National Climatic Data Center, 2000). The 

PHDI is an index that assesses regional long-term 

moisture supply. October and November of 1996 

had below average rainfall. From January to June 

1997, rainfall was near or above the long-term 

average for both stations. From August through 

October, precipitation was below the long-term 

average. The PHDI indicated an incipient to mild 

drought from September through December 1997. 

About 10 to 12 inches more rain than average was 

measured at the two stations during January 1998. 

Rainfall was near normal through April 1998, but 

then was below average until the storms of September 1998. Very little rain fell in May 1998. 

Figure 2. Instantaneous discharge of the Lower Atchafalaya River at Morgan City and Wax Lake Outlet at 

Calumet, and daily-average stage of the Lower Atchafalaya River at Morgan City, Louisiana, October 1996 

to December 1999.

Stage, Lower Atchafalaya River at Morgan City

Instantaneous discharge, Lower Atchafalaya River at Morgan City

Instantaneous discharge, Wax Lake Outlet at Calumet

1

2

3

4

5

6

7

8

0

Nov. 1

Dec. 1

Feb. 1

Mar. 1

Apr. 1

May 1

July 1

Aug. 1

Sept. 1

Jan. 1

Nov. 1

Dec. 1

Feb. 1

Mar. 1

Apr. 1

May 1

July 1

Aug. 1

Sept. 1

Jan. 1

Nov. 1

Dec. 1

Feb. 1

Mar. 1

Apr. 1

May 1

July 1

Aug. 1

Sept. 1

Jan. 1

Nov. 1

Dec. 1

700

600

500

400

300

200

100

0

DAILY-AVERA

GE 

STA

GE, IN FEET A

B

OVE NAVD88

INSTANTANEOUS DISCHARGE, IN 

CUBIC FEET PER SECOND (x 1,000)

1996 1997 1999 1998

Break in line indicates missing data
SURFACE-WATER HYDROLOGY 9

Even with the high rainfall in September, the 

South-Central Division was in a mild to moderate 

drought from May through the end of the year. The 

Southeast Division was in a moderate drought in 

July and August 1998. From October 1998 

through May 1999, rainfall was below average. 

Both divisions were in a moderate drought for most 

months during 1999.

Two events during the fall low-water season 

raised LAR at Morgan City stage for prolonged 

periods (fig. 4). During the last week of September 

and the first 2 weeks of October 1997, a period of 

strong south winds raised the stage along the coast 

and in the LAR at Morgan City. In early and midSeptember of 1998, Hurricane Earle (August 30-

September 1) and Tropical Storms Frances (September 10-12) and Hermine (September 19-20) 

were major storms that produced substantial precipitation and raised the stage along the coast and 

in the LAR at Morgan City. In contrast, the high 

LAR stage in September 1997 occurred without a 

major storm or substantial precipitation.

SURFACE-WATER HYDROLOGY

In this section, flow characteristics in the 

GIWW are discussed. Distribution and differences 

in discharge, stage, and water-level gradients, and 

suspended-sediment and salinity concentrations are 

described.

Flow Characteristics

Discharge in the GIWW was highest where 

it crossed the LAR and at WLO south of Calumet, 

Figure 3. Monthly total precipitation and departure from normal at Morgan City and Houma, Louisiana, 

and Palmer Hydrological Drought Index for the Southeast and South-Central Climatological Divisions in 

Louisiana, October 1996 to December 1999. PALMER HYDROLOGICAL DROUGHT INDEX -2

-4

4

2

0

Source: National Oceanic and Atmospheric Administration, 

National Climatic Data Center, 2000

MONTHLY PRECIPITATION, IN INCHES

20

15

10

5

0

-5

20

15

10

5

0

-5

Total

Departure from normal

HOUMA

Total

Departure from normal

MORGAN CITY

 3.00 to 3.99 = very moist

 2.00 to 2.99 = moderately moist

 1.99 to -1.99 = mid-range

-2.00 to -2.99 = moderate drought

-3.00 to -3.99 = severe drought

Southeast Division

South-Central Division
10 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

and decreased with increasing distance east and 

west (table 2). At the Avoca Island Cutoff Channel, discharge ranged from 267 to 31,100 ft3

/s; the 

average discharge was 12,200 ft3

/s. At Bayou 

Penchant south of Morgan City, flow was usually 

to the southeast, away from the Avoca Island Cutoff Channel. The average discharge was 

3,250 ft3/s. Measured discharge at Bayou Boeuf 

at railroad bridge at Amelia ranged from 9,110 ft3

/s 

flowing north into the Verret Subbasin to 15,300 

ft3/s flowing south from the basin toward the 

GIWW. The average discharge was 1,000 ft3/s 

entering the GIWW from the Verret Subbasin. 

Discharge was usually to the east and averaged 

about 5,700 ft3

/s in the GIWW near Bay Wallace 

east of Morgan City and the GIWW west of HNC 

at Houma. Discharge of 922 ft3/s to the west was 

measured at the former site. Discharge always was 

to the east in the GIWW west of HNC at Houma. 

Discharge measured in the GIWW east of 

HNC at Houma averaged 2,610 ft3

/s and ranged 

from 1,100 ft3

/s, flowing westward toward the 

HNC, to 4,640 ft3

/s flowing eastward toward 

Bayou Lafourche. Discharge in the GIWW west 

and east of Bayou Lafourche at Larose was similar 

in magnitude and direction and averaged about 

2,200 ft3

/s to the east. Eastward discharge of as 

much as 5,320 ft3

/s toward Bayou Perot was measured. Flow westward toward Houma was 

observed on two occasions in the GIWW, with a 

maximum discharge of 1,860 ft3

/s east of Bayou 

Lafourche at Larose.

In Barataria Basin, to the east of Larose, 

instantaneous discharge was measured in the 

GIWW west of the intersection with Bayou Perot, 

and in Bayou Perot just south of the intersection on 

several occasions. The measurements were made 

Figure 4. Daily resultant wind speed and direction at New Orleans, Louisiana, daily-average stage at 

selected sites in coastal Louisiana, and daily total precipitation at Morgan City, Louisiana, from 

September 1 to November 30, 1997 and 1998.

Lower Atchafalaya River at Morgan City

Houma Navigation Canal at Dulac

Vermilion Bay near Cypremort Point

Precipitation

Wind speed

Direction (0 and 360 degrees are north;

180 degrees is south)

EXPLANATION

DAILY TOTAL PRECIPITATION,

 IN INCHES

DAILY TOTAL PRECIPITATION,

 IN INCHES

DAILY-AVERAGE STAGE, IN 

FEET ABOVE NAVD88

DAILY-AVERAGE STAGE, IN 

FEET ABOVE NAVD88

4

5

3

2

1

0

4

3

2

1

0

-1

Sept. 1

Sept. 8

Sept. 15Sept. 22Sept. 29

Oct. 6

Oct. 13

Oct. 20Oct. 27

Nov. 3

Nov. 10Nov. 17Nov. 24

Dec. 1

Sept. 1

Sept. 8

Sept. 15Sept. 22Sept. 29

Oct. 6

Oct. 13Oct. 20Oct. 27

Nov. 3

Nov. 10Nov. 17Nov. 24

Dec. 1

0

2

4

6

8

10

0

2

4

6

8

10

Period of little rain, strong south winds

DAILY RESULTANT WIND 

SPEED, IN MILES PER HOUR

DAILY RESULTANT WIND 

SPEED, IN MILES PER HOUR

DAILY RESULTANT WIND 

DIRECTION, IN DEGREES

DAILY RESULTANT WIND 

DIRECTION, IN DEGREES360

270

180

90

0

24

20

16

12

8

4

0

360

270

180

90

0

24

20

16

12

8

4

0

1997 1998
SURFACE-WATER HYDROLOGY 11

Table 2. Summary of instantaneous discharge measurements made at selected sites along or 

near the Gulf Intracoastal Waterway (GIWW) in south-central Louisiana, 1997-99

[Discharge measurements are in cubic feet per second. Negative discharge indicates flow toward Morgan City or northward flow.]

Map 

number 

(pl. 1) Site name

Instantaneous discharge

Number 

of 

measurements

Mean 

(±standard 

deviation) Median

Range

Minimum Maximum

East of Lower Atchafalaya River

Between Lower Atchafalaya 

River and Houma Navigation 

Canal 

10 Avoca Island Cutoff Channel south 

of Morgan City

18 12,200 ± 9,790 9,400 267 31,100

11 Bayou Penchant south of Morgan 

City

18 3,250 ± 2,370 2,620 260 7,570

12 Bayou Boeuf at railroad bridge at 

Amelia

17 1,000 ± 6,720 768 -9,110 15,300

13 GIWW near Bay Wallace east of 

Morgan City

18 5,600 ± 3,940 5,630 -922 10,600

16 GIWW west of Houma Navigation 

Canal at Houma 

19 5,770 ± 3,190 4,980 845 11,800

East of Houma Navigation Canal

17 GIWW east of Houma Navigation 

Canal at Houma 

19 2,610 ± 1,740 2,200 -1,100 4,640

20 GIWW west of Bayou Lafourche at 

Larose

19 2,220 ± 1,760 2,510 -1,580 5,320

21 GIWW east of Bayou Lafourche at 

Larose

19 2,190 ± 1,830 2,670 -1,860 4,930

West of Lower Atchafalaya River 

Between Lower Atchafalaya 

River and Wax Lake Outlet

8 GIWW at Lower Atchafalaya River 

south of Morgan City

13 13,400 ± 9,380 12,200 3,910 35,200

7 GIWW east of Wax Lake Outlet 

near Calumet

18 9,230 ± 3,350 8,890 4,430 16,200

West of Wax Lake Outlet

5 GIWW west of Wax Lake Outlet 

south of Calumet

16 9,460 ± 5,610 10,300 2,050 20,300

4 GIWW east of Jaws Bay west of 

Franklin 

18 8,230 ± 4,110 6,600 3,560 17,000

3 GIWW west of Jaws Bay near 

Cypremort

18 3,310 ± 2,160 2,850 1,000 10,200

2 GIWW at Cypremort 17 1,350 ± 2,550 2,090 - 2,830 4,830
12 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Table 3. Supplemental instantaneous discharge measurements made at the Gulf Intracoastal 

Waterway (GIWW) west of Bayou Perot, in Bayou Perot south of the Gulf Intracoastal Waterway, 

and in the Houma Navigation Canal at Dulac, south-central Louisiana, 1997-99

[Positive discharge is to the east or south. --, no data; NAVD88, North American Vertical Datum of 1988]

Date

Stage, Lower Atchafalaya River at Morgan 

City (site 9)

(feet above NAVD88)

Instantaneous discharge (cubic feet per second)

GIWW 

west of 

Bayou Perot

(site 23) 

Bayou Perot 

south of 

GIWW

(site 25)

Houma Navigation Canal at 

Dulac

(site 29)

4-15-97 6.97 3,450 17,500 4.200

7-22-97 5.25 1,900 17,400 --

9-23-97 2.57 126 800 --

9-24-97 2.50 -- -- 4,690

11-19-97 1.50 -- -- 4,240

 1-28-98 5.56 -- -- 13,700

 6- 3-98 4.95 -- -- 5,980

 6-15-98 3.70 -- -- 4,670

 4-21-99 3.83 1,440 18,000 --

 6- 1-99 4.44 1,050 18,400 --

 7-14-99 3.64 1,300 -12,000 --

 8-26-99 1.98 709 10,100 --

10- 7-99 2.43 1,220 29,300 --

11- 4-99 1.55 1,340 5,990 --

in conjunction with synoptic surveys of discharge 

along the GIWW, but were not made as frequently 

(table 3). In the GIWW west of Bayou Perot, discharge was to the east and ranged from 126 ft3

/s to 

more than 3,450 ft3/s. In Bayou Perot, discharge 

was much higher than in the GIWW and was measured at 29,300 ft3

/s flowing south and 12,000 ft3/s 

flowing north into Lake Salvador.

Instantaneous discharge was measured at 

two sites between the LAR and WLO at Calumet 

(pl. 1, table 2): GIWW at LAR south of Morgan 

City and the GIWW east of WLO at Calumet. 

Measured discharge always indicated flow from 

east to west in this short stretch of the GIWW. At 

the GIWW at LAR south of Morgan City, average 

discharge (13,400 ft3/s) and maximum discharge 

(35,200 ft3/s) were the highest of all 14 sites.

West of the WLO, discharge in the GIWW 

decreased toward the west (table 2). In the GIWW 

west of WLO south of Calumet, measurements 

ranged from 2,050 to 20,300 ft3/s, with an average 

of 9,460 ft3

/s. In the GIWW east of Jaws Bay west 

of Franklin, average measured discharge was 

8,230 ft3

/s, and reached a maximum of 

17,000 ft3

/s. In the GIWW west of Jaws Bay near 

Cypremort, the average discharge was 3,310 ft3/s, 

and flow was always to the west. In the GIWW at 

Cypremort, average discharge was 1,350 ft3/s. 

Maximum westward discharge was 4,830 ft3/s, but 

eastward discharge as high as 2,830 ft3/s toward 

Jaws Bay was measured.

Daily-average stage and discharge in Bayou 

Penchant south of Morgan City, Bayou Boeuf at 

railroad bridge at Amelia, and GIWW at Cypremort are presented (fig. 5). The gages did not start 

operating at the same time. At Bayou Penchant 

south of Morgan City, computed discharge tracked 

stage fluctuations, with fall discharge about half 

that during the spring (fig. 5). Flow was almost 

always to the southeast, with isolated days when 

there was a net flow of water northwest toward the 

Avoca Island Cutoff Channel. Water flowed both 

into (north) and out of (south) the Verret Subbasin 
SURFACE-WATER HYDROLOGY 13

through Bayou Boeuf (fig. 5). Flow was mostly to 

the south to the confluence of Bayou Boeuf and the 

GIWW. Discharge was not simply related to stage 

of the LAR at Morgan City. For example, April 

through July 1997 and January, February, and September 1998 were months when relatively more 

Verret Subbasin water was entering the GIWW and 

mixing with LAR water (fig. 5). In February and 

May 1999, discharge was small and often entered 

rather than drained the Verret Subbasin.

Hourly point-velocity was measured at 

Bayou L’Eau Bleu from May to December 1996 as 

part of a separate study. Although continuous discharge was not computed for this site, velocity data 

indicate flow mostly was to the south, away from 

Figure 5. Daily-average discharge and stage at Bayou Penchant south of Morgan City, Bayou Boeuf at 

railroad bridge at Amelia, and Gulf Intracoastal Waterway at Cypremort, Louisiana, October 1996 to 

December 1999.

0

1

2

3

4

0

1

2

3

4

-1

0

1

2

3

4

DAILY-AVERAGE 

S

TAGE, IN FEE

T A

BOVE NAV

D88

DAILY-AVERAGE DISCHARGE, IN CUBIC FEET PER SECOND

Bayou Penchant south of Morgan City

Bayou Boeuf at

railroad bridge

at Amelia

Gulf Intracoastal Waterway

at Cypremort

Break in line indicates 

missing data

Break in line 

indicates 

missing data

Positive discharge is to the southeast;

Negative discharge is to the northwest

Positive discharge

is to the south;

Negative discharge

is to the north

Positive discharge is to the west;

Negative discharge is to the east

0

2,000

- 2,000

4,000

6,000

8,000

10,000

12,000

0

-2,000

2,000

4,000

6,000

20,000

15,000

10,000

5,000

-5,000

-10,000

-15,000

OND J J FMAM J J ASON D D J FMAM J ASOND J FMAM J J ASON

OND J J FMAM J J ASON D D J FMAM J ASOND J FMAM J J ASON

OND J J ASONDJ FMAMJ J A SON D D J FMAMJ FMAMJ J A SON

1996 1997 1998 1999

0

Stage Discharge

EXPLANATION
14 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

the GIWW (fig. 6). This was true even in the fall 

months, when flow direction frequently reversed due 

to strong south winds. Available instantaneous discharge measurements have indicated between 200 

and 400 ft3/s of water flowing to the south (USGS 

miscellaneous measurements).

In the GIWW at Cypremort, daily-average 

discharge was usually to the west, toward Vermilion 

Bay. The exception was September 1998 (fig. 5), 

when discharge was eastward toward Jaws Bay. On 

average, fall discharge was smaller than earlier in the 

year. In the fall of 1999, discharge was much 

smaller than in the fall of 1997, and flow more frequently was to the east.

Discharge in Relation to Stage of the Lower 

Atchafalaya River and Precipitation

To evaluate how stage of LAR at Morgan City 

affects flow in the GIWW, the instantaneous discharge measurements made at all points along the 

GIWW were plotted against same-day average stage 

of the LAR at Morgan City (pl. 1). Measurements 

made after periods of heavy precipitation or little 

prior precipitation are highlighted to show the influence of precipitation on flow in the GIWW. 

Between Lower Atchafalaya River and Houma 

Navigation Canal

Water draining the Verret Subbasin through 

Bayou Boeuf at railroad bridge at Amelia and LAR 

water entering through the Avoca Island Cutoff 

Channel maintain eastward flow in the GIWW east 

of Morgan City. The amount of water flowing into 

Avoca Island Cutoff Channel from the LAR generally increased as stage of the LAR at Morgan City 

rose, but this relation was affected by how much 

water was leaving Bayou Boeuf at railroad bridge 

at Amelia (pl. 1). During periods of high southward flow in Bayou Boeuf at railroad bridge at 

Amelia, water entering the GIWW through Avoca 

Island Cutoff Channel was less than could be 

expected based solely on stage of the LAR at Morgan City. Thus, peak discharge of 25,000 to 

35,000 ft3/s in Avoca Island Cutoff Channel was 

measured when there was little water exiting the 

Verret Subbasin through Bayou Boeuf and stage of 

LAR at Morgan City was 4 to 5 ft above NAVD88. 

When stage of the LAR at Morgan City was 5.5 to 

7 ft above NAVD88, discharge in the Avoca Island 

Cutoff Channel was only 17,000 to 22,000 ft3

/s. In 

January 1998, more water was flowing south 

through Bayou Boeuf at railroad bridge at Amelia 

due to the unusually heavy precipitation. Inflow 

through the Avoca Island Cutoff Channel was close 

to zero even though the LAR at Morgan City was 

5.5 ft above NAVD88. Discharge in the Avoca 

Island Cutoff Channel during periods with little 

prior precipitation seemed to maintain the same 

relation with stage of the LAR at Morgan City as 

during other times (pl. 1).

The amount of water flowing south through 

Bayou Boeuf at railroad bridge at Amelia increased 

with precipitation (fig. 7). In 1997, total monthly 

rainfall was high for April, May, and June. Flow in 

Bayou Boeuf at railroad bridge at Amelia was to 

the south, and monthly-average discharge was 

between 8,000 and 10,000 ft3/s. In April, May, and 

June 1998, precipitation was low and monthlyaverage discharge was only 2,700 ft3/s to the south. 

In February 1999, there was very little rain, and 

monthly-average discharge in Bayou Boeuf was 

close to zero. LAR stage was still increasing during this time. In April 1999, there was even less 

rain than in February 1999, but discharge in Bayou 

Boeuf was slightly higher, possibly due to the fallFigure 6. Daily-average stage of the Lower Atchafalaya River at Morgan City and three-hour average 

velocity of Bayou L’Eau Bleu, Louisiana, May to 

December 1996.

DAILY-AVERA

GE 

STA

GE, 

IN FEET ABOVE NAVD 88

THREE-HOUR AVERAGE 

VELOCITY, IN FEET PER SECOND

0

1

2

3

4

5

6

7

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1996

EXPLANATION

Velocity, Bayou L’Eau Bleu

Stage, Lower Atchafalaya River at Morgan City

Break in line indicates missing data

Positive velocity is to the south

Negative velocity is to the north

May 1

May 29

June 26

July 24

Aug. 21

Sept. 18

Oct. 16

Nov. 13

Dec. 11
SURFACE-WATER HYDROLOGY 15

ing stage of the LAR at Morgan City. An unknown 

portion of the Verret Subbasin flow is generated by 

the Atchafalaya River through locks far upstream 

of Morgan City (Wells and Demas, 1977).

Bayou Penchant intersects the Avoca Island 

Cutoff Channel at the confluence with Bayou 

Chene (pl. 1). Bayou Penchant captures water 

from the Avoca Island Cutoff Channel that would 

otherwise be available for eastward flow in the 

GIWW and is one of two main conduits of water 

flowing into the interior of the western Terrebonne 

Basin (between Lower Atchafalaya River and 

Houma Navigational Canal) wetlands. Monthlyaverage discharge at Bayou Penchant south of 

Morgan City was always to the southeast (fig. 8), 

removing water from the Avoca Island Cutoff 

Channel. Monthly-average discharge at Bayou 

Penchant south of Morgan City had a positive relation with stage of the LAR at Morgan City. High 

discharge occurred even when there was little precipitation, for example in April and May 1998 and 

April 1999. High precipitation may have contributed to the high average discharge in January 1998, 

but an increase in precipitation did not appear to 

increase discharge at Bayou Penchant south of 

Morgan City in October 1999. The lower discharge measured in 1999 compared to 1998 

appeared to result from deployment of new instrumentation and a new stage and point-velocity flow 

relation rather than natural phenomena such as less 

rainfall, although it rained less in 1999 than in 

1998. The disconnect between stage and discharge 

from September to October 1998 is shown by the 

continuous data plotted in figure 5. The data prior 

to October 1998 probably overestimate actual discharge. The new stage-discharge relation has been 

in use since September 1998.

Figure 7. Monthly-average stage of the Lower Atchafalaya River at Morgan City and Bayou Boeuf at railroad bridge at Amelia, monthly total precipitation at Morgan City, and monthly-average discharge at 

Bayou Boeuf at railroad bridge at Amelia, Louisiana, October 1996 to December 1999.

MONTHLY TOTAL PRECIPITATION,

IN INCHES

2,000

0

-2,000

4,000

6,000

8,000

10,000

20,000

M

O

N

T

HLY-AVERAGE DIS

C

HARGE, 

IN CUBIC FEET PER SECOND

MONTHLY-AVERAGE STAGE, 

IN FEET ABOVE NAVD88

0

2

4

6

8

0

4

8

12

16

20

24

28
16 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

The GIWW near Bay Wallace east of Morgan City is the other main conduit of water from 

the LAR at Morgan City and the Verret Subbasin. 

Here, water usually moved eastward toward the 

HNC. Discharge in both Bayou Penchant south of 

Morgan City and in the GIWW near Bay Wallace 

east of Morgan City increased predictably with 

increases in the stage of the LAR at Morgan City 

(pl. 1). Discharge in the GIWW near Bay Wallace 

east of Morgan City and west of the HNC at 

Houma was similar in magnitude and responded in 

similar fashion to changes in stage of the LAR at 

Morgan City. At stages between 2.5 and 4 ft 

above NAVD88, discharge increased rapidly to 

about 8,000 ft3

/s and, then, continued to increase 

more gradually to maximums of about 10,000 to 

12,000 ft3/s. Discharge measured after the heavy 

rainfall in January 1998 was within the range of 

measurements made at other times. Extended 

periods with little rain did not alter the relation 

between stage and discharge substantially. The 

GIWW between Bay Wallace and Houma has few 

sections with well-defined levees. Rather than 

being restricted to the GIWW, water easily can 

leave the GIWW at the many breaks in the levees. 

When the LAR at Morgan City was 3 ft or less 

above NAVD88, measured discharge in the 

GIWW near Bay Wallace east of Morgan City frequently was less than that measured west of the 

HNC at Houma. Discharge at the latter site generally remained above 4,000 ft3/s. Flow reversals 

(flow toward the LAR) were observed several 

times in the GIWW near Bay Wallace east of Morgan City during low stage of the LAR at Morgan 

City, but were never observed west of the HNC at 

Houma. 

Figure 8. Monthly-average stage of the Lower Atchafalaya River at Morgan City and Bayou Penchant south 

of Morgan City, monthly total precipitation at Morgan City, and monthly-average discharge of Bayou Penchant south of Morgan City, Louisiana, October 1996 to December 1999.

0

2

4

6

8

2,000

0

-2,000

4,000

6,000

8,000

10,000

MONTHLY-AVERAGE DISCHARGE, 

IN CUBIC FEET PER SECOND

MONTHLY-AVERAGE STAGE, 

IN FEET ABOVE NAVD88

0

4

8

12

16

20

24

28

MONTHLY TOTAL PRECIPITATION,

IN INC

H

E

S

Stage, Lower Atchafalaya River at Morgan City

Stage, Bayou Penchant south of Morgan City

Precipitation, Morgan City
SURFACE-WATER HYDROLOGY 17

Water leaves the GIWW and flows into the 

western Terrebonne Basin wetlands through Bayou 

Penchant south of Morgan City, as previously discussed, and through Bayou Copasaw and Minor’s 

Canal farther east. The latter two waterways capture a percentage of the flow in the GIWW during 

high stage of the LAR at Morgan City and convey 

water southward (fig. 9). Combined discharge in 

Bayou Copasaw and Minor’s Canal was about 10 

to 30 percent of the discharge in the GIWW.

Stage of the LAR at Morgan City was a good 

predictor of how much water was flowing into the 

GIWW, but conditions of little or heavy prior precipitation determined the proportions and routing 

of the main sources of the water. In general, unless 

there was a severe drought, water in the GIWW 

east of the confluence with Bayou Chene contained 

at least some Bayou Boeuf water. During periods 

of heavy precipitation, some of the water from 

Bayou Boeuf moved southward through Bayou 

Chene and combined with LAR water flowing into 

Bayou Penchant south of Morgan City from the 

Avoca Island Cutoff Channel south of Morgan City 

(author’s personal observations during 1997-99). 

Periods of heavy precipitation and low LAR stage 

increased the proportion of water from the Verret 

Subbasin flowing south along this route. 

East of Houma Navigation Canal

Discharge in the GIWW at sites between 

Houma and Larose increased to about 4,000 ft3

/s as 

the stage of the LAR increased from 2.5 to 4 ft 

above NAVD88 (pl. 1). If discharge in the GIWW 

east of the HNC predictably increases as stage of 

the LAR at Morgan City increases to 4 ft or more 

above NAVD88, it was not readily apparent from 

the measurements made during the present study. 

Discharge in the GIWW east of the HNC at Houma 

and east and west of Bayou Lafourche at Larose 

was similar. An estimate of the volume of water 

flowing east was made by averaging all discharge 

measurements when stage of the LAR at Morgan 

City was 3 ft above NAVD88. In the GIWW east 

of the HNC at Houma, average discharge was 

about 3,100 ft3

/s. In the GIWW east and west of 

Bayou Lafourche at Larose, the discharge was 

about 2,880 ft3

/s. Bayou Lafourche typically carries about 200 to 250 ft3

/s of water southward to 

the GIWW at Larose (Garrison and others, 1996). 

A small amount of water may leave the GIWW to 

the south through Bayou Lafourche.

Heavy precipitation during January 1998 

reduced flow in the GIWW east of the HNC at 

Houma (pl. 1). This could be due to runoff causing 

increased stage in the Barataria Basin. Increased 

stage in the Barataria Basin would impede eastward flow in the GIWW and route more water 

southward through available outlets such as Bayou 

L’Eau Bleu and the HNC. This effect would be 

magnified by the extra water Bayou Lafourche 

would be carrying following heavy precipitation. 

Periods with little prior precipitation did not appear 

to affect the relation between discharge and stage 

of the LAR at Morgan City. During low river 

stage, eastward flow in the GIWW past the HNC 

varied from near zero to 2,000 ft3

/s. Flow occasionally was reversed (westward flow) in the 

GIWW east of the HNC at Houma, and west and 

east of Bayou Lafourche at Larose.

Between Lower Atchafalaya River and Wax Lake 

Outlet

Discharge in the GIWW at LAR south of 

Morgan City increased rapidly with increased stage 

in a linear manner, reaching a peak of about 

35,000 ft3/s when stage of the LAR at Morgan 

City was 7 ft above NAVD88 (pl. 1). At that 

stage, about 380,000 ft3/s was flowing south in the 

LAR at Morgan City. At stages higher than 5 ft 

above NAVD88, discharge in the GIWW east of 

WLO at Calumet ranged from 5,000 to about 

10,000 ft3

/s, but did not show a direct relation to 

stage. As the LAR at Morgan City increased from 

5 to 7 ft above NAVD88, much of the additional 

water flowing into the GIWW south of Morgan 

City drained south through the connecting bayous, 

rather than continuing on to WLO. The heavy 

rains of January 1998 seemed to increase the 

amount of water flowing in this segment of the 

GIWW, but discharge was not clearly lower during 

periods of little prior precipitation. When the LAR 

at Morgan City was less than 3 ft above NAVD88, 

discharge in the GIWW east of WLO near Calumet 

and in the LAR south of Morgan City usually was 

similar in magnitude, but varied more or less independently of stage of the LAR at Morgan City.

West of the Wax Lake Outlet

Discharge in the GIWW west of WLO south 

of Calumet increased linearly with increases in 

stage of the LAR at Morgan City when stage was 
Figure 9. Instantaneous discharge at sites between the Lower Atchafalaya River and Houma Navigation 

Canal, Louisiana, April, June, and July 1999.

18 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99
SURFACE-WATER HYDROLOGY 19

2.5 ft or more above NAVD88 (pl. 1). When LAR 

at Morgan City was 7 ft above NAVD88, about 

20,000 ft3

/s flowed westward from the WLO into 

the GIWW. This was about 10 percent of the 

approximately 200,000 ft3/s flowing south in WLO 

at Calumet. When the LAR at Morgan City 

reached 5 ft or more above NAVD88, discharge in 

GIWW east of Jaws Bay west of Franklin 

decreased relative to discharge west of WLO south 

of Calumet, indicating the intervening stretch of 

the GIWW accumulated some of the inflowing 

water. When the LAR at Morgan City was less 

than 2 ft above NAVD88, the opposite generally 

was true; discharge in the GIWW east of Jaws Bay 

west of Franklin was between 5,000 and 

8,000 ft3

/s, but west of WLO south of Calumet was 

less than 5,000 ft3/s. When the LAR at Morgan 

City was at low stage, the segment of the GIWW 

between west of WLO south of Calumet and east 

of Jaws Bay west of Franklin contributed water 

flowing toward Jaws Bay that did not originate 

from WLO. Flow at both sites was to the west.

Discharge in the GIWW east and west of 

Jaws Bay after heavy precipitation was higher than 

expected from relations between other discharge 

measurements and LAR stage (pl. 1). Heavy antecedent precipitation likely caused this anomaly. 

Flow did not appear depressed after periods of little 

prior precipitation. The Charenton Canal intersects 

the GIWW at the opening to Jaws Bay and could 

influence the amount of water entering the GIWW 

and West Côte Blanche Bay. The canal may play 

an important role in maintaining flow in the 

GIWW during periods of low stage of the LAR at 

Morgan City or in adding large amounts of runoff 

to the GIWW when there is heavy precipitation. 

The USGS began monitoring discharge in this 

canal with a continuous velocity and stage gage in 

the summer of 1999; however, results were not 

included in this study.

Discharge in the GIWW was substantially 

less on the west side of Jaws Bay compared with 

discharge measured to the east of Jaws Bay 

(table 2). Discharge measured west of Jaws Bay 

near Cypremort was about 3,000 to 5,000 ft3

/s 

whenever stage of the LAR at Morgan City 

increased to 5 ft or more above NAVD88. Discharge here and 15 mi farther west in the GIWW at 

Cypremort usually was similar in magnitude and 

flow was always to the west, except during periods 

when the LAR at Morgan City was less than 2.5 ft 

above NAVD88. At such times, flow reversals at 

the latter site were frequent. The computed hourly 

discharge at the continuously recording gage in the 

GIWW at Cypremort was averaged by month and 

compared to total monthly precipitation at Morgan 

City, stage of the LAR at Morgan City, and stage in 

GIWW at Cypremort. Monthly-average discharge 

in the GIWW at Cypremort correlated positively 

with stage of the LAR at Morgan City. Lower discharge occurred in the fall; higher discharge 

occurred during the spring flood (fig. 10). Neither 

little nor heavy prior precipitation affected discharge during high LAR stage, remaining steady at 

about 2,000 ft3/s for much of the peak discharge. 

High Gulf of Mexico levels affected flow along 

this stretch of the GIWW. For example, in September 1998, the average monthly discharge was 

toward the east. Gulf of Mexico level was unusually high during this month because of the tropical 

storms (fig. 4), and appeared to disrupt the westerly 

flow in the GIWW between Jaws Bay and Cypremort.

Discharge along the GIWW was highest 

where it crossed the LAR and WLO. Water flowed 

east and west in the GIWW, in decreasing volumes 

with increasing distance from the source. Discharge tended to increase as same-day average 

stage of the LAR at Morgan City increased beyond 

3 ft above NAVD88 at most of the sites between 

HNC and Jaws Bay. When the LAR at Morgan 

City reached 3 to 4 ft above NAVD88, discharge in 

the GIWW became more predictable. The flow of 

freshwater along most of the GIWW studied was 

enhanced not so much by absolute high stage of the 

LAR at Morgan City as by the total time the LAR 

at Morgan City was 3 ft or more above NAVD88. 

The LAR at Morgan City is 3 ft or more above 

NAVD88 for about 7 months in an average year. 

Discharge in the Gulf Intracoastal Waterway 

from 1997-99

To the east of the LAR, monthly-average 

discharge from the Verret Subbasin through Bayou 

Boeuf at railroad bridge at Amelia varied from 

month to month and year to year, independently 

from stage of the LAR at Morgan City (table 4). 

For example, from April through June 1999, the 

volume of water flowing south through Bayou 

Boeuf and into the GIWW was about a third as 
20 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

much as in 1997 during the same months. The volume of eastward flow in the GIWW near Bay Wallace east of Morgan City did not decrease during 

these months between 1997 and 1999 by the same 

amount as in Bayou Boeuf. To compensate for the 

loss of inflowing water from Bayou Boeuf, proportionately more LAR water presumably was flowing 

into the GIWW through the Avoca Island Cutoff 

Channel in 1999 than in 1997, even though the 

spring flood of the LAR was greater in 1997. In 

1997, relatively more water from the LAR was 

routed southeasterly through Bayou Penchant 

south of Morgan City rather than flowing north and 

east toward Bay Wallace. The water flowing eastward in the GIWW past Bay Wallace at times consisted primarily of the Bayou Boeuf flow, and at 

other times substantially less.

Discharge past Houma, across Bayou 

Lafourche, and into Barataria Basin did not 

increase predictably with increasing stage of the 

LAR at Morgan City. Flow was always eastward 

whenever stage of the LAR at Morgan City was 3 

ft or more above NAVD88 (pl. 1). When stage of 

the LAR at Morgan City was 3 ft above NAVD88, 

average discharge in the GIWW east of the HNC at 

Houma was about 3,100 ft3

/s, and in the GIWW 

east and west of Bayou Lafourche at Larose, the 

discharge was about 2,880 ft3

/s. As little as 30 percent to more than 50 percent of the water reaching 

the HNC continued flowing east in the GIWW 

rather than flowing south in the HNC. As flow 

reaching the HNC increased, proportionately more 

water flowed south in the HNC rather than continued east in the GIWW. 

Figure 10. Monthly-average stage of the Lower Atchafalaya River at Morgan City and the Gulf Intracoastal 

Waterway at Cypremort, monthly total precipitation at Morgan City, and monthly-average discharge in the 

Gulf Intracoastal Waterway at Cypremort, Louisiana, October 1996 to December 1999. MONTHLY-AVERAGE DISCHARGE, IN CUBIC FEET PER SECOND MONTHLY-AVERAGE STAGE, IN FEET ABOVE NAVD88

MONTHLY TOTAL PRECIPITATION,

IN INCHES

16

0

4

8

12

20

24

28
SURFACE-WATER HYDROLOGY 21

Table 4. Monthly-average discharge calculated from regression equations and from stage-discharge relations for sites along or near the Gulf 

Intracoastal Waterway (GIWW) east of the Lower Atchafalaya River, 1997-99

[Stage is in feet above North American Vertical Datum of 1988. Discharge is in cubic feet per second. nd, missing data; --, not applicable] 1From stage-discharge relations. 2From regression equation (see appendix, fig. A1).

1997 1998 1999

Stage, 

Lower 

Atchafalaya 

River at 

Morgan 

City

Discharge Stage, 

Lower 

Atchafalaya 

River at 

Morgan 

City

Discharge Stage, 

Lower 

Atchafalaya 

River at 

Morgan 

City

Discharge

Month

Bayou 

Penchant 

south of 

Morgan 

City1

Bayou 

Boeuf 

at railroad 

bridge 

at 

Amelia1

GIWW 

near 

Bay 

Wallace 

east of 

Morgan 

City2

GIWW 

west of 

the 

Houma 

Navigation 

Canal at 

Houma2

Bayou 

Penchant 

south 

of Morgan 

City1

Bayou 

Boeuf 

at railroad 

bridge 

at 

Amelia1

GIWW 

near 

Bay 

Wallace 

east of 

Morgan 

City2

GIWW 

west of 

the 

Houma 

Navigation 

Canal 

at 

Houma2

Bayou 

Penchant 

south 

of Morgan 

City1

Bayou 

Boeuf 

at railroad 

bridge 

at 

Amelia1

GIWW 

near 

Bay 

Wallace 

east of 

Morgan 

City2

GIWW 

west of 

the 

Houma 

Navigation 

Canal 

at 

Houma2

Jan. 3.99 nd nd 8,060 6,990 4.00 nd 8,560 8,080 7,000 2.99 1,590 3,260 6,290 5,420Feb. 4.71 nd nd 9,080 8,120 4.59 nd 9,890 8,920 7,930 4.94 3,080 60 9,370 8,480Mar. 6.49 nd nd 11,000 10,900 5.20 nd 6,780 9,680 8,890 4.87 3,650 4,240 9,280 8,370Apr. 6.90 nd 8,640 11,400 11,600 5.54 nd 2,720 10,100 9,430 4.33 6,120 2,730 8,560 7,530 May 5.32 nd 9,680 9,820 9,080 5.93 nd 1,630 10,500 10,000 4.96 6,490 1,080 9,390 8,520June 4.74 nd 8,030 9,110 8,170 4.13 nd 3,260 8,270 7,210 3.97 4,740 3,190 8,030 6,960July 3.71 nd 7,240 7,620 6,550 4.02 nd nd 8,100 7,040 3.31 4,050 3,850 6,920 5,930Aug. 2.16 nd 2,660 -- -- 2.71 nd 2,060 -- -- 2.06 1,640 2,360 -- --

Sept. 2.13 nd 1,950 -- -- 2.75 nd 6,040 -- -- 2.01 1,000 2,940 -- --

Oct. 2.01 nd 2,180 -- -- 2.53 3,049 250 -- -- 1.91 1,070 3,910 -- --

Nov. 1.66 nd 3,090 -- -- 2.93 1,576 3,480 -- -- 1.68 500 3,380 -- --

Dec. 1.68 nd 4,030 -- -- 2.22 1,171 3,210 -- -- 1.71 940 3,730 -- --

Average (Jan.-July) nd nd 9,440 8,770 nd nd 9,090 8,220 4,240 2,630 8,260 7,320 Average (Aug.-Dec.) nd 2,780 -- -- nd 3,000 -- -- 1,030 3,260 -- --
22 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Table 5. Monthly-average discharge calculated from regression equations and from stage-discharge relations for sites along the Gulf Intracoastal Waterway (GIWW) west of the Lower Atchafalaya River, 1997-99 [Stage is in feet above the North American Vertical Datum of 1988. Discharge is in cubic feet per second. Positive discharge indicates flow to west. Negative discharge indicates flow to east. nd, missing data; --, not applicable]

1From regression equation (see appendix, fig. A2).2From stage-discharge relations.

1997 1998 1999

Month

Stage, 

Lower 

Atchafalaya 

River at 

Morgan 

City

Discharge Stage, 

Lower 

Atchafalaya 

River at 

Morgan 

City

Discharge Stage, 

Lower 

Atchafalaya 

River 

at Morgan 

City

Discharge

GIWW 

west of 

Wax 

Lake 

Outlet 

south of 

Calumet1

GIWW 

east of 

Jaws 

Bay 

west of 

Franklin1

Across 

Jaws 

Bay1

GIWW 

at 

Cypremort2

GIWW 

west of 

Wax 

Lake 

Outlet 

south of 

Calumet1

GIWW 

east of 

Jaws 

Bay 

west of 

Franklin1

Across 

Jaws 

Bay1

GIWW 

at 

Cypremort2

GIWW 

west of 

Wax 

Lake 

Outlet 

south of 

Calumet1

GIWW 

east of 

Jaws 

Bay 

west of 

Franklin1

Across 

Jaws 

Bay1

GIWW 

at 

Cypremort2

Jan. 3.99 10,700 9,630 6,570 nd 4.00 10,700 9,660 6,590 nd 2.99 7,140 7,080 4,690 nd

Feb. 4.71 13,200 11,100 7,650 nd 4.59 12,800 10,900 7,480 2,160 4.94 14,000 11,500 7,960 1,740Mar. 6.49 19,500 13,900 9,740 nd 5.20 15,000 12,000 8,300 2,100 4.87 13,800 11,400 7,870 1,720Apr. 6.90 21,000 14,500 10,100 nd 5.54 16,200 12,500 8,710 1,950 4.33 11,900 10,400 7,100 1,250May 5.32 15,400 12,200 8,450 nd 5.93 17,500 13,100 9,150 2,340 4.96 14,100 11,600 7,990 1,700June 4.74 13,300 11,200 7,700 nd 4.13 11,200 9,940 6,800 1,650 3.97 10,600 9,600 6,500 1,680July 3.71 9,700 8,990 6,100 nd 4.02 10,800 9,700 6,620 2,260 3.31 8,300 7,970 5,350 1,630Aug. 2.16 -- -- -- 1,580 2.71 -- -- -- 1,570 2.06 -- -- -- 1,000Sept. 2.13 -- -- -- 1,150 2.75 -- -- -- -130 2.01 -- -- -- 160Oct. 2.01 -- -- -- 930 2.53 -- -- -- nd 1.91 -- -- -- 310Nov. 1.66 -- -- -- 1,560 2.93 -- -- -- nd 1.68 -- -- -- 310Dec. 1.68 -- -- -- nd 2.22 -- -- -- nd 1.71 -- -- -- 950

Average (Jan.-July) 14,700 11,600 8,040 nd 13,500 11,100 7,660 nd 11,400 9,930 6,780 ndAverage (Aug.-Dec.) -- -- -- nd -- -- -- nd -- -- -- 540
SURFACE-WATER HYDROLOGY 23

Monthly-average discharge decreased westward from Morgan City, with a sharp decrease 

across Jaws Bay at the opening to West Côte 

Blanche Bay (table 5). From January to August, 

about 60 percent of the volume of water entering 

the GIWW west of WLO near Calumet left the 

GIWW at Jaws Bay. More than 10 percent of the 

original volume reached the GIWW at Cypremort, 

but the source of the westward flowing water was 

not necessarily all derived from WLO. 

Measuring discharge directly with a gage 

continuously recording velocity and stage is preferable to using estimates derived from regression 

equations. Indirect estimates using the stage of the 

LAR at Morgan City and instantaneous discharge 

measurements are subject to increased error. The 

regression equations used to indirectly estimate 

discharge have a relatively high correlation coefficient and meet the needs of this study. There is 

value to the numbers generated by the regressions 

to evaluate in general terms what contributes to the 

flow of a particular segment in the GIWW. When 

the LAR at Morgan City is 3 ft or more above 

NAVD88, the GIWW effectively distributes freshwater and sediments from the LAR and WLO to 

points in coastal Louisiana 30 to 50 mi east and 

west of Morgan City. 

Flow Reversals

Events that push Gulf of Mexico water in an 

inland direction can result in short periods of flow 

reversal in the GIWW and periods when the 

GIWW can move saltwater laterally. These can be 

driven by elevated stage associated with tropical 

storms and/or winds. Under these conditions, the 

north-south trending HNC and Bayou Perot can 

convey saltwater in an inland direction into the 

GIWW. From there, the water can then move laterally in the GIWW following the normal gradients.

Elevated stage in the Gulf of Mexico can 

cause flow reversals in the GIWW, canals, and bayous, and push salinity in an inland direction. For 

example, in the last week of September and for the 

first 2 weeks of October 1997, strong offshore 

winds pushed more saline water toward the fresher 

parts of coastal Louisiana (fig. 5). At the HNC at 

Dulac, daily average discharge was in an inland 

direction from October 7-13. Salinity at Little 

Lake near Cut Off increased markedly during the 

same time, indicating northward flow of more 

saline waters from the Gulf of Mexico (fig. 11). 

Three synoptic surveys of surface-water salinity in 

the GIWW between Larose and Lafitte, La., were 

done during this time (fig. 11). Data collected during the surveys indicate that some of the saltier 

water moving north flowed east and west into the 

GIWW where it crossed Bayou Perot. Salinities 

greater than 14 ppt were measured in the GIWW 

west of Bayou Perot just after passage of a weather 

front from October 13-14. Salinity reached 8 ppt 

in Segnette Waterway. The water intruded through 

the canal, and not through Lake Salvador. Under 

these conditions of rapidly inflowing water from 

the south, flow was temporarily westward in the 

GIWW from Bayou Perot toward Bayou 

Aerial view looking south beyond the intersection of the Gulf Intracoastal Waterway and Bayou Perot toward Little Lake, Louisiana.
24 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Figure 11. Daily-average salinity in the Gulf Intracoastal Waterway east of Houma Navigation Canal at 

Houma, east of Larose, and in Little Lake near Cut Off, and daily-average velocity and stage of the 

Houma Navigation Canal at Dulac in September and October 1997; synoptic surveys of salinity in the 

Gulf Intracoastal Waterway and associated surface waters in Louisiana, October 9, 14, and 17, 1997.

Gulf Intracoastal Waterway east of Larose

Gulf Intracoastal Waterway east of

Houma Navigation Canal at Houma

Little Lake near Cut Off

EXPLANATION

0.6 Salinity, in parts per thousand

Positive velocity indicates southward flow;

negative velocity indicates northward flow

VELO

C TI Y, NI FEET

PER SEC

OND

STAGE, IN FEET 

ABOVE NGVD29

1

-1

0

2.0

2.5

1.5

3.5

3.0

1997

SALINITY, IN PARTS 

PER THOUSAND

0

25

20

15

5

10

Oct. 9

Oct. 14

Oct. 17

Sept. 19Sept. 26

Oct. 3

Oct. 10Oct. 17Oct. 23

0

0 10

5

5 KILOMETERS

MILES

Index Map

Velocity

Stage

Houma Navigation Canal at Dulac

Enlarged

area

TEXAS

MISSISSIPPI

ARKANSAS

90°00'

29°30'

29°45'

Bayou

 

des Allemands

Bayou Lafourche

Lake

Salvador

Little

Lake

MISS I SS IPPI RIVER

INTRACOASTAL WATERWAY

GULF

BARATARIA WATERWAY

2.0

0.0 3.0

0.0

0.0

Bayou Perot

4.0 4.0

4.0 4.0

Oct. 9, 1997

90°30' 15'

NEW ORLEANS

SEGNETTE 

WATERWAY

90°00'

Bayou

 

des Allemands

Bayou Lafourche

Lake

Salvador

NEW ORLEANS

Little

Lake

MISS I SS IPPI RIVER

INTRACOASTAL WATERWAY

GULF

BARATARIA WATERWAY

5.0

8.0

3.2

9.4

7.0

0.6

0.4

Bayou Pe or t

7.5

14.5

14.5

9.2

11.2 7.9

10.9 9.1

8.5

3.4

Oct. 14, 1997

29°30'

29°45'

90°30' 15'

SEGNETTE 

WATERWAY Bayou

 

des Allemands

Bayou Lafourche

Lake

Salvador

NEW ORLEANS

Little

Lake

MISS I SS IPPI RIVER

INTRACOASTAL WATERWAY

GULF

BARATARIA WATERWAY

3.2 5.4

Bayou Perot

5.1

3.7 2.9

5.0

3.3 5.2

5.1

7.8

6.4

4.9 4.2

5.9

5.7 5.3

5.2

5.2

Oct. 17, 1997

29°30'

29°45'

90°30' 15' 90°00'

SEGNETTE 

WATERWAY

Louisiana Oil Spill Coordinator, Office of the Governor, Louisiana GIS CD: A Digital Map of the State, Version 2.0
SURFACE-WATER HYDROLOGY 25

Lafourche, and flow was eastward in the GIWW 

from the HNC toward Bayou Lafourche. After 

passage of the weather front, eastward flow along 

the entire stretch of the GIWW between the HNC 

and Bayou Perot resumed.

Net Gain or Loss of Water Along Major 

Segments

To better understand the surface-water 

hydrology of the GIWW and its implications for 

regional freshwater availability, it is helpful to consider (1) whether individual segments of the 

GIWW contribute water to the surrounding wetlands or drain water from them, and (2) whether 

this characteristic changes depending on inflowing 

river water or antecedent precipitation. Three segments of the GIWW east of Morgan City between 

Bay Wallace and Bayou Lafourche and three segments of the GIWW west of Morgan City between 

the WLO and Cypremort were characterized. Net 

inflow or outflow can occur in any of these segments. Net inflow occurs when more water enters 

than leaves from one or both ends of a segment and 

flows into the surrounding wetlands. In this 

instance, the GIWW functions as a freshwater 

diversion, contributing to the water budget of the 

wetlands. Net outflow occurs when more water 

leaves than enters a segment through one or both 

ends. In this case, the GIWW drains some water 

from the wetlands. Water from the surrounding 

wetlands contributes flow to the GIWW.

For the three segments east of Morgan City 

(segments A, B, C; fig. 12), the difference in 

instantaneous discharge between segment start and 

segment end was plotted with the instantaneous 

discharge at GIWW near Bay Wallace east of Morgan City. The complex interactions of flow from 

Bayou Boeuf, Bayou Chene, and the Avoca Island 

Cutoff Channel were the reason the analysis started 

at GIWW near Bay Wallace east of Morgan City 

and not closer to the LAR. For the three segments 

west of Morgan City (segments D, E, F; fig. 13), 

the difference in instantaneous discharge at the 

start and end of each segment was plotted with 

instantaneous discharge in the GIWW west of 

WLO near Calumet. For all six segments the difference in instantaneous discharge at the start and 

end of each segment also was plotted with the 21-

day average discharge at Bayou Boeuf at railroad 

bridge at Amelia preceding each set of measurements. The 21-day average discharge at Bayou 

Boeuf at railroad bridge at Amelia was assumed to 

indicate influence of local runoff on flow in the 

GIWW.

During slightly more than half the synoptic 

trips, instantaneous discharge was greater in the 

GIWW west of the HNC at Houma than near Bay 

Wallace (segment A, fig. 12). Adjacent wetlands 

contributed water to the GIWW along this segment 

during high and low stage of the LAR at Morgan 

City. On two occasions, water flowed both west 

toward Morgan City and east toward the HNC. 

When the 21-day average discharge was less than 

4,500 ft3

/s at Bayou Boeuf at railroad bridge at 

Amelia, this segment acted both as a source and 

sink of water (fig. 12). The segment tended to 

store inflowing water when the LAR at Morgan 

City was 3 ft or more above NAVD88. When the 

LAR at Morgan City was less than 3 ft above 

NAVD88, more water tended to flow out of this 

segment than entered. When the 21-day average 

discharge of water from Bayou Boeuf was greater 

than 6,000 ft3

/s, there was a net outflow of water to 

adjacent wetlands from the segment of the GIWW 

between Bay Wallace and west of the HNC.

The GIWW was a source of water to the 

HNC in segment B (fig. 12) during almost all the 

synoptic trips. The volume of water entering the 

HNC generally increased as eastward flow in the 

GIWW near Bay Wallace east of Morgan City 

increased, but the relation between LAR stage and 

discharge from the GIWW to the HNC was variable. For example, when the LAR at Morgan City 

was 4 ft or more above NAVD88, discharge from 

the GIWW to the HNC ranged from near zero to 

greater than 8,000 ft3/s. The relation between local 

runoff and the amount of water that left the GIWW 

at the HNC was more direct. As the 21-day average discharge at Bayou Boeuf at railroad bridge at 

Amelia increased, the amount of water leaving the 

GIWW through the HNC increased steadily, 

regardless of whether stage of the LAR at Morgan 

City was high or low. Local runoff appears to contribute to the flow of water in the GIWW near the 

HNC.

The wetlands adjacent to the GIWW 

between the HNC and Bayou Lafourche stored and 
26 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Figure 12. Relation between net inflow (positive) or outflow (negative) of water in three segments of the 

Gulf Intracoastal Waterway (GIWW) east of Morgan City and instantaneous discharge in the GIWW near 

Bay Wallace east of Morgan City and the 21-day average discharge at Bayou Boeuf at railroad bridge at 

Amelia, Louisiana, 1997-99.

INSTANTANEOUS DISCHARGE, IN CUBIC FEET 

PER SECOND, OF GIWW NEAR BAY WALLACE 

EAST OF MORGAN CITY

(Positive flow is to the east)

21-DAY AVERAGE DISCHARGE, IN CUBIC FEET PER 

SECOND, OF BAYOU BOEUF AT RAILROAD BRIDGE 

AT AMELIA

(Positive flow is to the south)

6,000

4,000

2,000

0

-2,000

-4,000

-6,000

-8,000

6,000

4,000

2,000

0

-2,000

-4,000

-6,000

-8,000

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

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

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

4,000

0

-4,000

12,000

8,000

-4,000

6,000

4,000

2,000

0

-2,000

8,000

DIFFERENCE IN INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND

GIWW east of Houma Navigation 

Canal at Houma minus GIWW west 

of Bayou Lafourche at Larose 

GIWW west of Houma Navigation 

Canal at Houma minus GIWW 

east of Houma Navigation Canal 

at Houma 

GIWW near Bay Wallace east of Morgan

City minus GIWW west of Houma 

Navigation Canal at Houma

Symbol color indicates corresponding stage of Lower Atchafalaya River at Morgan City: 

[magenta ( ) 4 feet or more above NAVD88, dark blue ( ) between 3 and 4 feet above 

NAVD88, light blue ( ) 3 feet or less above NAVD88] 

A, B, and C are segments along the GIWW (see text)

0 4,000 8,000 12,000

0 4,000 8,000 12,000

0 4,000 8,000 12,000

EXPLANATION
SURFACE-WATER HYDROLOGY 27

Figure 13. Relation between net inflow (positive) or outflow (negative) of water in three segments of the 

Gulf Intracoastal Waterway (GIWW) west of the Wax Lake Outlet and instantaneous discharge in the 

GIWW west of Wax Lake Outlet south of Calumet and the 21-day average discharge at Bayou Boeuf at 

railroad bridge at Amelia, Louisiana, 1997-99.

6,000

4,000

2,000

0

-2,000

-4,000

-6,000

-8,000

6,000

4,000

2,000

0

-2,000

-4,000

-6,000

-8,000

4,000

0

-4,000

12,000

8,000

4,000

0

-4,000

12,000

8,000

6,000

4,000

2,000

0

-2,000

-4,000

8,000

6,000

4,000

2,000

0

-2,000

-4,000

8,000

0 4,000 8,000 12,000

0 4,000 8,000 12,000

0 5,000 10,000 15,000 20,000 0 4,000 8,000 12,000

0 5,000 10,000 15,000 20,000

0 5,000 10,000 15,000 20,000

INSTANTANEOUS DISCHARGE, IN CUBIC FEET 

PER SECOND, AT GIWW WEST OF WAX LAKE 

OUTLET SOUTH OF CALUMET

(Positive flow is to the west) 

GIWW west of Jaws Bay near Cypremort minus GIWW at Cypermort GIWW east of Jaws Bay west of Franklin minus GIWW west 

of Jaws Bay near Cypremort

GIWW west of Wax Lake Outlet south

of Calumet minus GIWW east of

Jaws Bay west of Franklin

21-DAY AVERAGE DISCHARGE, IN CUBIC FEET 

PER SECOND AT BAYOU BOEUF AT RAILROAD 

BRIDGE AT AMELIA

(Positive flow is to the south)

DIFFERENCE IN IN

STANTANEOU

S

DIS

C

HARGE, IN CU

BIC FEET PER 

SECON

D

D

E

F

D

E

F

Symbol color indicates corresponding stage of Lower Atchafalaya River at Morgan City: 

[magenta ( ) 4 feet or more above NAVD88, dark blue ( ) between 3 and 4 feet above 

NAVD88, light blue ( ) 3 feet or less above NAVD88] 

D, E, and F are segments along the GIWW (see text)

EXPLANATION
28 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

drained water (segment C, fig. 12). Typically, 

when the LAR at Morgan City was less than 3 ft 

above NAVD88, water was stored in this segment. 

The volume stored did not increase or decrease 

depending on the initial amount of water flowing 

eastward at GIWW near Bay Wallace east of Morgan City. Less frequently, this segment of the 

GIWW released water to the adjacent parts of the 

GIWW. When local runoff was low as indicated 

by the 21-day average discharge in Bayou Boeuf at 

railroad bridge at Amelia, the GIWW between the 

HNC and Bayou Lafourche and adjacent wetlands 

appeared to store the inflowing water. As local 

runoff increased, more water appeared to leave this 

segment by flowing eastward. The data suggest 

that as local runoff increases, this segment is a 

source of water to adjacent parts of the GIWW. 

The relation is reasonably direct for the available 

data. 

The GIWW between WLO and Jaws Bay 

shifted from draining water to Jaws Bay to storing 

water as instantaneous discharge in the GIWW 

west of WLO south of Calumet increased in segment D (fig. 13). During low discharge, more 

water left the GIWW at Jaws Bay than entered the 

GIWW from the WLO. Water from the Charenton 

Drainage Canal and adjacent wetlands contributed 

to flow in the GIWW. Local runoff did not appear 

to influence whether more or less water was stored 

along this segment (fig. 13).

With one exception, water flowed from the 

GIWW to Jaws Bay in segment E during all synoptic trips (fig. 13). About 2,000 to 3,000 ft3/s or 

more water was leaving the GIWW east of Jaws 

Bay west of Franklin than was entering west of 

WLO south of Calumet when the LAR at Morgan 

City was 3 ft or less above NAVD88. The volume 

of water leaving increased steadily as flow entering 

the GIWW west of WLO south of Calumet 

increased. When the LAR at Morgan City was 4 ft 

or less above NAVD88, the volume of water leaving the GIWW at Jaws Bay appeared to increase as 

local runoff increased. When the LAR at Morgan 

City was 4 ft or more above NAVD88, a relation 

between local inflow and instantaneous discharge 

across Jaws Bay was not evident. 

Water entered from the west and was stored 

in segment F between Jaws Bay and Cypremort, 

when the LAR at Morgan City was 3 ft or less 

above NAVD88, but the volume progressively 

decreased as flow into the GIWW from WLO 

increased (fig. 13). When the LAR at Morgan City 

was 4 ft or more above NAVD88, the volume of 

water entering this segment at Jaws Bay generally 

was similar to the volume leaving at Cypremort 

indicating throughflow conditions. The exception 

occurred during the high precipitation in January 

1998, when almost 6,000 ft3

/s of water was stored 

along the segment of the GIWW between Jaws Bay 

and Cypremort. As the 21-day average discharge 

at Bayou Boeuf at railroad bridge at Amelia 

increased from 2,000 ft3/s to more than 8,000 ft3/s, 

the rate at which water was stored in the GIWW 

between the Jaws and Cypremort increased by 

about 1,000 to 2,000 ft3/s. Two linear curves with 

the same slope show this trend. The data indicate 

that local runoff also accumulates in and contributes to flow along this segment of the GIWW. 

The segments of the GIWW east of Morgan 

City and west of WLO differ hydrologically in 

whether they contribute water to, or drain water 

from adjacent wetlands. In the segments closest to 

the source (segments A and D), water entering the 

GIWW west of WLO south of Calumet was stored 

in increasing volumes in the area between WLO 

and Jaws Bay as inflow increased. To the east of 

Morgan City in segment A, the wetlands between 

the GIWW near Bay Wallace east of Morgan City 

and west of the HNC at Houma more frequently 

contributed water to the GIWW than stored water 

from the GIWW. Water left the GIWW both at 

Jaws Bay and at the HNC. The volume of water 

leaving the GIWW at Jaws Bay in segment E 

increased as the volume of river water entering the 

GIWW west of WLO south of Calumet increased. 

The volume of water leaving the GIWW through 

the HNC in segment B increased as discharge at 

Bayou Boeuf at railroad bridge at Amelia 

increased. The increase in flow into the HNC was 

influenced less by the volume of water entering the 

GIWW near Bay Wallace east of Morgan City (fig. 

12). The GIWW between Jaws Bay and Cypremort (segment F) rapidly changed from contributing water to the wetlands to mainly net 

throughflow as the inflow into the GIWW west of 

WLO near Calumet increased from low to high. At 

low inflow, water entering this segment was from 

west of Cypremort and west of Jaws Bay. East of 

Morgan City, the volume of water flowing east-
SURFACE-WATER HYDROLOGY 29

ward near Bay Wallace east of Morgan City did not 

influence storage or release of water in the GIWW 

in segment C between the HNC and Bayou 

Lafourche. Local runoff appeared to change storage characteristics of the two segments farthest 

from WLO and the LAR but in opposite ways. 

West of WLO, the volume of water stored in segment F increased as runoff increased. Increasing 

local runoff appeared to increase the net volume of 

water leaving segment C of the GIWW between 

Houma and Larose.

Magnitude, Duration, and Timing of Flow

The difference in elevation between the 

stage of the LAR or WLO and surrounding waters 

controls the magnitude, duration, and timing of the 

flow in the GIWW east and west of Morgan City. 

In an earlier section of the report, it was demonstrated that discharge in the GIWW becomes more 

predictable whenever the stage of the LAR at Morgan City is 3 to 4 ft or more above NAVD88. The 

period when this condition is met varies considerably from year to year. An examination of the 

stage record of the LAR allows inferences to be 

drawn on flow characteristics of the GIWW. Figure 14 shows stages of the LAR at Morgan City 

averaged from 1989-99, for years when the spring 

flood of the Atchafalaya River was exceptionally 

high or low, and for a period of sustained moderate 

flow.

Under average conditions, stage of the LAR 

at Morgan City remains 3 ft or more above 

NAVD88 from about January to mid-July, and 4 ft 

or more above NAVD88 for about 6 weeks less. In 

2000, annual discharges of the Mississippi River 

and Atchafalaya River were among the 10 lowest 

on record. Stage of the LAR at Morgan City 

exceeded 4 ft or more above NAVD88 only briefly 

and was 3 ft or more above NAVD88 from April to 

mid-May and for about 2 weeks in July. During 

1997, which was a record flood year, stage of the 

LAR at Morgan City reached 8 ft above NAVD88. 

For a brief period flow along the GIWW would 

have been exceptionally high, but because LAR 

stage was below the long-term average from July 

through December 1997, the total volume of river 

water moving laterally in the GIWW was not much 

different from an average year. To sustain flow and 

maximize the volume of freshwater delivered to 

coastal Louisiana marshes, stage of the LAR at 

Morgan City should remain 3 ft or more above 

NAVD88 for prolonged periods. This occurred in 

Figure 14. Daily-average stage of the Lower Atchafalaya River at Morgan City, Louisiana, for characteristic flows of the Lower Atchafalaya River.

D

AILY-AV

ERAG

E

STAG

E, IN

FEET ABOVE NAVD88

MONTH

Break in line indicates missing data

Jan. 1

Feb. 1

M

ar. 1

A

pr. 1

M

ay 1

Ju yl 1

Aug. 1

Sept. 1

N

ov. 1

Dec. 1

Jan. 1

0

1

2

3

4

5

6

7

8

EXPLANATION

CHARACTERISTIC FLOW:

Average (1989-98)

High spring flood (1997)

Sustained moderate flow (1993)

Low spring flood (2000)

Shaded area indicates approximate

stages at or above which flow in the

Gulf Intracoastal Waterway 

becomes predictable
30 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

1993, when the LAR at Morgan City was 4 ft or 

more above NAVD88 for almost 11 months in 

1993 and was below 3 ft above NAVD88 for less 

than 2 weeks. 

The percent of time in a year that stage of the 

LAR at Morgan City was between 3 ft above 

NAVD88 and the maximum elevation for that time 

period was tabulated in 1⁄2-ft increments for the 

four characteristic flows described above (fig. 15). 

The regression equations relating discharge at 

selected sites along the GIWW with stage of the 

LAR at Morgan City (see appendix) were then 

applied to estimate the discharge at selected sites 

along the GIWW. The percent of time in the year a 

discharge equaled or exceeded indicated levels in 

the GIWW near Bay Wallace east of Morgan City, 

west of HNC at Houma, east of Jaws Bay west of 

Franklin, and across Jaws Bay was calculated for 

characteristic flows in the LAR when stage in the 

LAR was 3 ft or more above NAVD88. The relations in figure 15 suggest the time during a year at 

characteristic flows indicated discharge is expected 

to occur at the selected sites. The amount of freshwater flowing in the GIWW influences sediment 

transport and salinity in coastal Louisiana (discussed in later sections of this report) and has 

implications for wetland restoration.

Figure 15. Percent of time in a year discharge at selected sites is equaled or exceeded for characteristic flows of the Lower Atchafalaya River, Louisiana. PERCENT OF TIME IN A YEAR DISCHARGE IS EQUALED OR EXCEEDED

CALCULATED DISCHARGE, IN 

CUBIC FEET PER SECOND

EAST OF MORGAN CITY

WEST OF WAX LAKE OUTLET

CALCULATED DISCHARGE, IN 

CUBIC FEET PER SECOND

0 4,000 8,000 12,000 16,000 20,000

0

10

20

30

40

50

60

70

80

90

100

Average (1989-98)

H

ustained moderate flow(1993)

L

igh spring flood (1997)

S

ow spring flood(2000)

EXPLANATION

Discharge calculated from regression

equations relating instantaneous discharge

to stage of the Lower Atchafalaya River at 

Morgan City listed in the appendix

in 0.5-ft (feet) increments when stage 3 ft or 

more above NAVD88

CHARACTERISTIC FLOW:

Near Bay Wallace east of Morgan City

0 4,000 8,000 12,000 16,000 20,000

0

10

20

30

40

50

60

70

80

90

100

Across Jaws Bay

0 4,000 8,000 12,000 16,000 20,000

0

10

20

30

40

50

60

70

80

90

100

West of Houma Navigation 

Canal at Houma

0 4,000 8,000 12,000 16,000 20,000

0

10

20

30

40

50

60

70

80

90

100
SURFACE-WATER HYDROLOGY 31

In 2000, stage of the LAR was 3 ft or less 

above NAVD88 about 80 percent of the time; discharge was predictable for less than 20 percent of 

the time. Peak discharge was 25 to 60 percent 

lower than in the other three time periods. For the 

time periods 1989-98 and 1997, discharge was predictable 55 to 60 percent of the time. In 1997, 

when the Atchafalaya River experienced a large 

spring flood, calculated discharge was greater than 

for the three other time periods about 45 percent of 

the time. In 1993, discharge was predictable for 

more than 80 percent of the time. For average conditions (1989-98) and in 1993, discharge in the 

GIWW near Bay Wallace east of Morgan City was 

between 9,000 and 11,000 ft3/s for at least 40 percent of the time; peak discharge for the two time 

periods was about 70 percent of that reached in 

1997. For average conditions and in 1993, about 

6,000 to 8,000 ft3

/s of water discharged across the 

Jaws Bay opening at least 40 percent of the time 

(fig. 15). In 1997, between 12,000 and 16,000 ft3

/s 

of water flowed across the Jaws Bay opening at 

least 40 percent of the time. In 2000, at most 4,000 

to 6,000 ft3/s of water flowed across Jaws Bay, and 

this maximum flow occurred less than 20 percent 

of the time. 

 Stage in the LAR has increased steadily 

since the 1950's and in 1999 was about 2 to 2.5 ft 

higher (Harley Winer, USACE, oral commun., 

1999). The downstream emergence of the Atchafalaya Delta in the late 1970's and the aggradation of 

the river bed are the principal reasons for this 

increase. As the time when the stage of the LAR is 

3 to 4 ft or more above NAVD88 increases, and the 

duration of stage differences between the LAR and 

canals and bayous in adjacent waterbodies 

increases, the volume of water moving laterally in 

the GIWW also will increase. The percent of time 

when the stage of the LAR is greater than 3 ft 

above NAVD88 could increase in the future with 

further aggradation of the river bed. 

Discharge in the GIWW at Bay Wallace 

measured during 1997-99 is higher relative to a 

given stage of the LAR at Morgan City than in a 

series of measurements made in 1983 and 1984 by 

the USACE (Donnel and Lederer, 1992) (fig. 16). 

The increase is substantial. Some of the increase 

may be attributed to widening and deepening of the 

Avoca Island Cutoff Channel in the early 1980's 

(N.J. Powell, USACE, oral commun., 1999). 

The distance from the GIWW that coastal 

wetlands have been affected by freshwater entering 

from the GIWW has increased since the 1970’s. 

Vegetation surveys of coastal Louisiana marshes 

made at regular intervals indicate less salt-tolerant 

species have expanded in a southeasterly direction 

in western Terrebonne Basin wetlands (Visser and 

others, 1999). Average monthly salinity measured 

during 6 consecutive years in the 1970’s, and again 

from 1995-99, at Caillou Lake (Sister Lake) southwest of Dulac, shows a freshening in the spring 

months for the more recent time period (fig. 17). 

Figure 16. Relation between instantaneous discharge in the Gulf Intracoastal Waterway west of 

Houma Navigation Canal at Houma and same-day 

average stage of the Lower Atchafalaya River at Morgan City, Louisiana, 1983-84 and 1997-98.

Figure 17. Average monthly salinity at Caillou 

Lake (Sister Lake) southwest of Dulac, Louisiana, 

from 1972-77 and from 1995-99. AVERAGE M

ONTHLY SALINITY, 

IN PARTS PER THOUSAND

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

0

5

10

15

20

1972-77

1995-99

Brackets show±1 standard error

SAME-DAY AVERAGE STAGE, 

IN FEET ABOVE NAVD88

INSTA

NTA

NEOUS DISC

H

ARGE, NI

CUBIC FEET PER SEC

O

N

D

0 12345678

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0

Measurements by U.S. Army Corps of Engineers (1983-84) 

Discharge = 1,400 + 900*LAR; r =0.62

Measurements made by U.S. Geological Survey (1997-98)

Discharge = 570 + 1,600*LAR, r = 0.73 (LAR is stage of the 

Lower Atchafalaya River at Morgan City)

2

2

EXPLANATION
32 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

recorded at all sites. Water-level gradients were 

steepest closest to the LAR and WLO and 

decreased rapidly with increasing distance from the 

two sources. Water-level gradients along the 

GIWW were most pronounced during the spring 

flood and were attenuated in the fall months. The 

September storms in 1998 raised stage along the 

entire study reach. Elevated Gulf of Mexico levels 

contributed to the high stage in the GIWW, by 

direct influence and by impeding flow of accumulated runoff from the heavy precipitation. 

East of Morgan City, more stage data were 

available from which to infer water-level gradients 

in the GIWW and adjacent water bodies. Monthlyaverage stage at the Avoca Island Cutoff Channel, 

Bayou Boeuf at railroad bridge at Amelia, and 

Bayou Penchant south of Morgan City were very 

similar and tracked stage of the LAR at Morgan 

City (fig. 19). Differences in stage between the 

LAR at Morgan City and these three sites were 

greater than 3 ft during the spring flooding, but 

decreased to less than 1 ft in the fall low-water season. The limited data available for the GIWW near 

Bay Wallace east of Morgan City during the spring 

floods for 1998-99 indicate stage is about 0.3-0.5 ft 

lower compared to the three sites farther west when 

the LAR at Morgan City was at flood stage. The 

difference in stage was greater in 1998, when the 

spring flood was greater, than in 1999. Along this 

segment of the GIWW, stage is lowest east of the 

HNC at Houma. There was a steeper rise in stage 

in the GIWW near Bay Wallace east of Morgan 

City in response to rising stage of the LAR than 

east of the HNC at Houma when the LAR at Morgan City was 4 ft or more above NAVD88. The 

water-level gradient between the two sites became 

steeper as the stage of the LAR at Morgan City 

increased. From August to December, water-level 

gradients among the sites in the western Terrebonne Basin were at their lowest. 

Monthly-average stage at sites along the 

GIWW between Morgan City and Houma 

responded to those of the LAR at Morgan City 

when the LAR at Morgan City was about 2.75 to 

3 ft or more above NAVD88 (fig. 20). In the 

GIWW east of HNC at Houma, the response was 

small, with an average increase of about 0.3 to 

0.5 ft as the LAR at Morgan City increased from 3 

to 7 ft above NAVD88. In contrast to the sites 

Figure 18. Average monthly salinity in the Gulf 

Intracoastal Waterway at the intersections with 

Houma Navigation Canal and Bayou Lafourche at 

Larose, Louisiana, from 1957-60, 1972-77, and 

1983-88.

In the GIWW at the intersection with the 

HNC and in Larose, salinity in the spring averaged 

about 0.2 ppt from 1957-60, prior to the construction of the HNC, and again more recently, from 

1983-88 (fig. 18). From 1972-77, salinity was 

higher in the spring, almost reaching 1 ppt. The 

flow of freshwater in the GIWW presumably lowered the salinity in the spring months during 1983-

88. In the fall, this freshwater flow was not available, and average monthly salinity was greater than 

1 ppt.

Stage and Water-Level Gradients

Flow in the GIWW is influenced by the differences in stage between the LAR or WLO and 

the interconnecting network of canals, bayous, and 

other surface-water bodies along and near the 

GIWW. Differences in stage and water-level gradients in the GIWW and adjacent waterways are vital 

to understanding flow in the GIWW. 

Stage at all the sites along the GIWW 

tracked the stage of the LAR at Morgan City 

(figs. 19-22). The spring flood on the LAR was Jan. Feb. Mar. Apr. May July Aug. Sept. Nov. Dec. Jan. Feb. Mar. Apr. May July Aug. Sept. Nov. Dec.

4

3

2

1

0

4

3

2

1

0

 AVERAGE SALINITY, IN 

PARTS PER THOUSAND

MONTHLY

Data source: U.S. Army Corp of Engineers, New Orleans District

EXPLANATION

1957-60 1972-77 1983-88

Intersection with Bayou Lafourche at Larose

Intersection with Houma Navigation Canal

Brackets show 1standard error ±
SURFACE-WATER HYDROLOGY 33

Figure 19. Monthly-average stage for the Lower Atchafalaya River at Morgan City, Avoca Island Cutoff 

Channel south of Morgan City, Bayou Penchant south of Morgan City, Bayou Boeuf at railroad bridge at 

Amelia, and in the Gulf Intracoastal Waterway (GIWW) near Bay Wallace east of Morgan City and east of 

Houma Navigation Canal at Houma, Louisiana, October 1996 to December 1999.

closer to the LAR, the response of stage in the 

GIWW east of the HNC in Houma was more variable, especially when the LAR at Morgan City was 

less than 5 ft above NAVD88. Stage in the GIWW 

east of HNC at Houma responded in a similar manner as stage in the HNC at Dulac. There appeared 

to be two relations between stage in the Avoca 

Island Cutoff Channel, Bayou Penchant south of 

Morgan City, Bayou Boeuf at railroad bridge at 

Amelia, and GIWW east of HNC at Houma, 

depending on whether the LAR was in flood or not. 

East of Houma, stage decreased toward 

Bayou Lafourche, both in Company Canal, and in 

the GIWW east of Bayou Lafourche (fig. 21). The 

site at Company Canal is located about a tenth of a 

mile from Bayou Lafourche and is about 7 mi 

upstream of the GIWW-Bayou Lafourche intersection. Stage in Company Canal was higher than in 

the GIWW at Bayou Lafourche. 

Stage also decreased southward from 

Houma along the HNC. The gradient in stage in 

the GIWW from Bayou Lafourche to Bayou Perot 

was very low. During spring peak flow of the 

LAR, the gradient was highest. High stage in September 1998 was characteristic of all sites in this 

part of coastal Louisiana. At sites near Morgan 

City, the spring peak in the LAR influenced stage 

more than at distant sites. When gradients are low, 

for example in the fall, a small error in datum 

rather than real differences in stage may be the 

cause of the observed gradient. Stage in the sites to 

the east of Houma appeared to track stage in the 

HNC at Dulac more closely than the LAR at Morgan City with the exception of the GIWW-Bayou 

Perot intersection (fig. 21).

Stage for sites between the WLO and Vermilion Bay near Cypremort Point are shown in figure 22. The record at all five sites is incomplete. 

Lower Atchafalaya River at Morgan City

Avoca Island Cutoff Channel south of Morgan City

Bayou Penchant south of Morgan City

Bayou Boeuf at railroad bridge at Amelia

GIWW near Bay Wallace east of Morgan City

GIWW east of Houma Navigation Canal at Houma

M

ONT

H

LY-AV

E

RAG

E

STAG

E, IN F

E

ET A

B

O

V

E NAV

D88

Break in line indicates missing data

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

0

1

2

3

4

5

6

7

8
34 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Moreover, stage in the GIWW west of Wax Lake 

Outlet near Calumet is from a gage surveyed to 

NAVD88, but not by the USGS. Measured stage 

may deviate from the actual stage by some 

unknown amount. Data from Vermilion Bay near 

Cypremort Point indicate the Gulf of Mexico levels. 

Stage in the GIWW west of WLO near Calumet was between 0.5 and 2 ft higher than east of 

Jaws Bay west of Franklin. In the GIWW between 

east of Jaws Bay west of Franklin and Cypremort, 

the maximum difference in stage that occurred during the annual river flood was about 1 ft. This difference did not occur in the fall and winter months 

during periods when the river was not in flood. 

Stage in the GIWW at Cypremort and in Vermilion 

Bay was similar. 

Monthly-average stages along the GIWW 

are plotted with stage for WLO at Calumet and for 

Vermilion Bay near Cypremort Point (fig. 20). 

Figure 20. Relation between monthly-average stage at selected sites east of Morgan City and monthlyaverage stage of the Lower Atchafalaya River at Morgan City and Houma Navigation Canal at Dulac; and 

between monthly-average stage at selected sites on the Gulf Intracoastal Waterway west of Wax Lake 

Outlet and monthly-average stage of Wax Lake Outlet at Calumet and Vermilion Bay near Cypremort 

Point, Louisiana, October 1996 to December 1999.

Between Lower Atchafalaya River

and Houma Navigation Canal East of Houma Navigation Canal

MONTHLY-AVERAGE STAGE, LOWER ATCHAFALAYA 

RIVER AT MORGAN CITY, IN FEET ABOVE NAVD88

MONTHLY-AVERAGE STAGE, HOUMA NAVIGATION 

CANAL AT DULAC, IN FEET ABOVE NGVD29

MONT

HLY-AVERAGE STAGE, IN FEET ABOVE NAV

D88

MONTHLY-AVERAGE STAGE, WAX LAKE 

OUTLET AT CALUMET, IN FEET ABOVE NAVD88

MONTHLY-AVERAGE STAGE, 

VERMILION BAY NEAR CYPREMORT

POINT, IN FEET ABOVE NAVD88

EAST OF MORGAN CITY WEST OF WAX 

LAKE OUTLET

EXPLANATION EXPLANATION

GIWW west of Wax Lake Outlet 

south of Calumet

GIWW east of Jaws Bay west

of Franklin

GIWW at Cypremort

Avoca Island Cutoff Channel

south of Morgan City

Bayou Penchant south of 

Morgan City

Bayou Boeuf at railroad 

bridge at Amelia

GIWW near Bay Wallace east 

of Morgan City

GIWW east of Houma Navigation 

Canal at Houma

EXPLANATION

Company Canal at Highway 1

at Lockport

GIWW east of Bayou Lafourche

GIWW intersection with Bayou Perot

1

2

3

4

0

0 1 2 3 4

1

2

3

4

0 0 1 2 34

1

2

3

4

0

0 1 234

1

2

3

4

0

0 1 2 3 4 5678

1

2

3

4

0

0 1 234 5678

1

2

3

4

0

0 1 234 5 678
SURFACE-WATER HYDROLOGY 35

Figure 21. Monthly-average stage for the Lower Atchafalaya River at Morgan City, the Houma Navigation 

Canal at Dulac, Company Canal at Highway 1 at Lockport, and in the Gulf Intracoastal Waterway (GIWW) 

east of Houma Navigation Canal at Houma, east of Larose, and at the intersection with Bayou Perot, Louisiana, October 1996 to December 1999.

Figure 22. Monthly-average stage for the Wax Lake Outlet at Calumet, Vermilion Bay near Cypremort 

Point, and in the Gulf Intracoastal Waterway (GIWW) west of Wax Lake Outlet south of Calumet, east of 

Jaws Bay west of Franklin, and at Cypremort, Louisiana, October 1996 to December 1999.

Lower Atchafalaya River at Morgan City

GIWW east of the Houma Navigation Canal at Houma

Company Canal at Highway 1 at Lockport

GIWW east of Bayou Lafourche 

GIWW at the intersection with Bayou Perot 

Houma Navigation Canal at Dulac (NGVD29)

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

0

1

2

3

4

5

6

7

8

MONTHLY-AVERAGE STAGE, IN FEET ABOVE NAVD88

Wax Lake Outlet at Calumet

GIWW west of Wax Lake Out south of Calumet

GIWW east of Jaws Bay west of Franklin

GIWW at Cypermort

Vermilion Bay near Cypremort

Break in line indicates missing data

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

0

1

2

3

4

5

6

7

8

MONTHLY-AVERAGE STAGE, IN FEET ABOVE NAVD88
36 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Stage increased in the GIWW west of WLO near 

Calumet from 2 to over 4 ft as stage at WLO at 

Calumet increased from 2 to 7 ft. A similar 

increase was evident in the GIWW east of Jaws 

Bay west of Franklin, but the magnitude was 

smaller. As WLO at Calumet stage increased by 

4 ft, stage in the GIWW east of Jaws Bay west of 

Franklin increased by less than 1 ft. Stage in the 

GIWW at Cypremort did not track WLO at Calumet stage fluctuations; it tracked stage in Vermilion 

Bay near Cypremort Point. The two sites on either 

side of Jaws Bay also responded to changes in Vermilion Bay stage. Two separate relations between 

stage in the GIWW east of Jaws Bay west of Franklin and stage in Vermilion Bay near Cypremort 

Point were offset by about 0.5 ft, presumably 

depending on whether the LAR was in flood or not. 

Stage in the GIWW west of WLO near Calumet 

always increased as stage at Vermilion Bay near 

Cypremort increased, but stage was often much 

higher than would have occurred if Vermilion Bay 

was the only factor affecting stage in the GIWW.

The differences in stage between the LAR 

and WLO and between the GIWW and adjacent 

surface-water bodies, strongly influence the local 

and regional surface-water hydrology of large parts 

of coastal Louisiana. The stage differences, and 

the resulting flows affect the north-south, and eastwest movement of water, much of it fresh, in major 

bayous and canals that criss-cross the wetlands. 

Long-term measurements of stage and discharge in 

the GIWW and adjacent surface-water bodies are 

needed to more fully characterize the surface-water 

hydrology and to better incorporate the flows into 

the ongoing efforts relating to coastal wetland restoration. This is perhaps most easily achieved if 

stage and discharge measurements in the GIWW 

are integrated into long-term monitoring programs 

of coastal Louisiana waters. Studies are also 

needed to understand how the water flowing in the 

GIWW may affect the wetlands. To date, the effect 

of the freshwater flow and associated water-quality 

in the GIWW for the integrity and health of adjacent wetlands is largely unknown. 

Selected Water-Quality Constituents

The water flowing in the GIWW carries sediment and generally is fresh. The freshwater moderates salinity in receiving water bodies. This 

effect decreases with distance from the sources, the 

LAR and WLO, and with increasing distance from 

the GIWW. In rare instances when Gulf of Mexico 

water intrudes inland, the GIWW can also move 

more saline water laterally along the coast. 

Suspended Sediment

Suspended-sediment concentrations and discharge data are summarized in table 6. On the east 

side of the LAR, average suspended-sediment concentrations in the Avoca Island Cutoff Channel and 

Bayou Penchant south of Morgan City were about 

50 to 70 mg/L higher than at the next four sites to 

the east, averaging 162 mg/L (table 6). Suspendedsediment discharge ranged from 74 to 7,440 tons/d 

at Bayou Penchant south of Morgan City and was 

as much as 22,900 tons/d at the Avoca Island Cutoff Channel. Discharge at the latter site was variable. Bayou Boeuf at railroad bridge at Amelia 

consistently had the lowest suspended-sediment 

concentrations among all sites sampled. Concentrations ranged from 47 to 175 mg/L but averaged 

91 mg/L. Suspended-sediment discharge ranged 

from 2,990 tons/d moving into the Verret Subbasin 

to 4,250 tons/d moving out of the basin. In the 

GIWW near Bay Wallace east of Morgan City, and 

the GIWW west of the HNC at Houma, suspendedsediment concentrations averaged 114 and 

108 mg/L, respectively. In the GIWW near Bay 

Wallace east of Morgan City, an average of 

1,610 tons/d of suspended sediment were transported eastward, with about one-fourth of the suspended-sediment discharge entering at the Avoca 

Island Cutoff Channel.

In the GIWW west and east of Bayou 

Lafourche at Larose, suspended-sediment concentrations averaged about 134 and 161 mg/L, respectively. The higher average concentrations of 

suspended sediment to the east of Bayou Lafourche 

are likely due to the inflow and eastward movement of water from Bayou Lafourche. Average 

concentration is about 45 percent higher than in the 

GIWW near Bay Wallace east of Morgan City. 

Average suspended-sediment discharge was about 

1,150 tons/d flowing eastward into Barataria Basin, 

with maximums of 2,500 to 3,240 tons/d. 

Suspended-sediment concentrations at the 

two sites between WLO and the LAR were similar, 

about 172 to 183 mg/L. On average, about half the 

sediment discharge entering the GIWW from the 

LAR left the GIWW through the WLO. 
SURFACE-WATER HYDROLOGY 37

Table 6. Suspended-sediment concentrations and discharge at selected sites along or near the Gulf 

Intracoastal Waterway (GIWW) in south-central Louisiana, 1997-99

[Negative sign indicates discharge is toward Morgan City or to the north. mg/L, milligrams per liter]

Suspended sediment

Map 

number 

(pl. 1)

Site name

Number 

of 

samples

Concentration (mg/L) Discharge (tons per day)

 Average 

(±standard

 deviation)

Range Average

(±standard 

deviation)

Range

MinimumMaximumMinimumMaximum

East of Lower Atchafalaya River

Between Lower Atchafalaya River and Houma Navigation Canal

10 Avoca Island Cutoff Channel 

south of Morgan City

17 164±64 79 273 6,540±6,490 65 22,900

11 Bayou Penchant south of 

Morgan City

13 161±64 103 270 1,470±1,164 74 7,440

12 Bayou Boeuf at railroad 

bridge at Amelia

14 91±35 47 175 143±1,910 -2,990 4,250

13 GIWW near Bay Wallace east 

of Morgan City

15 114±33 68 180 1,610±1,050 -421 3,050

16 GIWW west of Houma Navigation Canal at Houma 15 108±30 75 182 1,760±1,120 187 4,320

East of Houma Navigation 

Canal

17 GIWW east of Houma Navigation Canal at Houma 15 117±41 68 225 994±550 -345 1,700

20 GIWW west of Bayou 

Lafourche at Larose

14 134±62 74 246 1,020±570 306 2,500

21 GIWW east of Bayou 

Lafourche at Larose

14 161±62 80 267 1,230±827 22 3,240

West of Lower Atchafalaya River

Between Lower Atchafalaya River and Wax Lake 

Outlet

8 GIWW at Lower Atchafalaya 

River south of Morgan City

13 183±69 85 286 8,690±6,650 900 19.900

7 GIWW east of Wax Lake Outlet near Calumet15 172±42 105 248 4,550±2,320 1,840 10,900

West of Wax Lake Outlet

5 GIWW west of Wax Lake 

Outlet south of Calumet

13 179±66 87 332 5,130±3,530 1,731 11,800

4 GIWW east of Jaws Bay west 

of Franklin

13 172±62 106 307 4,130±3,420 1,020 14,100

3 GIWW west of Jaws Bay near 

Cypremort

13 167±55 110 290 1,670±1,570 572 6,390

2 GIWW at Cypremort 12 191±65 127 325 509±1,440 -2,480 2,660
38 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Suspended-sediment concentrations averaged 179 to 172 mg/L at sites in the GIWW west of 

WLO south of Calumet and the GIWW east of 

Jaws Bay west of Franklin, respectively. Concentrations greater than 300 mg/L were measured at 

least once at the two sites. The average suspendedsediment discharge was about 15 percent less in the 

GIWW east of Jaws Bay west of Franklin compared to the GIWW west of WLO south of Calumet. In the GIWW west of Jaws Bay near 

Cypremort, average suspended-sediment concentrations were 167 mg/L. In the GIWW at Cypremort, average suspended-sediment concentrations 

were 191 mg/L. Reversed flow occurred here, with 

about 2,500 tons/d of suspended sediment moving 

east on at least one occasion. 

 Suspended sediment is important to the 

maintenance of wetlands. It provides nourishment 

to wetland plants and building materials for soil. 

The GIWW carries a substantial amount of suspended sediment to points east and west of Morgan 

City that may be of use in wetland restoration 

projects. The data collected from this study indicate that in addition to sediments from the Atchafalaya River, local sources of inflow such as Bayou 

Boeuf and Bayou Lafourche influence suspendedsediment concentrations in the GIWW. Resuspension of sediments presumably becomes more 

important farther away from the LAR and WLO, 

but the sustained flow in the GIWW also will help 

in moving the resuspended sediment.

Salinity 

Sites where salinity was measured in the 

study area were (1) directly on the GIWW, (2) 

hydrologically connected and in close proximity to 

the GIWW, and (3) only indirectly connected to the 

GIWW. Data were collected at some sites during 

most of the study, whereas data collection was discontinued or began operation at other sites during 

the study. Most sites were operated by the USGS. 

Data from sondes installed by DNR in mid-summer of 1998 on Bayou Penchant at Brady Canal, in 

the GIWW at a point about 5 mi east of Larose, and 

at the GIWW intersection with Bayou Perot also 

were used in this study.

During high stage of LAR at Morgan City, 

salinity in the GIWW east of HNC at Houma and 

east of Bayou Lafourche intersection at Larose 

generally was about 0.2 to 0.6 ppt (pl. 1). In the 

fall, salinity was mostly low, but increased briefly 

on a frequent basis, at times to 10 ppt. In the fall of 

1997 and 1999, the salinity increases were of 

longer duration than in the fall of 1998. Salinity in 

the HNC at Dulac had a similar seasonal pattern, 

but the brief salinity increases were higher, and 

more frequent. In 1996 and 1998, salinity did not 

decrease as much between the brief increases and 

the salinity was higher during the increases than in 

the intervening 2 years. Although less frequent, 

salinity increased even during periods of high stage 

of the LAR at Morgan City, most noticeably in 

April 1997, when stage of the LAR at Morgan City 

was near 8 ft above NAVD88.

Salinity was related to stage of the LAR at 

Morgan City at sites adjacent to and east of the 

LAR at Morgan City and distant from the GIWW 

(pl. 1). Salinity at Bayou du Large near Theriot 

decreased to about 1 ppt, when stage of the LAR at 

Morgan City was elevated. Salinity at Bayou Penchant at Brady Canal had brief increases of less 

than 5 ppt occurring in the fall, but otherwise was 

low. At Caillou Lake (Sister Lake) southwest of 

Dulac, salinity peaked at 15 to 25 ppt in the fall, 

when the stage of the LAR at Morgan City was 

low. When the LAR at Morgan City was in flood 

in the spring of 1997-98, salinity decreased to less 

than 4 ppt for extended periods.

At sites between the HNC and Bayou 

Lafourche, salinity was similar and responded less 

to changes in stage of the LAR at Morgan City than 

at the previously discussed sites with the exception 

of Bayou L’Eau Bleu (pl. 1). At this site in 1996-

97, salinity was low when the LAR was in flood 

but increased in the fall. 

Two sites, Vermilion Bay near Cypremort 

Point and Côte Blanche 4, provided the only available salinity data from the west side of the study 

area (pl. 1). Côte Blanche 4 is located in the wetlands east of Jaws Bay and south of the GIWW. 

Salinity was low during peak stage of the LAR at 

Morgan City in the spring. In 1997-98, salinity 

decreased to almost zero. In 1998, salinity started 

increasing before the river stage decreased. In 

mid-summer 1999 when the LAR was in flood, 

salinity was 2 to 3 ppt. In the fall of 1999, salinity 

reached the highest concentration for the study 

period at the two sites.
SURFACE-WATER HYDROLOGY 39

Salinity in the study area is affected by 

freshwater inflow from the LAR and WLO through 

the GIWW and by local precipitation and runoff. 

The freshwater from these sources mixes with saltwater from the Gulf of Mexico. The influence of 

river stage and precipitation on salinity was evaluated first by plotting monthly-average values of 

salinity at selected sites in relation to monthlyaverage stage of the LAR at Morgan City and 

WLO and monthly total precipitation. Sites were 

grouped depending on whether they were (1) on or 

near the GIWW or (2) at some distance from the 

GIWW. Next, pair-wise correlation coefficients 

between monthly-average salinity at a given site 

and stage of the LAR at Morgan City, monthly total 

precipitation at Morgan City, and the PHDI value 

were established (table 7). Only sites with the 

most complete data record were used for this analysis. 

Salinity at all four sites on or near the 

GIWW, both east and west of the LAR, was less 

than 4 ppt whenever the LAR was in flood 

(fig. 23). Water in the GIWW east of HNC at 

Houma and Côte Blanche 4 remained fresh during high river stage in 1997-1999, but salinity 

became slightly elevated in September, October, 

and November. Salinity was not as high in the 

fall of 1998 as in the fall of 1997 or 1999. Salinity at the HNC at Dulac and at Vermilion Bay near 

Cypremort Point ranged from 8 to 15 ppt during 

the fall of 1998. Salinity at both sites did not 

decrease as much in 1999 as in 1998. In late fall 

1999, salinity was between 12 and 14 ppt. In Vermilion Bay near Cypremort Point, the salinity pattern appeared to be low in late winter and high in 

September and October in response to, first, the 

spring peak in river flow, and then, the annual 

peak Gulf of Mexico level in the fall. The pattern changed in 1998 when salinity in September 

and October was lower than the salinity in July. 

The relatively low salinity at all four sites in fall 

1998 compared to fall of 1997 and 1999 was most 

likely due to the excess precipitation that fell 

along Louisiana’s coast during three storms in late 

September and early October 1998. Salinity 

increased for June and July, after the spring flood 

of the Mississippi and Atchafalaya Rivers 

receded. Salinity reached a maximum during the 

fall. Salinity increased from 2 ppt to greater than 

10 ppt from July to August 1999 at Vermilion Bay 

near Cypremort Point; the sensitivity to lack of 

rain may be seen by the prolonged high salinity in 

the late fall months of 1999 (fig. 24). Even 

though precipitation in October was high, the 

south-central Climate Division was in a moderate 

to extreme drought during the entire fall (see fig. 

4). Cumulative annual rainfall was substantially 

below average for 1996 and 1999 and above average for the first few months of 1997 and in the 

mid-summer of 1998 (fig. 24).

Only data from the east side of the LAR 

were available to evaluate response of salinity to 

freshwater inflows at some distance from the 

GIWW (fig. 23). Salinity was similar and 

decreased at the three sites during the first six 

months of 1997. In mid-summer 1997 salinity was 

the lowest measured during the study. In 1998-99, 

salinity increased from a mid-winter low to a high 

in May and June. In the fall of 1998 and 1999, 

salinity increased gradually from September to 

December at Bayou Terrebonne at Bush Canal, 

south of Chauvin and at Grand Bayou tributary 

west of Galliano. This was unusual. Typically, 

salinity follows a bell-shaped curve for the fall 

months, such as occurred in 1996-97 for all three 

sites. The high rainfall that occurred in September 

1998 probably decreased salinity at these sites at 

this time. The lack of rain during the year likely 

underlay the atypical pattern of salinity seen at all 

sites in fall 1999. Salinity increased steadily after 

August and continued through December at sites 

distant from the GIWW (fig. 23), or increased rapidly and remained high at sites on or near the 

GIWW (fig. 23). Salinity during the first 5 months 

of 1999 was higher than during comparable times 

in the 2 previous years.

Caillou Lake (Sister Lake) southwest of 

Dulac is located between Four League Bay to the 

west and the HNC to the east. Salinity at this site 

ranged from 3 to about 20 ppt and was influenced 

by the stage and discharge of the LAR and seasonal 

variations in Gulf of Mexico levels. Typically 

salinity peaked in the spring and again in the fall. 

Annual highs occurred during the fall peak. In 

1996 and 1999, the fall peak was 2 to 4 ppt higher 

than in the intervening 2 years. Lowest monthlyaverage salinity occurred during the mid-summer 

low in 1997, after the record flood in spring 1997. 

The lowest salinity in 1999 occurred in August and 

was more than twice the lowest salinity in 1997, 

which occurred in July.
40 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Table 7. Correlation matrix for monthly-average salinity at selected sites, monthly-average stage of the Lower Atchafalaya River at Morgan City, Louisiana, monthly total precipitation at Morgan City, and Palmer Hydrological Drought Index [Individual Pearson correlation coefficient is the top number and significance level is the bottom number. A significant difference is indicated when the significance level is less than 0.05. Bold result indicates a significant correlation with the variable in column 2. GIWW, Gulf Intracoastal Waterway; NAVD88, North American Vertical Datum of 1988; <, less than] Map number (pl. 1) Variable, monthly-average salinity, in parts per thousand, at indi- cated site Monthly-average salinity (parts per thousand) Monthly-aver- age stage, Lower Atcha- falaya River at Morgan City (feet above NAVD88) Monthly total precipitation at Morgan City (inches) Palmer Hydro- logical Drought Index GIWW east of Houma Navigation Canal at Houma Vermilion Bay near Cypremort Point Houma Navigation Canal at Dulac Bayou Terrebonne at Bush Canal, south of Chauvin Caillou Lake (Sister Lake) southwest of Dulac Grand Bayou tributary west of Galliano 17 GIWW east of Houma Navigation Canal at Houma 0.65 <.001 0.87 <.001 0.62 <.001 0.67 <.001 0.08 .680 -0.49 .004 0.105 .607 -0.30 .093

1 Vermilion Bay near 

Cypremort Point

.73

<.001

.60

<.001

.64

<.001

.22.242

-.79

<.001

-.03

.856

-.61

<.001

29 Houma Navigation 

Canal at Dulac

.67

<.001

.78

<.001

.20.251

-.66

<.001

-.02

.92

-.34

0.037

28 Bayou Terrebonne at 

Bush Canal, south of Chauvin

.91

<.001

.78

<.001

-.49

.002

-.13

.442

-.51

<.001

30 Caillou Lake (Sister 

Lake) southwest of Dulac

.57

<.001

-.63

<.001

-.15

.380

-.42

.010

26 Grand Bayou tributary 

west of Galliano

-.30

.079

-.05

.769

-.53

<.001
SURFACE-WATER HYDROLOGY 41

Figure 23. Relation between monthly-average salinity at selected sites in south-central Louisiana and 

monthly total precipitation at Morgan City and monthly-average stage of Lower Atchafalaya River at Morgan City, October 1996 to December 1999.

10

0

5

15

20

MONT

HLY TOTAL 

PRECIPITATION, IN INCHES MONTHLY-AVERAGE SALINITY, IN PARTS PER THOUSAND

MONTHLY-AVERAGE STAGE, IN FEET ABOVE NAVD88

SITES ON OR NEAR THE GULF INTRACOASTAL WATERWAY

SITES AT SOME DISTANCE FROM THE GULF INTRACOASTAL WATERWAY

OND J J FMAM J J ASON D D J FMAM J ASOND J FMAM J J ASON

OND J JASOND J FMAMJ J A SON D D J FMAMJ FMAMJ J ASON

OND J J FMAM J J ASON D D J FMAM J ASOND J FMAM J J ASON

Salinity, Gulf Intracoastal Waterway east of Houma 

Navigation Canal at Houma

Salinity, Houma Navigation Canal at Dulac

Salinity, C te Blanche 4

Salinity, Vermilion Bay near Cypremort Point

^

Salinity, Grand Bayou tributary west of Galliano

Salinity, Bayou Terrebonne at Bush Canal, south of Chauvin

Salinity, Caillou Lake (Sister Lake) southwest of Dulac

Stage, Lower Atchafalaya River at Morgan City

EXPLANATION
42 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Bayou Terrebonne and Grand Bayou are 

located to the east of the HNC and to the south of 

the GIWW. Grand Bayou tributary west of Galliano is mostly cut off from freshwater sources 

except for rain and some inflow through Bayou 

L’Eau Bleu through the GIWW. Company Canal 

connects Bayou Terrebonne at Bush Canal, south 

of Chauvin with the GIWW at Houma. Bayou Terrebonne at Bush Canal, south of Chauvin is also 

connected indirectly to the HNC near Dulac and 

Bayou Grand Caillou by channels crossing Lake 

Boudreaux. In 1997, salinity at the two sites was 

similar. Salinity in Grand Bayou tributary west of 

Galliano was slightly higher during early summer; 

salinity in Bayou Terrebonne was higher during the 

fall. In May and June 1998, salinity was 2 to 5 ppt 

higher than the previous year at the same time. At 

Bayou Terrebonne at Bush Canal, south of Chauvin, salinity increased from July to December 

1998. The fall peak salinity was less than the previous year, probably due to local runoff. At Grand 

Bayou tributary west of Galliano, salinity increased 

from the fall of 1998 through April and May 1999 

but then decreased through June and July 1999.

Monthly-average salinity at the sites with the 

most complete record from both sides of the study 

area were plotted against monthly-average stage of 

the LAR at Morgan City and the HNC at Dulac 

(fig. 25) to determine the influence of river stage 

and Gulf of Mexico levels, respectively. Salinity at 

sites on or near the GIWW, east of the HNC at 

Houma, HNC at Dulac, Côte Blanche 4, and Vermilion Bay near Cypremort Point decreased with 

increases in stage of the LAR at Morgan City. 

When the LAR at Morgan City was 2 ft or more 

above NAVD88, monthly-average salinity in the 

GIWW east of HNC at Houma did not increase 

much above 1 ppt. With one exception, monthlyaverage salinity at HNC at Dulac was 2 ppt or less 

when the LAR at Morgan City was 3 ft or more 

above NAVD88. As stage of the LAR at Morgan 

City increased to 7 ft above NAVD88, salinity did 

not exceed progressively smaller values for Vermilion Bay near Cypremort Point. When the LAR at 

Morgan City was 5 ft or more above NAVD88, 

salinity was never greater than 3 ppt. However, 

salinity was frequently lower than a direct correlation with stage would predict, indicating that aside 

from LAR water, freshwater runoff and Gulf of 

Mexico stage also influenced salinity in Vermilion 

Bay near Cypremort Point. 

Stage of the LAR at Morgan City affected 

salinity at Caillou Lake (Sister Lake) southwest of 

Dulac and salinity at Vermilion Bay near Cypremort Point in much the same way. As stage of the 

LAR at Morgan City increased, maximum salinity 

decreased. Salinity at the other three sites at some 

distance from GIWW did not respond to changing 

stage of the LAR at Morgan City as directly, 

increasing and decreasing as stage increased at 

most stages of the LAR at Morgan City (fig. 25). 

However, salinity at all three sites was consistently 

higher when the LAR at Morgan City was 7 ft 

above NAVD88 than when the LAR at Morgan 

City was 5 ft above NAVD88. More data could aid 

in verifying this relation. There may be an indirect 

effect of seasonally higher Gulf of Mexico levels 

along the coast of Louisiana. Discharges from the 

Mississippi River and the LAR are highest in April 

and May. Onshore winds last for several weeks 

during much of the same time and push Gulf of 

Mexico water into estuaries along the coast (Byrne 

and others, 1976). Freshwater input in the form of 

precipitation and variations in Gulf of Mexico level 

affected salinity at all four sites. At Grand Bayou 

tributary west of Galliano and at Bayou Terrebonne 

at Bush Canal, south of Chauvin, there was a relation below which salinity did not decrease as stage 

of the HNC at Dulac increased. Such a relation 

was less apparent at Bayou Jean Lacroix near Montegut, La.

Figure 24. Cumulative monthly precipitation by 

year (1996-99) and average (1961-90) for the 

South-Central Climatological Division in Louisiana. CUMULATIVE MONTHLY PRECIPITATION, IN INCHES

0

10

20

30

40

50

60

70

Jan.

Feb.

Mar.

Apr.

May

July

Aug.

Sept.

Nov.

Dec.

Data from Louisiana State Office of Climatology

1996

1997

1998

1999

Average (1961-1990)
SURFACE-WATER HYDROLOGY 43

Table 7 lists pair-wise regression coefficients 

for monthly-average salinity at selected sites and 

monthly-average stage of the LAR at Morgan City, 

monthly total precipitation at Morgan City, and the 

PHDI. Pair-wise regression coefficients also are 

listed for monthly-average salinity among sites. 

Salinity at Vermilion Bay at Cypremort Point and 

Caillou Lake (Sister Lake) southwest of Dulac correlated significantly with stage of the LAR at Morgan City. Vermilion Bay near Cypremort Point 

was sensitive to drought as well, as indicated by 

the significant negative correlation between 

monthly-average salinity and PHDI. Salinity at 

Grand Bayou tributary west of Galliano showed 

the highest correlation with the PHDI and the least 

with stage of the LAR at Morgan City, which indicates isolation from the GIWW freshwater and 

strong climatic influence on salinity. Monthly total 

precipitation and salinity were not significantly 

correlated at any stations. There were significant 

correlations in monthly-average salinity among all 

stations.

Figure 25. Relation between monthly-average salinity at eight sites in coastal Louisiana and monthlyaverage stage of Lower Atchafalaya River at Morgan City and Houma Navigation Canal at Dulac, Louisiana, October 1996 to December 1999.

10

0

5

15

20

0 1 2 3 4 5 6 7 8

10

0

5

15

20

0 1 23456 78

10

0

5

15

20

0.0 0.5 1.0 1.5 2.0 2.5

10

0

5

15

20

0.0 0.5 1.0 1.5 2.0 2.5

SITES ON OR NEAR THE GULF INTRACOASTAL WATERWAY

SITES AT SOME DISTANCE FROM THE GULF INTRACOASTAL WATERWAY

MONTHLY-AVERAGE STAGE, 

LOWER ATCHAFALAYA RIVER AT

 MORGAN CITY, IN FEET ABOVE NAVD88

MONTHLY-AVERAGE STAGE, 

LOWER ATCHAFALAYA RIVER AT

 MORGAN CITY, IN FEET ABOVE NAVD88

MONTHLY-AVERAGE STAGE, 

HOUMA NAVIGATION CANAL AT 

DULAC, IN FEET ABOVE NGVD29

MONTHLY-AVERAGE STAGE, 

HOUMA NAVIGATION CANAL AT 

DULAC, IN FEET ABOVE NGVD29

Grand Bayou tributary west of Galliano

Bayou Terrebonne at Bush Canal south 

of Chauvin

Caillou Lake (Sister Lake) southwest of Dulac

Bayou Jean Lacroix near Montegut

EXPLANATION

MONTHLY-AVERAGE SALINITY, 

IN PARTS PER THOUSAND

MONTHLY-AVERAGE SALINITY, 

IN PARTS PER THOUSAND

Gulf Intracoastal Waterway east of 

Houma Navigation Canal

Houma Navigation Canal at Dulac

EXPLANATION

Côte Blanche 4

Vermilion Bay near Cypremort Point
44 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

The GIWW influences salinity in a wide part 

of coastal Louisiana and in many instances in a 

predictable manner. The significant correlations 

between salinity at selected sites and stage of the 

LAR have important implications for managing 

and restoring coastal wetlands in Louisiana. The 

relations between salinity and stage of the LAR can 

be used with relations presented previously in this 

report between stage of the LAR and discharge at 

selected sites to better understand availability of 

freshwater inflow or lack of inflow at specific locations along the Louisiana coast.

SUMMARY AND CONCLUSIONS

The flow of freshwater and suspended sediments from the Lower Atchafalaya River (LAR) 

and Wax Lake Outlet (WLO) into and along the 

Gulf Intracoastal Waterway (GIWW) and selected 

adjacent surface-water bodies between Cypremort 

and Larose in south-central Louisiana from October 1996 to December 1999 was characterized 

using instantaneous and continuous discharge 

measurements, and measurements of suspendedsediment concentrations. The GIWW parallels the 

entire Louisiana coast near the wetland/upland 

interface. Following natural hydraulic gradients, 

the GIWW captures water from the southward 

flowing LAR and the WLO where it crosses those 

waterways, and distributes the freshwater and sediment to points east and west. Water-level gradients of surface waters in the study area were 

determined and the potential for flow evaluated. 

The influence of freshwater flow in the GIWW on 

current and historical salinity patterns in the 

GIWW and adjacent surface-waters also was evaluated.

The LAR was the primary source of water 

to the GIWW east of Morgan City. East of Morgan City, an average of 12,200 ft3

/s (cubic feet 

per second) of water flowed from the LAR 

directly into the Avoca Island Cutoff Channel. 

Measured discharge ranged from 267 to 

31,100 ft3

/s entering the Avoca Island Cutoff 

Channel. Drainage from the Verret Subbasin 

through Bayou Boeuf at railroad bridge at Amelia contributed an average of 1,000 ft3

/s to flow 

east of Morgan City. Measured discharge ranged 

from 9,110 ft3/s entering to 15,300 ft3

/s draining 

the Verret Subbasin. Bayou Penchant south of 

Morgan City, connecting to the Avoca Island Cutoff Channel south of the GIWW, drained on average 3,250 ft3

/s of varying proportions of LAR 

and Verret Subbasin water in a southeasterly 

direction. A combination of LAR and Verret 

Subbasin water flowed east in the GIWW near 

Bay Wallace east of Morgan City and west of the 

HNC (Houma Navigation Canal) at Houma, averaging about 5,700 ft3

/s and frequently reaching at 

least 8,000 ft3/s at both sites. 

Average discharge in the GIWW east of the 

HNC at Houma was about 2,610 ft3

/s in an eastward direction, and ranged from 1,100 ft3/s flowing west toward the HNC to 4,640 ft3/s flowing 

east toward the intersection of the GIWW. Discharge in the GIWW east of Bayou Lafourche at 

Larose averaged 2,220 ft3/s, and peaked at 

5,320 ft3/s, also flowing to the east. Flow 

towards Houma was observed on two occasions, 

with a maximum discharge of 1,860 ft3/s. At 

sites west of the HNC, discharge in the GIWW 

increased predictably with increasing LAR stage. 

This was not the case to the east of the HNC.

Flow was always to the west between the 

LAR and WLO. Average discharge was about 

13,400 ft3

/s in the GIWW at LAR south of Morgan City. The highest discharge measurement of 

any synoptic trip was made here, at 35,200 ft3/s. 

Discharge increased predictably with increasing 

LAR stage. In the GIWW east of WLO near 

Calumet, average discharge was 9,460 ft3/s. Discharge did not appear to increase with increasing 

LAR stage beyond 5 ft above NAVD88, indicating increasing volumes of water flowed south 

toward Atchafalaya Bay when this condition was 

met.

Flow was always to the west in the 

GIWW west of WLO south of Calumet, and discharge ranged from 2,050 to 20,300 ft3

/s with an 

average of 9,460 ft3

/s. Flow was to the west in 

the GIWW east of Jaws Bay west of Franklin, 

and average discharge was 8,230 ft3

/s, with a 

maximum of 17,000 ft3

/s. Discharge at both 

these locations on the GIWW increased predictably with increasing stage of the LAR at Morgan City. In the GIWW west of Jaws Bay near 

Cypremort, average discharge was 3,310 ft3/s 

and flow was always to the west. In the GIWW 

at Cypremort, average discharge was 1,350 ft3

/s 

to the west, but discharge as high as 2,830 ft3/s 

was measured flowing eastward to Jaws Bay.
SUMMARY AND CONCLUSIONS 45

Discharge along the GIWW was highest 

where it crossed the LAR and WLO. Water 

flowed east and west in the GIWW, in decreasing volumes with increasing distance from the 

source. Discharge tended to increase as sameday average stage of the LAR at Morgan City 

increased beyond 3 ft (feet) above NAVD88 at 

most of the sites between HNC and Jaws Bay. 

At sites in the GIWW east of HNC, discharge 

did not increase predictably. For all measurements made when the LAR at Morgan City was 

3 ft or more above NAVD88, average discharge 

was about 3,100 ft3

/s in the GIWW east of HNC 

at Houma and 2,880 ft3/s east of Bayou 

Lafourche at Larose. When the LAR at Morgan 

City reached 3 to 4 ft above NAVD88, discharge in the GIWW became more predictable. 

The flow of freshwater along most of the GIWW 

studied was enhanced not so much by absolute 

high stage of the LAR at Morgan City as by the 

total time the LAR at Morgan City was 3 ft or 

more above NAVD88. The LAR at Morgan City 

is 3 ft or more above NAVD88 for about 7 

months in a normal year. 

The GIWW not only moves freshwater 

along the coast, but also carries saltwater when 

there are several days of reversed flow in northto-south trending ship channels such as the HNC. 

Saltwater then can flow laterally in the GIWW 

when normal hydraulic gradients resume and 

reach areas which contain marsh plants that have 

a low tolerance for salt. 

Localized runoff and drainage contributed 

to flow along segments of the GIWW on both 

sides of Morgan City. When the LAR at Morgan City was less than 3 ft above NAVD88, 

water flowed along the prevailing gradients, to 

the east between Bay Wallace and the HNC and 

to the west between WLO and Jaws Bay. However, frequently more water left the GIWW at 

the downstream end of the segments than 

entered, indicating that flow was sustained by 

water draining from areas adjacent to the GIWW. 

Substantial volumes of water left the GIWW 

through the HNC and at the opening to Jaws 

Bay. As local runoff increased, the volume of 

water leaving the GIWW through the HNC 

increased, whereas at Jaws Bay, the volume of 

water leaving the GIWW increased steadily as 

water entering the GIWW west of WLO near 

Calumet increased. As local runoff increased, 

the GIWW between Houma and Bayou 

Lafourche shifted from storing to draining water. 

In contrast, the GIWW between Jaws Bay and 

Cypremort stored increasing volumes of water as 

runoff increased. Here, there was a net inflow 

of water at low LAR stage which decreased rapidly and steadily as the volume of river water 

entering at WLO increased.

The flow of water from the LAR to the 

GIWW has changed over time. Aggradation of 

the bed of the LAR since the 1950’s has raised 

water levels where the river joins the Avoca 

Island Channel Cutoff by about 2 to 2.5 ft. The 

aggraded riverbed has increased the time when 

the stage of LAR at Morgan City is at least 3 ft 

above NAVD88. Lateral flow in the GIWW 

presently (1999) occurs for longer periods than in 

the 1950's. Infilling of the riverbed also has 

increased the absolute difference in stage 

between the LAR and adjacent waters, so that the 

GIWW predictably can move water to points farther away from the source. During 1997-99, 

more water was flowing in the GIWW west of 

HNC at Houma relative to a given stage of the 

LAR at Morgan City than at the same location in 

1982-83. 

Flow in the GIWW results from differences in stage along the GIWW and adjacent surface-water bodies. In the coastal zone, stage 

narrowly fluctuates around Gulf of Mexico levels most of the time. Where the GIWW crosses 

the LAR and WLO, stage typically reaches 3 to 4 

ft above NAVD88. Stage differences between 

LAR and the rest of the coast induce flow in the 

GIWW and allow LAR water to penetrate distant reaches of the coast for several months each 

year. Monthly-average stage at sites on the 

GIWW east of Morgan City to the HNC and 

west of Morgan City to Jaws Bay tracked stage 

of the LAR reasonably well. The correlation was 

better and the response greater closer to the LAR. 

The spring flood of the LAR was the dominant influence on flow in and near the GIWW, raising water levels high above levels in surrounding 

waters. Seasonal and event-driven fluctuations in 

Gulf of Mexico levels changed stage along the 

coast and impacted gradients. Stage in and near the 

GIWW also was affected by winds, precipitation, 
46 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

and storms. Together, the factors influenced waterlevel gradients along the GIWW and controlled the 

direction and movement of water from the LAR to 

the GIWW and from the GIWW to adjacent 

waters.

East of the LAR, suspended-sediment concentrations averaged about 162 mg/L (milligrams 

per liter) at the two sites closest to the LAR, at the 

Avoca Island Cutoff Channel and at Bayou Penchant south of Morgan City. The water entering 

the GIWW through Bayou Boeuf at railroad bridge 

at Amelia averaged 91 mg/L of suspended sediments. The water in the GIWW near Bay Wallace 

east of Morgan City to the HNC was intermediate 

in concentrations (108 to 114 mg/L), reflecting the 

mixture of the two water types. Concentrations of 

suspended sediment in the GIWW near Bay Wallace east of Morgan City, and on both sides of the 

HNC were comparable, but in the GIWW east of 

Larose they increased by about 45 percent. To the 

west of WLO, suspended-sediment concentrations 

averaged between 167 and 191 mg/L. 

Freshwater flow affected salinity patterns in 

the GIWW and areas immediately adjacent to it 

in the study area. Monthly-average salinity at 

many coastal sites was correlated with monthlyaverage stage of the LAR at Morgan City. In the 

GIWW east of HNC at Houma and at Côte 

Blanche 4 salinity was near zero whenever stage 

of the LAR at Morgan City was 2 ft or more 

above NAVD88. At HNC at Dulac and Vermilion Bay near Cypremort Point, maximum 

monthly-average salinity progressively decreased 

as stage of the LAR at Morgan City increased. 

When the LAR at Morgan City was 5 ft or more 

above NAVD88, salinity remained below 3 parts 

per thousand. At these four sites, salinity during 

the fall months was appreciably higher than during the rest of the year. Salinity patterns at sites 

distant from the GIWW--Caillou Lake (Sister 

Lake), HNC at Dulac, and Vermilion Bay near 

Cypremort Point--were similar, with maximum 

salinity decreasing with increasing stage of the 

LAR at Morgan City. 

The differences in stage between the intersections of the LAR and WLO with the GIWW 

and the GIWW, and between the GIWW and 

adjacent surface-water bodies, strongly influence 

the local and regional surface-water hydrology of 

large parts of coastal Louisiana. The stage differences, and the resulting flows affect the north to 

south, and east to west movement of water, much 

of it fresh, in major bayous and canals that crisscross the wetlands. Long-term measurements of 

stage and discharge in the GIWW and adjacent 

surface-water bodies are needed to more fully 

characterize the surface-water hydrology and to 

better incorporate the flows into the ongoing 

efforts relating to coastal wetland restoration. 

This is perhaps most easily achieved if stage and 

discharge measurements in the GIWW are integrated into long-term monitoring programs of 

coastal Louisiana waters. Studies are also needed 

to understand how the water flowing in the 

GIWW may affect the wetlands. To date, the 

effect of the freshwater flow and associated 

water-quality in the GIWW for the integrity and 

health of adjacent wetlands is largely unknown. 

When the LAR at Morgan City is 3 or 

more ft above NAVD88, the GIWW effectively 

distributes freshwater and sediments from the 

LAR and WLO to points in coastal Louisiana 30 

to 50 mi east and west of Morgan City. The 

freshwater and sediments, some of which originate indirectly from the Mississippi River, are 

important building blocks for wetlands and could 

prove valuable in ongoing efforts to restore 

coastal Louisiana.

SELECTED REFERENCES

Arcement, G.J., Jr., 1988, Discharge and suspended-sediment data for the lower Atchafalaya River, Atchafalaya Bay, and Wax Lake 

Outlet, Louisiana, 1980-82: U.S. Geological 

Survey Open-File Report 87-553, 30 p.

Boesch, D.F., Josselyn, M.N., Ashish, J.M., Morris, J.R., Nuttle, W.K., Simenstad, C.A., and 

Swift, D.J.P., 1994, Scientific assessment of 

coastal wetland loss, restoration and management in Louisiana: Journal of Coastal 

Research, Special Issue no. 20, 103 p.

Byrne, P., Borengasser, M., Drew, G., Muller, R.A., 

Smith, B.L., Jr., and Wax, C., 1976, Barataria 

Basin—hydrological and climatological processes: Baton Rouge, La., Louisiana State 

University Center for Wetland Resources, 

Sea Grant Publication no. LSU-T-76-010, 

58 p.
SELECTED REFERENCES 47

Chabreck, R.H., 1972, Vegetation, water, and soil 

characteristics of the Louisiana Coastal 

Region: Baton Rouge, La., Louisiana State 

University Agricultural Experimental Station, Bulletin 664, 72 p.

Coleman, J.M., 1988, Dynamic changes and processes in the Mississippi River delta: Geological Society of America Bulletin, v. 100, 

p. 999-1015.

Craig, N.J., Turner, R.E., and Day, J.W., Jr., 1980, 

Wetland losses and their consequences in 

coastal Louisiana: Zeitschrift zur Geomorphologie Neue Folge, v. 34, p. 225-241.

Donnel, B.P., and Lederer, J.V., Jr., 1992, The 

Atchafalaya River Delta, Report 12, Twodimensional modeling of alternative plans 

and impacts on the Atchafalaya Bay and Terrebonne marshes: Vicksburg, Ms., U.S. 

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

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48 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

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APPENDIX 49

APPENDIX:

Regression Equations
50 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

Figure A1. Regressions of daily-average stage of the Lower Atchafalaya River at Morgan City 

against same-day instantaneous discharge measurements in the Gulf Intracoastal Waterway 

west of Houma navigation Canal at Houma and near Bay Wallace east of Morgan City.

Bay Wallace

Y Data

2000

4000

6000

8000

10000

12000

Gulf Intracoastal Waterway near 

Bay Wallace east of Morgan City

Discharge = 6110*ln (LAR)-393; r =0.90, p< 0.001 2

0 1234 5 6 7 8

Gulf Intracoastal Waterway west of

Houma Navigation Canal at Houma

Discharge = 1570* LAR + 727; r =0.64, p< 0.001 2

0 678 1 2 345

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

0

2,000

4,000

6,000

8,000

10,000

12,000

DAILY-AVERAGE STAGE OF LOWER ATCHAFALAYA RIVER 

AT MORGAN CITY, IN FEET ABOVE NAVD88

INSTANTANE

O

US DIS

C

H

A

R

GE, IN 

C

UBI

C FEET PER SE

C

OND

EXPLANATION

LAR is daily-average stage of the Lower Atchafalaya River at Morgan City

r is correlation coefficient, p is level of significance 2
APPENDIX 51

Figure A2. Regressions of daily-average stage of the Lower Atchafalaya River at Morgan City 

against same-day instantaneous discharge measurements at selected sites in the Gulf Intracoastal Waterway west of Wax Lake Outlet.

DAILY-AVERAGE STAGE OF LOWER ATCHAFALAYA 

RIVER AT MORGAN CITY, IN FEET ABOVE NAVD88

INSTANTANE

OUS DISCHAR

GE, IN CUBIC FEET PER SEC

OND

EXPLANATION

LAR is daily-average stage of the Lower Atchafalaya River at Morgan 

City; r is correlation coefficient, p is level of significance 2

0 6 123 4 5 7 8

0 678 12345

0

4,000

8,000

12,000

16,000

20,000

24,000

Discharge = 3440*LAR-3540; r =0.96, p<0.001 2

Gulf Intracoastal Waterway west of 

Wax Lake Outlet south of Calumet

Across Jaws Bay 

Discharge = 6510*ln(LAR)-2450; r =0.84 p<0.001 2

0

2,000

4,000

6,000

8,000

10,000

12,000

0

4,000

8,000

12,000

16,000

0 678 12345

Gulf Intracoastal Waterway east 

of Jaws Bay west of Franklin

Discharge = 8870*ln(LAR)-2640; r =0.91, p<0.001 2