Surface-Water Hydrology of the Gulf Intracoastal Waterway in South-Central Louisiana, 1996-99

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

Prepared in cooperation with the U.S. ARMY CORPS OF ENGINEERS NEW ORLEANS DISTRICT SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1672 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

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

Contents iii 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 iv Contents 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

Contents v 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)

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

By Christopher M. Swarzenski

ABSTRACT Calumet and 8,230 ft3/s east of Jaws Bay west of Franklin. Average discharge in the GIWW west of The flow of freshwater and suspended sedi- Jaws Bay near Cypremort was 3,310 ft3/s and at ment from the Lower Atchafalaya River (LAR) Cypremort was 1,350 ft3/s. Average discharge was and Wax Lake Outlet (WLO) into and along the to the west at all four locations, but discharge as Gulf Intracoastal Waterway (GIWW) and selected high as 2,830 ft3/s was measured flowing eastward adjacent surface-water bodies between Cypremort toward Jaws Bay in the GIWW at Cypremort. and Larose in south-central Louisiana, from Octo- ber 1996 to December 1999, was characterized In bayous and canals in most of coastal Lou- using instantaneous and computed continuous dis- isiana, including the GIWW, stage narrowly fluctu- charge measurements and measurements of sus- ates around the Gulf of Mexico level. Where the pended-sediment concentrations. The GIWW GIWW crosses the LAR and WLO, stage can reach parallels the entire Louisiana coast near the wet- 3 ft (feet) or more above the North American Verti- land/upland interface. Following natural hydraulic cal Datum of 1988 (NAVD88). Flow in the GIWW gradients, the GIWW captures water and sediment results from these differences in stage. When the from the southward flowing LAR and the WLO LAR at Morgan City reached 3 to 4 ft above where it crosses those waterways, and distributes NAVD88, flow in the GIWW became more pre- this freshwater and sediment to points east and dictable. Discharge at most sites between the HNC west. and Jaws Bay increased in varying amounts as stage of the LAR at Morgan City increased beyond East of Morgan City, La., an average of 3 ft above NAVD88. At sites in the GIWW east of 3 12,200 ft /s (cubic feet per second) of water flowed HNC, discharge did not increase predictably. For from the LAR into the Avoca Island Cutoff Chan- all measurements made when the LAR at Morgan nel. The LAR was the primary source of water to City was 3 ft or more above NAVD88, average dis- the GIWW east of Morgan City. Drainage from the charge was about 3,100 ft3/s in the GIWW east of Verret Subbasin through Bayou Boeuf contributed HNC at Houma and 2,880 ft3/s east of Bayou 3 an average of 1,000 ft /s to the eastward flow in the Lafourche at Larose. The LAR at Morgan City is GIWW. Eastward flow in the GIWW near Bay 3 ft or more above NAVD88 for about 7 months in Wallace east of Morgan City and to the west of the a normal year. Houma Navigation Canal (HNC) at Houma, La., averaged about 5,700 ft3/s. Average flow in the When the LAR at Morgan City was less than GIWW east of the HNC at Houma was 2,610 ft3/s 3 ft above NAVD88, water in the GIWW flowed to the east, and 2,200 ft3/s east of Bayou Lafourche along the prevailing water-level gradients, to the at Larose, also to the east. east between Bay Wallace and the HNC and to the west between WLO and Jaws Bay. However, local Measured discharge in the GIWW was runoff and drainage from areas adjacent to the always to the west between the LAR and WLO. GIWW became more significant in maintaining Water entered this stretch of the GIWW from the flow at low LAR stage. Discharge was consis- LAR. The WLO was the primary source of water tently higher in the GIWW west of the HNC at to the GIWW west of WLO. Discharge in the Houma than farther west near Bay Wallace east of 3 GIWW averaged 9,460 ft /s west of WLO south of Morgan City, when the LAR at Morgan City was

ABSTRACT 1 less than 3 ft above NAVD88. Westward flow in modifications have played a major role in the rapid the GIWW between Houma and Morgan City was conversion of land to open water that has charac- observed near Bay Wallace east of Morgan City but terized much of coastal Louisiana for the past 50 or was never observed west of the HNC at Houma. more years (Gagliano and others, 1981; Turner and Discharge in the GIWW east of Jaws Bay west of Cahoon, 1988; Boesch and others, 1994; Reed, Franklin, La., frequently was higher than in the 1995). The two largest modifications to the flow of GIWW west of WLO south of Calumet, La., at low freshwater and sediment into and through coastal LAR stage. Louisiana wetlands have been the leveeing of the Mississippi River and the construction of the Gulf East of the LAR, suspended-sediment con- Intracoastal Waterway (GIWW) in the early part of centrations averaged about 162 mg/L (milligrams the 20th century. The confinement of the Missis- per liter) at the two sites closest to the LAR, Avoca sippi River between levees in much of southeastern Island Cutoff Channel and Bayou Penchant south Louisiana has reduced and in places eliminated the of Morgan City. Suspended-sediment concentra- frequent over-bank flooding that historically tions averaged 91 mg/L in the water entering the brought freshwater and sediment to coastal wet- GIWW through Bayou Boeuf at railroad bridge at lands. The GIWW has modified surface-water Amelia. The water in the GIWW near Bay Wallace hydrology in coastal Louisiana in ways that pres- east of Morgan City to the HNC was intermediate ently are not well understood. in suspended-sediment concentrations (108- 114 mg/L), reflecting the mixture of the two water The GIWW is the largest continuous ship types. Suspended-sediment concentrations in the channel in coastal Louisiana, traversing the entire GIWW near Bay Wallace east of Morgan City and Louisiana coast along the wetland/upland interface east and west of the HNC were comparable, but in (fig. 1). Following natural hydraulic gradients, the the GIWW east of Larose, they increased by about GIWW captures water from the Lower Atchafalaya 45 percent. To the west of WLO, suspended-sedi- River (LAR) and the Wax Lake Outlet (WLO) and ment concentrations averaged between 167 and distributes it to parts of Louisiana's coastal wet- 191 mg/L. lands during the annual spring flood (Paille, 1997). This flow potentially could support coastal restora- Freshwater flow directly affected salinity tion efforts with a minimum of cost (R.F. Paille, patterns in the GIWW and adjacent surface-water U.S. Fish and Wildlife Service, oral commun., bodies. Monthly-average salinity at many sites, 1999). Distinct from its potential for ecological both in and adjacent to the GIWW, was related to benefits, the GIWW also provides an exit point for monthly-average stage of the LAR at Morgan City. flood waters carried through the Atchafalaya River When the LAR at Morgan City is 3 or more Basin. However, the flow of freshwater and trans- ft above NAVD88, the GIWW effectively distrib- port of suspended sediment in the GIWW and the utes freshwater and sediment from the LAR and relation of these activities to stage of the LAR are WLO to points in coastal Louisiana 30 to 50 miles not well understood. east and west of Morgan City. The freshwater and In 1997, the U.S. Geological Survey (USGS) sediment, some of which originate indirectly from began to operate a network of data-collection sites the Mississippi River, are valuable resources for that monitor velocity, stage, and salinity in the building wetlands and could be incorporated into GIWW and several connecting waterways. This ongoing efforts to restore coastal Louisiana. effort was initiated in cooperation with Federal agencies concerned with flood control and environ- INTRODUCTION mental quality, principally the U.S. Army Corps of Engineers (USACE), but also including the U.S. Regional and local modifications to surface- Fish and Wildlife Service (USFWS), Natural water hydrology in coastal Louisiana have dis- Resources Conservation Service, the U.S. Environ- rupted the natural inflow of freshwater and sedi- mental Protection Agency, and the Coastal Wet- ment to wetlands for many years (Mendelssohn lands Protection, Planning, and Restoration Act and others, 1984; Turner and Cahoon, 1988). The task force. From 1997-99 the primary data-collec-

2 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 94° 93° 92° 91° 90° 89°

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Louisiana Oil Spill Coordinator Office of the Governor,

Louisiana GIS CD: A Digital Map of the State, Version 2.0 0 15 30 45 60 MILES

0 15 30 45 60 KILOMETERS

Figure 1. Coastal Louisiana showing the Mississippi River, Atchafalaya River, and the Gulf Intracoastal Waterway. tion effort focused on the GIWW between Cypre- in the study area are discussed to evaluate flow mort, La., to the west, and Larose, La., to the east characteristics. Suspended-sediment concentra- (pl. 1). Occasionally, measurements east of Larose tions and discharge are discussed. The present-day were made in Bayou Perot and in the GIWW just salinity patterns are discussed in relation to flow west of Bayou Perot. In 1998, the USGS, in coop- and local runoff as are changes to historical salinity eration with the USACE, began a study to evaluate patterns affected by flow in selected areas along the collected data and describe hydrologic condi- the coast. tions in the GIWW in south-central Louisiana. Quantifying the amount of water flowing in Purpose and Scope the GIWW and how this relates to sediment transport and salinity conditions in coastal Louisiana This report characterizes the flow of water may help State and Federal agencies in designing and suspended sediment from the LAR and WLO coastal wetland restoration projects and imple- into and along the GIWW and in selected adjacent menting flood-control measures. surface-water bodies between Cypremort and Larose from October 1996 to December 1999 Description of Study Area (pl. 1). The relative importance of the two primary sources of water sustaining flow along the The GIWW is part of the Intracoastal Water- GIWW, the Atchafalaya River (LAR and WLO) way, a continuous system of navigation channels and local precipitation and runoff, are evaluated. connecting the Atlantic and Gulf Coast States. The The distribution of stage and water-level gradients channels permit extensive ship-borne commerce

INTRODUCTION 3 between the states. The USACE maintains a chan- GIWW might lose or gain water along such seg- nel depth of at least 20 ft and a width of at least ments. A different type of segment (2) occurs 200 ft for the GIWW in the study area. The part of where the GIWW crosses a waterway, such as a the GIWW in the study area is in south-central canal or bayou. Substantial amounts of water can Louisiana and extends from Cypremort on the west enter or leave the GIWW in a very short distance. to the intersection of the GIWW and Bayou Perot Jaws Bay, on the west side of Morgan City, and the on the east. The Verret Subbasin of Terrebonne HNC are major examples of the second type of Basin (hereain referred to as the Verret Subbasin), segment along the GIWW in the study area. West to the north and east of Morgan City, La., drains of WLO, Jaws Bay separates two segments that into the GIWW and is included in the study area convey water. This arrangement of conveyance, (pl. 1). Plate 1 also shows the Teche-Vermilion, opening, and conveyance is mirrored on the east Terrebonne, Timbalier, and Barataria interdistribu- side of the LAR starting east of the confluence of tary Basins, which are linked by the GIWW in the Bayou Chene and the GIWW. The lateral flow of study area. Houma, La., and Morgan City are water in the GIWW between Bay Wallace and regional population centers. Bayou Lafourche is split into two segments at Houma by the HNC. The Gulf Intracoastal Waterway Physiography of Coastal Louisiana East of Morgan City, surface flow in the LAR is hydraulically disconnected from the The study area lies mostly within the coastal GIWW by the Bayou Boeuf locks at Morgan City zone. The Louisiana Gulf Coast contains about and the Avoca Island levee. The levee was com- 5,800 mi2 of wetlands (Chabreck, 1972), most of pleted around 1952 (N.J. Powell, USACE, oral which occur in the Mississippi River Delta Plain commun., 2001). Water in the LAR is prevented (fig. 1). The wetlands form a continuum from from flowing eastward between the Bayou Boeuf inland swamps and fresh marsh to salt marsh, locks and the levee’s termination at the Avoca based on tolerances of the emergent vegetation to Island Cutoff Channel, about 10 mi south of Mor- flooding and the local open-water salinity (Gos- gan City (pl. 1). The Avoca Island Cutoff Channel selink, 1984). Local salinity depends on the mix- connects the LAR to the GIWW. Because of pre- ing of marine waters from the Gulf of Mexico and vailing hydraulic gradients, the Avoca Island Cut- freshwater runoff from river flow and precipitation. off Channel conveys a portion of LAR water in a The coastal wetlands were formed by active depo- northeasterly direction toward the GIWW. The sition of mineral sediment carried by the Missis- Avoca Island Cutoff Channel becomes Bayou sippi River during the last 6,000 to 8,000 years, Chene where it joins with Bayou Penchant, prior to after sea level stabilized to approximate current reaching the GIWW. Bayou Boeuf connects to the levels (Kolb and Van Lopik, 1958; Frazier, 1967), GIWW between the Bayou Boeuf locks at Morgan and more recently in freshwater areas, through City and the confluence of the GIWW and Bayou accumulations of organic matter almost exclu- Chene. Bayou Boeuf is the primary hydrologic sively derived locally (Swarzenski and others, connection between the GIWW and the Verret Sub- 1991). The Mississippi River Delta Plain is com- basin. The GIWW extends eastward to Houma, prised of six to seven overlapping lobate deltas across the Houma Navigation Canal (HNC) and Bayou Lafourche toward Bayou Perot and New (Frazier, 1967). The delta lobes formed as the Mis- Orleans, Louisiana. West of Morgan City, the sissippi River switched channels to follow a more GIWW connects to the LAR just below the city hydraulically efficient path to the Gulf of Mexico. and extends westward past Jaws Bay and Cypre- The switching of channels and loci of delta build- mort to Weeks Bay. ing occurred about every 1,000 years, and has cre- ated a physiography of large areas of low-elevation Much of the GIWW between Cypremort and wetlands, termed inter-distributary basins, sepa- Bayou Perot can be categorized as one of two rated by natural levees which decrease in elevation hydrologically distinct types of segments. There toward the Gulf of Mexico (pl. 1). The natural are longer segments (1) where lateral flow, that is levees act as hydrologic barriers to the cross-basin flow parallel to the coast, predominates. The flow of water. Cross-basin flow is generally

4 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 restricted to man-made canals that traverse these at the same location, were used to describe flow at levees, such as the GIWW. In the study area, the selected sites along the GIWW (table 1, pl. 1). GIWW hydrologically links the Teche-Vermilion, Between April 1997 and December 1999, 13 to 19 Terrebonne, Timbalier, and Barataria inter-distribu- measurements were made at these sites at varying tary Basins (pl. 1). stages of the LAR. Measurements were made along the entire length of the GIWW from Cypre- Major Rivers mort to Bayou Perot on synoptic trips of 2 to 4 days. The measurements were made with a 600- The Mississippi River and the Atchafalaya kilohertz boat-mounted acoustic Doppler current River are major sources of freshwater to the Loui- profiler (ADCP) (Simpson and Oltmann, 1993). In siana coast in general, and in varying amounts to the GIWW at Cypremort, at Bayou Penchant south the study area. Because of the extensive levee sys- of Morgan City, and at Bayou Boeuf at railroad tem, the Mississippi River mostly flows into the bridge at Amelia, La., stage and point-velocity Gulf of Mexico, and enters coastal Louisiana wet- were recorded hourly (continuously) by onsite lands only indirectly as Gulf of Mexico waters data-recording equipment. A relation between work their way into the estuaries of the various these variables and the instantaneous discharge bays. The Atchafalaya River has a higher hydrau- measurements across the entire channel cross-sec- lic gradient and shorter path to the Gulf of Mexico tion was developed to compute continuous dis- than the Mississippi River but is prevented from charge (Morlock, 2001). The data from these sites capturing the entire flow of the Mississippi River were tabulated as hourly discharge values. While by the Old River Control Structure (fig. 1). Since instantaneous discharge measurements were being completion of the structure in the 1950's, about 30 made in the GIWW, suspended sediment usually percent of Mississippi River volume is diverted was sampled at points in the center, left, and right into the Atchafalaya River (Wells and Demas, of the channel cross section with a depth-integrat- 1977, p. 6). This water comprises most of the flow ing sampler (Edwards, 1999). There are not corre- of the Atchafalaya River, and flow from the Red sponding suspended-sediment measurements for River provides some additional water (fig. 1). The every discharge measurement. water flows through the Atchafalaya Basin and enters the Gulf of Mexico through WLO and the Flow direction can vary at and between sites LAR. in the low-gradient coastal area so a sign conven- tion was used to label flow direction at the sites. Approach and Methods Flow away from Morgan City and toward the Gulf of Mexico was labeled positive. To the east of Synoptic measurements of discharge and Morgan City, positive flow is to the east and south suspended sediment were made along the GIWW with the exception of the Avoca Island Cutoff in the study area during different times of the year. Channel. Here, positive flow is to the northeast, The synoptic data were supplemented by continu- away from the LAR (pl. 1). West of Morgan City, ous data collected at three sites from which hourly positive flow is to the west; negative flow indicates discharge was computed. Water-level gradients in flow toward Morgan City. the study area were established by comparing stage at sites along and adjacent to the GIWW. Salinity Monthly-average discharge for the study fluctuations at coastal sites were used to track period was determined at four synoptic sites along freshwater inflow from the GIWW. Table 1 lists all the GIWW--west of HNC at Houma, near Bay Wal- sites used for this report and the type of data pre- lace east of Morgan City, west of WLO south of sented. Site locations are shown on plate 1. Calumet, and east of Jaws Bay west of Franklin-- for the difference in discharge between the GIWW Discharge and Suspended Sediment east and west of Jaws Bay (across Jaws Bay) and for the three recording sites at Bayou Boeuf at rail- Instantaneous discharge measurements and road bridge at Amelia, Bayou Penchant south of computed continuous discharge, derived from rela- Morgan City, and the GIWW at Cypremort. At the tions between instantaneous discharge and hourly continuously recording sites, discharge data were recording point-velocity and stage measurements averaged by month. At the other site, relations

INTRODUCTION 5 s, 1 ------(conventional levels) (method) ng System; USACE, U.S. U.S. USACE, ng System; Datum: survey agency NGVD29 NAVD88: DNR (GPS) DNR NAVD88: NAVD88: DNR (GPS) DNR NAVD88: NAVD88: USGS (GPS) NAVD88: USGS (GPS) NAVD88: NAVD88: USGS (GPS) NAVD88: NAVD88: DNR (GPS) DNR NAVD88: NAVD88: USGS (GPS) NAVD88: NAVD88: USGS (GPS) NAVD88: USGS (GPS) NAVD88: NAVD88: USGS (GPS) NAVD88: USGS (GPS) NAVD88: NAVD88: USGS (GPS) NAVD88: USGS (GPS) NAVD88: USACE NAVD88: USGS (GPS) NAVD88: hmark at Patterson, sediment Suspended 88; GPS, Global Positioni FWS, U.S. Fish and Wildlife Service; --, not applicable; applicable; not --, Service; Wildlife and Fish U.S. FWS, ngle first order benc as part of this study, using a si of as part this study, partment of Environmental Quality; US Quality; of Environmental partment Continuous dataContinuous data Instantaneous Intracoastal Waterway (GIWW) and adjacent Intracoastal surface-water Waterway bodie -- X X X X ------X ------X ------X ------X ------X X ------X ------X X ------X X ------X X -- X -- X ------X X X X -- -- X ------X X ------X ------X ------X ------X -- X X ------X ------X -- X X ------X ------X X ------X -- -- X X X ------X X ------X X -- X ------X X X X X X X -- XXX--XX XXX--XX XXX--XX Discharge Velocity Stage Salinity Discharge DEQ, Louisiana DEQ, De owner ntional leveling; USGS survey ntional was done rtmentNatural of Resources; ana Department of Wildlife and Fisheries; NAVD88, North American Vertical Datum of 19 Datum Vertical American North NAVD88, Fisheries; and Wildlife of Department ana t near Calumet USGS/USACE y 1 at Lockporty 1 at USGS/USACE idge at Amelia USGS/USACE Site name Operator/ plished using GPS and conve Vertical Datum of 1929] 1929] Datum of Vertical Data-collection sites and type of data collected along the Gulf 1 Point Bay near Cypremort Vermilion USGS/DWF 2 at GIWW Cypremort89 at GIWW Lower Atchafalaya RiverCity south of Morgan City Atchafalaya Lower Riverat Morgan USGS/USACE USGS/USACE USGS/USACE 34 west GIWW of Jaws Bay near Cypremort5 east GIWW of Jaws Bay west of Franklin6 Lake Outlet south GIWWof Calumet west of Wax 7 Lake Outlet at Calumet Wax Lake Outle east GIWW of Wax USGS/USACE USGS/USACE USGS/USACE USGS/USACE 11 City Morgan of Penchant south Bayou USGS/USACE 29 Houma Navigation Canal at Dulac USGS/USACE 28 Chauvin south of at Canal, Bush Terrebonne Bayou USGS/DNR 27 Bayou Jean Lacroix near Montegut USGS/DNR 26 Grand Bayou tributary west of Galliano USGS/DNR 25 Bayou Perot south of the GIWW USGS 24 Perot and Bayou GIWW of Intersection DNR 23 GIWW west of Bayou Perot USGS 22 GIWW east of Larose DNR 21 GIWW east of Bayou Lafourche at Larose USGS/USACE 20 GIWW west of Bayou Lafourche at Larose USGS/USACE 1819 Canal at Highwa Company Bleu L’Eau Bayou USGS/USFWS 17 GIWW east of Houma Navigation Canal at Houma USGS/USACE 16 GIWW west of Houma Navigation Canal at Houma USGS/USACE 15 at BayouTheriot du Large USGS/DEQ 14 Canal Penchant at Brady Bayou DNR 32 Little Lake near Cut Off USGS/DNR 12 br at railroad Boeuf Bayou 13 CityMorgan east of near Bay Wallace GIWW USGS/USACE 31 Côte Blanche 4 DNR 10 City Channel south of Morgan Island Cutoff Avoca USGS/USACE 30 Caillou Lake (Sister Lake) southwest of Dulac USGS/DWF Map Elevation surveys were accom Elevation (pl. 1) 1 number NGVD29, National Geodetic Geodetic NGVD29, National Army Corps of Engineers; DNR, Louisiana Depa Engineers; DNR, Louisiana of Army Corps Table 1. Louisi [USGS, U.S. Geological Survey; DWF, Louisiana. south-central Louisiana

6 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 between instantaneous discharge measurements the HNC at Dulac was not part of the USGS GPS and same-day average stage of the LAR at Morgan survey, but was surveyed by the USGS from a City were used to develop regression equations. nearby benchmark to the NGVD29. Two sites The difference in same-day discharge across Jaws were surveyed by Louisiana Department of Natural Bay was used in the regression from this location. Resources (DNR) to NAVD88 datum. The impor- Only measurements made when the LAR at Mor- tance of reliable elevation data in establishing gra- gan City was 2 ft or more above NAVD88 were dients increases with distance from the LAR, as the used in developing the regression equations. Dis- gradient becomes smaller. Small differences in charge measurements in the GIWW after the high gradient may be attributed to differences in surveys rainfall event in 1998 also were eliminated from rather than actual differences in measured stage. the regressions because of the explainable departure from expected values. Monthly-average dis- Monthly-average values for stage were used charge was computed for those months when the in this comparison to determine water-level gradi- LAR at Morgan City averaged 3 ft or more above ents across the study area. To the east of Morgan NAVD88. The appendix contains the equations City, stage data from LAR at Morgan City and used at the respective stations. Stage of the LAR at HNC at Dulac were used as the upstream and Morgan City was 3 ft or more above NAVD88 for downstream points. To the west of WLO, stage the first 7 months of 1997, 1998, and 1999. data from WLO at Calumet and Vermilion Bay near Cypremort Point were used as the upstream Data from the individual synoptic trips were and downstream points. used to determine if and under what conditions wetlands or waterways adjacent to a specific seg- Acknowledgments ment of the GIWW were storing water from or The author extends his thanks to Troy Con- releasing water to that particular segment. Dis- stance and Nancy J. Powell of the U.S. Army charge measured at the downstream end of a partic- Corps of Engineers and Ronald F. Paille of the ular segment was subtracted from that entering the U.S. Fish and Wildlife Service for their support same segment for each synoptic trip. This differ- with this project. Paille was among the first to rec- ence was plotted against either the discharge enter- ognize the potential value of the flow of freshwa- ing the GIWW from the respective river source or ter and sediments in the GIWW to restoration of the prior 21-day average discharge at Bayou Boeuf coastal Louisiana wetlands. The field work and at railroad bridge at Amelia. The 21-day discharge data collection were carried out by C. Kevin Labbe at Bayou Boeuf at railroad bridge at Amelia was and Andrew Tarver of the USGS office in Baton used to approximate runoff generated from local Rouge. rainfall entering the GIWW and coastal Louisi- ana. Negative values indicate water from the sur- HYDROLOGIC CONDITIONS rounding wetlands is draining into the GIWW and DURING STUDY leaving the segment. Positive values indicate less water is flowing out of the segment than entering; Stage and discharge measured in LAR at water is being stored in the segment. Morgan City and discharge measured at WLO at Calumet from October 1996 to December 1999 Stage and Water-Level Gradients have similar hydrographs (fig. 2). Stage of LAR at Gages measuring stage are distributed at Morgan City reached almost 8 ft above NAVD88 sites throughout the study area (table 1). Data from for a brief time in April. The spring rise started in these sites were used to compare stage and estab- late February 1997 and continued through mid- lish water-level gradients in the GIWW and July. After July, stage and discharge remained low selected adjacent bayous and canals. All gages until early January 1998. The spring flood in 1998 used in this study were surveyed to NAVD88, with started earlier and, although not reaching the levels a Global Positioning System (GPS) using a first of 1997, remained high until late July. Stage dur- order benchmark at Patterson, La., as part of this ing the low water season (August through Decem- study except where noted (table 1). The gage in ber) in 1998 was slightly higher than in 1997. In

HYDROLOGIC CONDITIONS DURING STUDY 7 8 700 Stage, Lower Atchafalaya River at Morgan City Instantaneous discharge, Lower Atchafalaya River at Morgan City 7 600 Instantaneous discharge, Wax Lake Outlet at Calumet

6 VE NAVD88

O 500 B 5

Break in line indicates missing data 400

4 E, IN FEET A

G 300 TA

S 3 E G 200 2 INSTANTANEOUS DISCHARGE, IN CUBIC FEET PER SECOND (x 1,000) (x SECOND FEET CUBIC PER -AVERA Y 1 100 DAIL

0 0 Apr. 1 Apr. 1 Apr. 1 Apr. July 1 July 1 July 1 July Jan. 1 1 Mar. May 1 Jan. 1 1 Mar. May 1 Jan. 1 1 Mar. May 1 Nov. 1 Nov. 1 Feb. 1 Nov. 1 Feb. 1 Nov. 1 Feb. 1 Nov. Dec. 1 Dec. 1 Dec. 1 Dec. 1 Dec. Aug. 1 Aug. 1 Aug. 1 Sept. 1 Sept. 1 Sept. 1 Sept. 1996 19971998 1999 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.

1999, the spring flood also developed later than in Precipitation also affects flow in the GIWW. 1997 and had a smaller volume. During the low Monthly precipitation data are shown for weather water season, the stage of LAR at Morgan City stations at Morgan City and Houma (fig. 3). Data was at least 3 ft above NAVD88 only in 1996. In for the Palmer Hydrological Drought Index (PHDI) November and December 1999, stage of LAR at for the South-Central Division, which extends west Morgan City frequently was less than 1.5 ft above from Morgan City halfway to the Texas border, and NAVD88. for the Southeast Division, which extends east of Morgan City across the Mississippi River to the Peak measured instantaneous discharge for border with Mississippi, also are presented the period of study was about 380,000 ft3/s for (National Oceanic and Atmospheric Administra- LAR at Morgan City and 220,000 ft3/s for WLO at tion National Climatic Data Center, 2000). The Calumet for a combined discharge of about PHDI is an index that assesses regional long-term 600,000 ft3/s. The LAR at Morgan City received moisture supply. October and November of 1996 from 43 to 75 percent of the combined discharge, had below average rainfall. From January to June with most of the variability occurring when LAR at 1997, rainfall was near or above the long-term Morgan City was 3 ft or less above NAVD88. For average for both stations. From August through all measurements, an average of 59 percent ± 5 October, precipitation was below the long-term percent of the combined discharge entered the average. The PHDI indicated an incipient to mild LAR at Morgan City. The average percentage of drought from September through December 1997. Atchafalaya River flow entering LAR at Morgan About 10 to 12 inches more rain than average was City determined by measurements during this measured at the two stations during January 1998. study is similar to the division of discharge deter- Rainfall was near normal through April 1998, but mined by measurements made between 1980 and then was below average until the storms of Sep- 1982 (Arcement, 1988). tember 1998. Very little rain fell in May 1998.

8 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 20 15 Total MORGAN CITY Departure from normal 10 5 0 -5

20 Total 15 HOUMA Departure from normal 10 5

MONTHLY PRECIPITATION, IN INCHES IN PRECIPITATION, MONTHLY 0 -5

4 3.00 to 3.99 = very moist 2.00 to 2.99 = moderately moist 2 1.99 to -1.99 = mid-range -2.00 to -2.99 = moderate drought -3.00 to -3.99 = severe drought 0

-2 South-Central Division

DROUGHT INDEX Southeast Division

PALMER HYDROLOGICAL -4

Source: National Oceanic and Atmospheric Administration, National Climatic Data Center, 2000

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.

Even with the high rainfall in September, the were major storms that produced substantial pre- South-Central Division was in a mild to moderate cipitation and raised the stage along the coast and drought from May through the end of the year. The in the LAR at Morgan City. In contrast, the high Southeast Division was in a moderate drought in LAR stage in September 1997 occurred without a July and August 1998. From October 1998 major storm or substantial precipitation. through May 1999, rainfall was below average. Both divisions were in a moderate drought for most SURFACE-WATER HYDROLOGY months during 1999. In this section, flow characteristics in the Two events during the fall low-water season GIWW are discussed. Distribution and differences raised LAR at Morgan City stage for prolonged in discharge, stage, and water-level gradients, and periods (fig. 4). During the last week of September suspended-sediment and salinity concentrations are and the first 2 weeks of October 1997, a period of described. strong south winds raised the stage along the coast and in the LAR at Morgan City. In early and mid- Flow Characteristics September of 1998, Hurricane Earle (August 30- September 1) and Tropical Storms Frances (Sep- Discharge in the GIWW was highest where tember 10-12) and Hermine (September 19-20) it crossed the LAR and at WLO south of Calumet,

SURFACE-WATER HYDROLOGY 9 1997 1998

360 24 360 24 20 20 270 270 16 16 180 12 180 12 8 8 90 90 4 4 DAILY RESULTANT WIND RESULTANT DAILY WIND RESULTANT DAILY DAILY RESULTANT WIND RESULTANT DAILY WIND RESULTANT DAILY DIRECTION, IN DEGREES IN DIRECTION, 0 0 DEGREES IN DIRECTION, 0 0 SPEED, IN HOURMILES PER SPEED, IN HOURMILES PER

4 10 5 10 Period of little rain, strong south winds 3 8 4 8

2 6 3 6

1 4 2 4 IN INCHES IN INCHES IN 0 2

FEET ABOVE ABOVE NAVD88 FEET 2

FEET ABOVE NAVD88 ABOVE FEET 1 DAILY-AVERAGE STAGE, IN STAGE, DAILY-AVERAGE DAILY-AVERAGE STAGE, IN DAILY-AVERAGE

-1 0 PRECIPITATION, TOTAL DAILY 0 0 TOTAL PRECIPITATION,DAILY Oct. 6 Oct. Dec. 1 Nov. 3 Nov. Sept. 1 Sept. 8 Oct. 13 Oct. 20 Oct. 27 Oct. Oct. 6 Oct. Dec. 1 Nov. 10 Nov. 17 Nov. 24 Nov. Nov. 3 Nov. Sept. 1 Sept. 15 Sept. 22 Sept. 29 Sept. Sept. 8 Sept. Oct. 13 Oct. 20 Oct. 27 Oct. Nov. 10 Nov. 17 Nov. 24 Nov. Sept. 15 Sept. 22 Sept. 29

EXPLANATION Lower Atchafalaya River at Morgan City Wind speed Houma Navigation Canal at Dulac Direction (0 and 360 degrees are north; 180 degrees is south) Vermilion Bay near Cypremort Point Precipitation

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.

and decreased with increasing distance east and Discharge measured in the GIWW east of west (table 2). At the Avoca Island Cutoff Chan- HNC at Houma averaged 2,610 ft3/s and ranged nel, discharge ranged from 267 to 31,100 ft3/s; the from 1,100 ft3/s, flowing westward toward the average discharge was 12,200 ft3/s. At Bayou HNC, to 4,640 ft3/s flowing eastward toward Penchant south of Morgan City, flow was usually Bayou Lafourche. Discharge in the GIWW west to the southeast, away from the Avoca Island Cut- and east of Bayou Lafourche at Larose was similar off Channel. The average discharge was in magnitude and direction and averaged about 3,250 ft3/s. Measured discharge at Bayou Boeuf 2,200 ft3/s to the east. Eastward discharge of as at railroad bridge at Amelia ranged from 9,110 ft3/s much as 5,320 ft3/s toward Bayou Perot was mea- flowing north into the Verret Subbasin to 15,300 sured. Flow westward toward Houma was ft3/s flowing south from the basin toward the observed on two occasions in the GIWW, with a GIWW. The average discharge was 1,000 ft3/s maximum discharge of 1,860 ft3/s east of Bayou entering the GIWW from the Verret Subbasin. Lafourche at Larose. Discharge was usually to the east and averaged about 5,700 ft3/s in the GIWW near Bay Wallace In Barataria Basin, to the east of Larose, east of Morgan City and the GIWW west of HNC instantaneous discharge was measured in the at Houma. Discharge of 922 ft3/s to the west was GIWW west of the intersection with Bayou Perot, measured at the former site. Discharge always was and in Bayou Perot just south of the intersection on to the east in the GIWW west of HNC at Houma. several occasions. The measurements were made

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

Instantaneous discharge Number Range Mean Map of number (±standard Site name measure- Median Minimum Maximum (pl. 1) deviation) ments

East of Lower Atchafalaya River Between Lower Atchafalaya River and Houma Navigation Canal 10 Avoca Island Cutoff Channel south 18 12,200 ± 9,790 9,400 267 31,100 of Morgan City 11 Bayou Penchant south of Morgan 18 3,250 ± 2,370 2,620 260 7,570 City 12 Bayou Boeuf at railroad bridge at 17 1,000 ± 6,720 768 -9,110 15,300 Amelia 13 GIWW near Bay Wallace east of 18 5,600 ± 3,940 5,630 -922 10,600 Morgan City 16 GIWW west of Houma Navigation 19 5,770 ± 3,190 4,980 845 11,800 Canal at Houma East of Houma Navigation Canal 17 GIWW east of Houma Navigation 19 2,610 ± 1,740 2,200 -1,100 4,640 Canal at Houma 20 GIWW west of Bayou Lafourche at 19 2,220 ± 1,760 2,510 -1,580 5,320 Larose 21 GIWW east of Bayou Lafourche at 19 2,190 ± 1,830 2,670 -1,860 4,930 Larose

West of Lower Atchafalaya River Between Lower Atchafalaya River and Wax Lake Outlet 8 GIWW at Lower Atchafalaya River 13 13,400 ± 9,380 12,200 3,910 35,200 south of Morgan City 7 GIWW east of Wax Lake Outlet 18 9,230 ± 3,350 8,890 4,430 16,200 near Calumet West of Wax Lake Outlet 5 GIWW west of Wax Lake Outlet 16 9,460 ± 5,610 10,300 2,050 20,300 south of Calumet 4 GIWW east of Jaws Bay west of 18 8,230 ± 4,110 6,600 3,560 17,000 Franklin 3 GIWW west of Jaws Bay near 18 3,310 ± 2,160 2,850 1,000 10,200 Cypremort 2 GIWW at Cypremort 17 1,350 ± 2,550 2,090 - 2,830 4,830

SURFACE-WATER HYDROLOGY 11 in conjunction with synoptic surveys of discharge of 9,460 ft3/s. In the GIWW east of Jaws Bay west along the GIWW, but were not made as frequently of Franklin, average measured discharge was (table 3). In the GIWW west of Bayou Perot, dis- 8,230 ft3/s, and reached a maximum of charge was to the east and ranged from 126 ft3/s to 17,000 ft3/s. In the GIWW west of Jaws Bay near more than 3,450 ft3/s. In Bayou Perot, discharge Cypremort, the average discharge was 3,310 ft3/s, was much higher than in the GIWW and was mea- and flow was always to the west. In the GIWW at 3 sured at 29,300 ft3/s flowing south and 12,000 ft3/s Cypremort, average discharge was 1,350 ft /s. 3 flowing north into Lake Salvador. Maximum westward discharge was 4,830 ft /s, but eastward discharge as high as 2,830 ft3/s toward Instantaneous discharge was measured at Jaws Bay was measured. two sites between the LAR and WLO at Calumet (pl. 1, table 2): GIWW at LAR south of Morgan Daily-average stage and discharge in Bayou City and the GIWW east of WLO at Calumet. Penchant south of Morgan City, Bayou Boeuf at Measured discharge always indicated flow from railroad bridge at Amelia, and GIWW at Cypre- east to west in this short stretch of the GIWW. At mort are presented (fig. 5). The gages did not start the GIWW at LAR south of Morgan City, average operating at the same time. At Bayou Penchant discharge (13,400 ft3/s) and maximum discharge south of Morgan City, computed discharge tracked (35,200 ft3/s) were the highest of all 14 sites. stage fluctuations, with fall discharge about half that during the spring (fig. 5). Flow was almost West of the WLO, discharge in the GIWW always to the southeast, with isolated days when decreased toward the west (table 2). In the GIWW there was a net flow of water northwest toward the west of WLO south of Calumet, measurements Avoca Island Cutoff Channel. Water flowed both ranged from 2,050 to 20,300 ft3/s, with an average into (north) and out of (south) the Verret Subbasin

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]

Instantaneous discharge (cubic feet per second) Stage, Lower Atchafa- Date laya River at Morgan GIWW Bayou Perot Houma Naviga- City (site 9) west of south of tion Canal at (feet above NAVD88) Bayou Perot GIWW Dulac (site 23) (site 25) (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 --

12 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 4 12,000 Bayou Penchant south of Morgan City 10,000 3 8,000

6,000 2 Break in line indicates missing data 4,000 2,000 1 Positive discharge is to the southeast; 0 Negative discharge is to the northwest 88

D 0 - 2,000 OND JJFMAMJJASOND J FMAMJ ASONDJ FMAMJJASON D

4 20,000 OVE NAV

B 15,000 A

T 3 Bayou Boeuf at railroad bridge 10,000 EE

F at Amelia 5,000 2 0 AGE, IN T

S -5,000 1 Positive discharge is to the south; Negative discharge is to the north -10,000

0 -15,000 OND JJFMAMJJASOND J FMAMJ ASONDJ FMAMJJASON D -AVERAGE -AVERAGE Y 4 6,000 AIL D Gulf Intracoastal Waterway Break in line 3 at Cypremort indicates 4,000 DISCHARGE, CUBIC IN PER FEET SECOND DAILY-AVERAGE missing data 2 2,000 1

0 0 Positive discharge is to the west; Negative discharge is to the east -1 -2,000 OND JFMAMJ JASONDJFMAMJ J ASOND J FMAMJ JASON D 1996 1997 1998 1999

EXPLANATION Stage Discharge 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. through Bayou Boeuf (fig. 5). Flow was mostly to May 1999, discharge was small and often entered the south to the confluence of Bayou Boeuf and the rather than drained the Verret Subbasin. GIWW. Discharge was not simply related to stage of the LAR at Morgan City. For example, April Hourly point-velocity was measured at through July 1997 and January, February, and Sep- Bayou L’Eau Bleu from May to December 1996 as tember 1998 were months when relatively more part of a separate study. Although continuous dis- Verret Subbasin water was entering the GIWW and charge was not computed for this site, velocity data mixing with LAR water (fig. 5). In February and indicate flow mostly was to the south, away from

SURFACE-WATER HYDROLOGY 13 the GIWW (fig. 6). This was true even in the fall Between Lower Atchafalaya River and Houma months, when flow direction frequently reversed due Navigation Canal to strong south winds. Available instantaneous discharge measurements have indicated between 200 Water draining the Verret Subbasin through and 400 ft3/s of water flowing to the south (USGS Bayou Boeuf at railroad bridge at Amelia and LAR miscellaneous measurements). water entering through the Avoca Island Cutoff Channel maintain eastward flow in the GIWW east of Morgan City. The amount of water flowing into

7 3.0 Avoca Island Cutoff Channel from the LAR gener- Break in line indicates missing data ally increased as stage of the LAR at Morgan City 6 Positive velocity is to the south 2.5 Negative velocity is to the north rose, but this relation was affected by how much E,

G water was leaving Bayou Boeuf at railroad bridge 5 2.0 TA at Amelia (pl. 1). During periods of high south- S

E 4 1.5 ward flow in Bayou Boeuf at railroad bridge at G Amelia, water entering the GIWW through Avoca 3 1.0 Island Cutoff Channel was less than could be -AVERA

Y 2 0.5 expected based solely on stage of the LAR at Mor- L

AI gan City. Thus, peak discharge of 25,000 to THREE-HOUR AVERAGE D IN FEETABOVE 88 NAVD 1 0.0 35,000 ft3/s in Avoca Island Cutoff Channel was VELOCITY, IN FEET PER SECOND VELOCITY, 0 -0.5 measured when there was little water exiting the Verret Subbasin through Bayou Boeuf and stage of May 1 July 24 May 29 Oct. 16 Oct. Dec. 11 Aug. 21 Nov. 13 Nov.

June 26 LAR at Morgan City was 4 to 5 ft above NAVD88. Sept. 18 1996 When stage of the LAR at Morgan City was 5.5 to EXPLANATION 7 ft above NAVD88, discharge in the Avoca Island 3 Velocity, Bayou L’Eau Bleu Cutoff Channel was only 17,000 to 22,000 ft /s. In Stage, Lower Atchafalaya River at Morgan City January 1998, more water was flowing south through Bayou Boeuf at railroad bridge at Amelia Figure 6. Daily-average stage of the Lower Atchafa- laya River at Morgan City and three-hour average due to the unusually heavy precipitation. Inflow velocity of Bayou L’Eau Bleu, Louisiana, May to through the Avoca Island Cutoff Channel was close December 1996. to zero even though the LAR at Morgan City was 5.5 ft above NAVD88. Discharge in the Avoca In the GIWW at Cypremort, daily-average Island Cutoff Channel during periods with little discharge was usually to the west, toward Vermilion prior precipitation seemed to maintain the same Bay. The exception was September 1998 (fig. 5), relation with stage of the LAR at Morgan City as when discharge was eastward toward Jaws Bay. On during other times (pl. 1). average, fall discharge was smaller than earlier in the The amount of water flowing south through year. In the fall of 1999, discharge was much Bayou Boeuf at railroad bridge at Amelia increased smaller than in the fall of 1997, and flow more fre- with precipitation (fig. 7). In 1997, total monthly quently was to the east. rainfall was high for April, May, and June. Flow in Discharge in Relation to Stage of the Lower Bayou Boeuf at railroad bridge at Amelia was to Atchafalaya River and Precipitation the south, and monthly-average discharge was between 8,000 and 10,000 ft3/s. In April, May, and To evaluate how stage of LAR at Morgan City June 1998, precipitation was low and monthly- 3 affects flow in the GIWW, the instantaneous dis- average discharge was only 2,700 ft /s to the south. charge measurements made at all points along the In February 1999, there was very little rain, and GIWW were plotted against same-day average stage monthly-average discharge in Bayou Boeuf was of the LAR at Morgan City (pl. 1). Measurements close to zero. LAR stage was still increasing dur- made after periods of heavy precipitation or little ing this time. In April 1999, there was even less prior precipitation are highlighted to show the influ- rain than in February 1999, but discharge in Bayou ence of precipitation on flow in the GIWW. Boeuf was slightly higher, possibly due to the fall-

14 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 8 28

6 20

16 4 12 IN INCHES IN 8 2

IN FEET ABOVEIN NAVD88 4 MONTHLY-AVERAGE STAGE, STAGE, MONTHLY-AVERAGE 0 0 MONTHLY TOTAL PRECIPITATION, ,

E 20,000

ARG 10,000 H C 8,000 IS D

E 6,000

RAG 4,000 E V 2,000 LY-A H

T 0 N IN CUBICINSECOND FEET PER O -2,000 M

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. ing stage of the LAR at Morgan City. An unknown tion with stage of the LAR at Morgan City. High portion of the Verret Subbasin flow is generated by discharge occurred even when there was little pre- the Atchafalaya River through locks far upstream cipitation, for example in April and May 1998 and of Morgan City (Wells and Demas, 1977). April 1999. High precipitation may have contributed to the high average discharge in January 1998, Bayou Penchant intersects the Avoca Island but an increase in precipitation did not appear to Cutoff Channel at the confluence with Bayou increase discharge at Bayou Penchant south of Chene (pl. 1). Bayou Penchant captures water Morgan City in October 1999. The lower dis- from the Avoca Island Cutoff Channel that would charge measured in 1999 compared to 1998 otherwise be available for eastward flow in the appeared to result from deployment of new instru- GIWW and is one of two main conduits of water mentation and a new stage and point-velocity flow flowing into the interior of the western Terrebonne relation rather than natural phenomena such as less Basin (between Lower Atchafalaya River and rainfall, although it rained less in 1999 than in Houma Navigational Canal) wetlands. Monthly- 1998. The disconnect between stage and discharge average discharge at Bayou Penchant south of from September to October 1998 is shown by the Morgan City was always to the southeast (fig. 8), continuous data plotted in figure 5. The data prior removing water from the Avoca Island Cutoff to October 1998 probably overestimate actual dis- Channel. Monthly-average discharge at Bayou charge. The new stage-discharge relation has been Penchant south of Morgan City had a positive rela- in use since September 1998.

SURFACE-WATER HYDROLOGY 15 8 28 Stage, Lower Atchafalaya River at Morgan City 24 Stage, Bayou Penchant south of Morgan City 6 Precipitation, Morgan City 20 S 16 E H 4 12 IN INC IN 2 8 4 INABOVE FEET NAVD88

MONTHLY-AVERAGE STAGE, STAGE, MONTHLY-AVERAGE 0

0 TOTAL PRECIPITATION, MONTHLY

10,000

8,000

6,000

4,000

2,000

IN CUBIC FEET PER SECOND -2,000 MONTHLY-AVERAGE DISCHARGE, MONTHLY-AVERAGE

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 Pen- chant south of Morgan City, Louisiana, October 1996 to December 1999.

The GIWW near Bay Wallace east of Mor- measurements made at other times. Extended gan City is the other main conduit of water from periods with little rain did not alter the relation the LAR at Morgan City and the Verret Subbasin. between stage and discharge substantially. The Here, water usually moved eastward toward the GIWW between Bay Wallace and Houma has few HNC. Discharge in both Bayou Penchant south of sections with well-defined levees. Rather than Morgan City and in the GIWW near Bay Wallace being restricted to the GIWW, water easily can east of Morgan City increased predictably with leave the GIWW at the many breaks in the levees. increases in the stage of the LAR at Morgan City When the LAR at Morgan City was 3 ft or less (pl. 1). Discharge in the GIWW near Bay Wallace above NAVD88, measured discharge in the east of Morgan City and west of the HNC at GIWW near Bay Wallace east of Morgan City fre- Houma was similar in magnitude and responded in quently was less than that measured west of the similar fashion to changes in stage of the LAR at HNC at Houma. Discharge at the latter site gener- Morgan City. At stages between 2.5 and 4 ft ally remained above 4,000 ft3/s. Flow reversals above NAVD88, discharge increased rapidly to (flow toward the LAR) were observed several about 8,000 ft3/s and, then, continued to increase times in the GIWW near Bay Wallace east of Mor- more gradually to maximums of about 10,000 to gan City during low stage of the LAR at Morgan 12,000 ft3/s. Discharge measured after the heavy City, but were never observed west of the HNC at rainfall in January 1998 was within the range of Houma.

16 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Water leaves the GIWW and flows into the Heavy precipitation during January 1998 western Terrebonne Basin wetlands through Bayou reduced flow in the GIWW east of the HNC at Penchant south of Morgan City, as previously dis- Houma (pl. 1). This could be due to runoff causing cussed, and through Bayou Copasaw and Minor’s increased stage in the Barataria Basin. Increased Canal farther east. The latter two waterways cap- stage in the Barataria Basin would impede east- ture a percentage of the flow in the GIWW during ward flow in the GIWW and route more water high stage of the LAR at Morgan City and convey southward through available outlets such as Bayou water southward (fig. 9). Combined discharge in L’Eau Bleu and the HNC. This effect would be Bayou Copasaw and Minor’s Canal was about 10 magnified by the extra water Bayou Lafourche to 30 percent of the discharge in the GIWW. would be carrying following heavy precipitation. Periods with little prior precipitation did not appear Stage of the LAR at Morgan City was a good to affect the relation between discharge and stage predictor of how much water was flowing into the of the LAR at Morgan City. During low river GIWW, but conditions of little or heavy prior pre- stage, eastward flow in the GIWW past the HNC cipitation determined the proportions and routing varied from near zero to 2,000 ft3/s. Flow occa- of the main sources of the water. In general, unless sionally was reversed (westward flow) in the there was a severe drought, water in the GIWW GIWW east of the HNC at Houma, and west and east of the confluence with Bayou Chene contained east of Bayou Lafourche at Larose. at least some Bayou Boeuf water. During periods of heavy precipitation, some of the water from Between Lower Atchafalaya River and Wax Lake Bayou Boeuf moved southward through Bayou Outlet Chene and combined with LAR water flowing into Bayou Penchant south of Morgan City from the Discharge in the GIWW at LAR south of Avoca Island Cutoff Channel south of Morgan City Morgan City increased rapidly with increased stage (author’s personal observations during 1997-99). in a linear manner, reaching a peak of about Periods of heavy precipitation and low LAR stage 35,000 ft3/s when stage of the LAR at Morgan increased the proportion of water from the Verret City was 7 ft above NAVD88 (pl. 1). At that Subbasin flowing south along this route. stage, about 380,000 ft3/s was flowing south in the LAR at Morgan City. At stages higher than 5 ft East of Houma Navigation Canal above NAVD88, discharge in the GIWW east of Discharge in the GIWW at sites between WLO at Calumet ranged from 5,000 to about 3 Houma and Larose increased to about 4,000 ft3/s as 10,000 ft /s, but did not show a direct relation to the stage of the LAR increased from 2.5 to 4 ft stage. As the LAR at Morgan City increased from above NAVD88 (pl. 1). If discharge in the GIWW 5 to 7 ft above NAVD88, much of the additional east of the HNC predictably increases as stage of water flowing into the GIWW south of Morgan the LAR at Morgan City increases to 4 ft or more City drained south through the connecting bayous, above NAVD88, it was not readily apparent from rather than continuing on to WLO. The heavy the measurements made during the present study. rains of January 1998 seemed to increase the Discharge in the GIWW east of the HNC at Houma amount of water flowing in this segment of the and east and west of Bayou Lafourche at Larose GIWW, but discharge was not clearly lower during was similar. An estimate of the volume of water periods of little prior precipitation. When the LAR flowing east was made by averaging all discharge at Morgan City was less than 3 ft above NAVD88, measurements when stage of the LAR at Morgan discharge in the GIWW east of WLO near Calumet City was 3 ft above NAVD88. In the GIWW east and in the LAR south of Morgan City usually was of the HNC at Houma, average discharge was similar in magnitude, but varied more or less inde- about 3,100 ft3/s. In the GIWW east and west of pendently of stage of the LAR at Morgan City. Bayou Lafourche at Larose, the discharge was about 2,880 ft3/s. Bayou Lafourche typically car- West of the Wax Lake Outlet 3 ries about 200 to 250 ft /s of water southward to Discharge in the GIWW west of WLO south the GIWW at Larose (Garrison and others, 1996). of Calumet increased linearly with increases in A small amount of water may leave the GIWW to stage of the LAR at Morgan City when stage was the south through Bayou Lafourche.

SURFACE-WATER HYDROLOGY 17 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 2.5 ft or more above NAVD88 (pl. 1). When LAR when the LAR at Morgan City was less than 2.5 ft at Morgan City was 7 ft above NAVD88, about above NAVD88. At such times, flow reversals at 20,000 ft3/s flowed westward from the WLO into the latter site were frequent. The computed hourly the GIWW. This was about 10 percent of the discharge at the continuously recording gage in the approximately 200,000 ft3/s flowing south in WLO GIWW at Cypremort was averaged by month and at Calumet. When the LAR at Morgan City compared to total monthly precipitation at Morgan reached 5 ft or more above NAVD88, discharge in City, stage of the LAR at Morgan City, and stage in GIWW east of Jaws Bay west of Franklin GIWW at Cypremort. Monthly-average discharge decreased relative to discharge west of WLO south in the GIWW at Cypremort correlated positively of Calumet, indicating the intervening stretch of with stage of the LAR at Morgan City. Lower dis- the GIWW accumulated some of the inflowing charge occurred in the fall; higher discharge water. When the LAR at Morgan City was less occurred during the spring flood (fig. 10). Neither than 2 ft above NAVD88, the opposite generally little nor heavy prior precipitation affected dis- was true; discharge in the GIWW east of Jaws Bay charge during high LAR stage, remaining steady at west of Franklin was between 5,000 and about 2,000 ft3/s for much of the peak discharge. 8,000 ft3/s, but west of WLO south of Calumet was High Gulf of Mexico levels affected flow along less than 5,000 ft3/s. When the LAR at Morgan this stretch of the GIWW. For example, in Septem- City was at low stage, the segment of the GIWW ber 1998, the average monthly discharge was between west of WLO south of Calumet and east toward the east. Gulf of Mexico level was unusu- of Jaws Bay west of Franklin contributed water ally high during this month because of the tropical flowing toward Jaws Bay that did not originate storms (fig. 4), and appeared to disrupt the westerly from WLO. Flow at both sites was to the west. flow in the GIWW between Jaws Bay and Cypre- mort. Discharge in the GIWW east and west of Jaws Bay after heavy precipitation was higher than Discharge along the GIWW was highest expected from relations between other discharge where it crossed the LAR and WLO. Water flowed measurements and LAR stage (pl. 1). Heavy ante- east and west in the GIWW, in decreasing volumes cedent precipitation likely caused this anomaly. with increasing distance from the source. Dis- Flow did not appear depressed after periods of little charge tended to increase as same-day average prior precipitation. The Charenton Canal intersects stage of the LAR at Morgan City increased beyond the GIWW at the opening to Jaws Bay and could 3 ft above NAVD88 at most of the sites between influence the amount of water entering the GIWW HNC and Jaws Bay. When the LAR at Morgan and West Côte Blanche Bay. The canal may play City reached 3 to 4 ft above NAVD88, discharge in an important role in maintaining flow in the the GIWW became more predictable. The flow of GIWW during periods of low stage of the LAR at freshwater along most of the GIWW studied was Morgan City or in adding large amounts of runoff enhanced not so much by absolute high stage of the to the GIWW when there is heavy precipitation. LAR at Morgan City as by the total time the LAR The USGS began monitoring discharge in this at Morgan City was 3 ft or more above NAVD88. canal with a continuous velocity and stage gage in The LAR at Morgan City is 3 ft or more above the summer of 1999; however, results were not NAVD88 for about 7 months in an average year. included in this study. Discharge in the Gulf Intracoastal Waterway Discharge in the GIWW was substantially from 1997-99 less on the west side of Jaws Bay compared with discharge measured to the east of Jaws Bay To the east of the LAR, monthly-average (table 2). Discharge measured west of Jaws Bay discharge from the Verret Subbasin through Bayou near Cypremort was about 3,000 to 5,000 ft3/s Boeuf at railroad bridge at Amelia varied from whenever stage of the LAR at Morgan City month to month and year to year, independently increased to 5 ft or more above NAVD88. Dis- from stage of the LAR at Morgan City (table 4). charge here and 15 mi farther west in the GIWW at For example, from April through June 1999, the Cypremort usually was similar in magnitude and volume of water flowing south through Bayou flow was always to the west, except during periods Boeuf and into the GIWW was about a third as

SURFACE-WATER HYDROLOGY 19 28

12 IN INCHES IN 8

IN FEET ABOVE NAVD88 4 MONTHLY-AVERAGE STAGE, STAGE, MONTHLY-AVERAGE 0 MONTHLY TOTALPRECIPITATION, IN CUBIC FEET PER SECOND PER FEET CUBIC IN MONTHLY-AVERAGE DISCHARGE,

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.

much as in 1997 during the same months. The vol- Discharge past Houma, across Bayou ume of eastward flow in the GIWW near Bay Wal- Lafourche, and into Barataria Basin did not lace east of Morgan City did not decrease during increase predictably with increasing stage of the these months between 1997 and 1999 by the same LAR at Morgan City. Flow was always eastward amount as in Bayou Boeuf. To compensate for the whenever stage of the LAR at Morgan City was 3 loss of inflowing water from Bayou Boeuf, propor- ft or more above NAVD88 (pl. 1). When stage of tionately more LAR water presumably was flowing the LAR at Morgan City was 3 ft above NAVD88, into the GIWW through the Avoca Island Cutoff average discharge in the GIWW east of the HNC at Channel in 1999 than in 1997, even though the Houma was about 3,100 ft3/s, and in the GIWW spring flood of the LAR was greater in 1997. In east and west of Bayou Lafourche at Larose, the 1997, relatively more water from the LAR was discharge was about 2,880 ft3/s. As little as 30 per- routed southeasterly through Bayou Penchant cent to more than 50 percent of the water reaching south of Morgan City rather than flowing north and the HNC continued flowing east in the GIWW east toward Bay Wallace. The water flowing east- rather than flowing south in the HNC. As flow ward in the GIWW past Bay Wallace at times con- reaching the HNC increased, proportionately more sisted primarily of the Bayou Boeuf flow, and at water flowed south in the HNC rather than contin- other times substantially less. ued east in the GIWW.

20 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 2 at at the Gulf tion Canal GIWW GIWW Houma Naviga- west of west Houma 2 gan gan Bay Bay lace Wal- near Mor- City GIWW east of 1 Discharge at road Boeuf at rail- Bayou Bayou bridge bridge Amelia 1 gan gan Pen- City chant south Bayou Bayou of Mor- City falaya falaya Lower Lower Stage, Atcha- Morgan Morgan River at rge relations for sites along or near the 2 ot applicable] at the tion tion Canal GIWW Houma Naviga- Houma west of 2 gan Bay Bay lace Wal- near Mor- City GIWW east of 1 Discharge at road Boeuf at rail- Bayou bridge bridge Amelia cond. nd, missing data; n cond. --, 1 nd 3,000 -- -- 1,030 3,260 -- -- gan Pen- City chant south Bayou of Mor- City 2.712.752.53 nd2.93 nd 3,049 2,0602.22 1,576 6,040 -- 250 1,171 3,480 -- -- 3,210 ------2.06 -- 2.01 1,640 1.91 1.68 1,000 2,360 1,070 1.71 2,940 500 -- 3,910 940 -- 3,380 -- 3,730 ------falaya falaya Lower Lower Stage, Atcha- Morgan Morgan River at rge is in cubic feet per se is in cubic feet per rge regression equations and from stage-discha 2 ------the the tion tion GIWW Houma Naviga- Houma west of Canal at 2 gan Bay Bay lace Wal- near Mor- City GIWW east of 1 rtical Datum of 1988. Discha Discharge at road Boeuf Bayou at rail- bridge 1997 1998 1999 Amelia 1 ee appendix, fig. A1). appendix, fig. ee Pen- City chant Bayou Bayou Morgan Morgan south of of south City falaya falaya Lower Lower Stage, Atcha- Morgan Morgan River at Monthly-average discharge Monthly-average calculated from From stage-discharge relations. stage-discharge From (s regression equation From Month Jan.Feb. 3.99Mar. 4.71Apr. 6.49May nd 6.90June nd 5.32July nd nd 4.74Aug. nd nd 3.71 nd nd 8,060 2.16 8,640 nd 9,080 6,990 11,000 9,680 nd 11,400 8,120 8,030 nd 10,900 9,820 11,600 4.00 7,240 9,110 4.59 2,660 9,080 5.20 7,620 5.54 nd 8,170 -- nd 5.93 6,550 nd nd 8,560 4.13 9,890 6,780 8,080 4.02 nd 2,720 8,920 nd 9,680 10,100 7,000 1,630 nd 7,930 3,260 8,890 10,500 9,430 2.99 8,270 10,000 4.94 nd 4.87 1,590 4.33 7,210 8,100 3,080 4.96 3,650 6,120 3,260 7,040 6,490 3.97 4,240 2,730 60 6,290 4,740 1,080 3.31 9,280 8,560 5,420 9,370 3,190 4,050 9,390 8,370 7,530 8,480 8,520 8,030 3,850 6,960 6,920 5,930 Sept. 2.13Sept. Oct.Nov. nd 2.01Dec. 1.66 (Jan.-July)Average 1,950 1.68 (Aug.-Dec.)Average nd -- nd nd nd 2,180 nd 3,090 nd 2,780 -- 4,030 -- 9,440 -- -- 8,770 nd nd 9,090 8,220 4,240 2,630 8,260 7,320 [Stage is in feet above North American Ve above feet in is [Stage 1 2 Intracoastal Waterway (GIWW) east of the Lower Atchafalaya River, 1997-99 River, Atchafalaya the Lower of east (GIWW) Waterway Intracoastal Table 4.

SURFACE-WATER HYDROLOGY 21 2 at mort GIWW Cypre- 1 Bay Jaws Jaws Across Across

1 indicates flow indicates flow Discharge Bay Bay Jaws Jaws GIWW east of west of Franklin

1 ------540 tive discharge discharge tive ------950 Wax Wax Lake 11,400 9,930 6,780 nd Outlet Outlet GIWW west of south of Calumet 2.99 7,140 7,080 4,690 nd 1.71 gan City River falaya falaya Lower Lower Stage, Atcha- at Mor-

2 at nd nd scharge relations for sites along scharge relations for sites along the Gulf indicates flow to west. Nega to west. indicates flow mort GIWW Cypre-

1 -- nd Bay Jaws Jaws Across Across

1 Discharge Bay Bay Jaws Jaws GIWW GIWW east of west of Franklin

1 Wax Lake Outlet Outlet GIWW west of south of Calumet City falaya falaya Lower Lower Stage, Atcha- Morgan Morgan River at harge is in cubic feet per second. Positive discharge discharge Positive second. per feet is in cubic harge 2 om regression equations and from stage-di at mort GIWW Cypre- Cypre- 1 Bay Jaws Jaws Across Across 1 Vertical Datum of 1988. Disc Datum of Vertical Discharge Bay Bay Jaws Jaws GIWW east of west of Franklin 1 1997 1998 1999 ee appendix, fig. A2). fig. ee appendix, Wax Lake Outlet Outlet GIWW west of south of Calumet laya City Lower Lower Stage, Morgan Morgan River at Atchafa- Monthly-average discharge calculated Monthly-average fr Month Jan. 3.99 10,700 9,630 6,570 nd 4.00 10,700 9,660 6,590 Feb.Mar. 4.71Apr. 6.49May 13,200 6.90June 19,500 11,100 5.32July 21,000 13,900 4.74 7,650Aug. 15,400 14,500 3.71 9,740 nd 2.13Sept. 13,300 12,200 2.16 10,100 ndOct. 9,700 11,200 nd -- 8,450 4.59Nov. -- 2.01 nd 5.20 7,700 8,990Dec. 12,800 1.66 5.54 nd -- 15,000 6,100 -- 1.68 -- 10,900 5.93 16,200 nd -- 12,000 7,480 4.13 -- 17,500 -- 12,500 -- -- 8,300 2,160 4.02 11,200 1,150 -- 8,710 13,100 2,100 1,580 4.94 -- -- 10,800 9,150 1,950 9,940 2.75 4.87 -- 2.71 14,000 2,340 6,800 4.33 9,700 930 -- 13,800 -- 1,560 11,500 1,650 6,620 4.96 -- 11,900 2.53 11,400 nd 7,960 2.93 2,260 3.97 14,100 10,400 -- 7,870 1,740 -- -- 3.31 7,100 10,600 11,600 2.22 -- 1,720 -- 7,990 1,250 -- 8,300 9,600 -- -- 1,700 -- -130 6,500 7,970 1,570 -- 1,680 -- 2.01 5,350 -- 2.06 1,630 nd -- -- nd -- 1.91 nd -- 1.68 ------160 1,000 -- -- 310 310 Average (Jan.-July)Average (Aug.-Dec.)Average 14,700 -- 11,600 8,040 -- nd -- nd 13,500 11,100 7,660 -- -- From regression equation (s From regression equation relations. From stage-discharge [Stage is in feet above the North American the North feet above [Stage is in to east. nd, missing data; --, not applicable] --, data; missing nd, to east. Intracoastal Waterway (GIWW) west of the Lower Atchafalaya River, 1997-99 River, the Lower Atchafalaya of west (GIWW) Waterway Intracoastal 1 2 Table 5.

22 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Monthly-average discharge decreased west- GIWW can move saltwater laterally. These can be ward from Morgan City, with a sharp decrease driven by elevated stage associated with tropical across Jaws Bay at the opening to West Côte storms and/or winds. Under these conditions, the Blanche Bay (table 5). From January to August, north-south trending HNC and Bayou Perot can about 60 percent of the volume of water entering convey saltwater in an inland direction into the the GIWW west of WLO near Calumet left the GIWW. From there, the water can then move later- GIWW at Jaws Bay. More than 10 percent of the ally in the GIWW following the normal gradients. original volume reached the GIWW at Cypremort, but the source of the westward flowing water was Elevated stage in the Gulf of Mexico can not necessarily all derived from WLO. cause flow reversals in the GIWW, canals, and bayous, and push salinity in an inland direction. For Measuring discharge directly with a gage example, in the last week of September and for the continuously recording velocity and stage is prefer- first 2 weeks of October 1997, strong offshore able to using estimates derived from regression winds pushed more saline water toward the fresher equations. Indirect estimates using the stage of the parts of coastal Louisiana (fig. 5). At the HNC at LAR at Morgan City and instantaneous discharge Dulac, daily average discharge was in an inland measurements are subject to increased error. The direction from October 7-13. Salinity at Little regression equations used to indirectly estimate Lake near Cut Off increased markedly during the discharge have a relatively high correlation coeffi- same time, indicating northward flow of more cient and meet the needs of this study. There is saline waters from the Gulf of Mexico (fig. 11). value to the numbers generated by the regressions Three synoptic surveys of surface-water salinity in to evaluate in general terms what contributes to the the GIWW between Larose and Lafitte, La., were flow of a particular segment in the GIWW. When done during this time (fig. 11). Data collected dur- the LAR at Morgan City is 3 ft or more above ing the surveys indicate that some of the saltier NAVD88, the GIWW effectively distributes freshwater moving north flowed east and west into the water and sediments from the LAR and WLO to GIWW where it crossed Bayou Perot. Salinities points in coastal Louisiana 30 to 50 mi east and greater than 14 ppt were measured in the GIWW west of Morgan City. west of Bayou Perot just after passage of a weather front from October 13-14. Salinity reached 8 ppt Flow Reversals in Segnette Waterway. The water intruded through the canal, and not through Lake Salvador. Under Events that push Gulf of Mexico water in an these conditions of rapidly inflowing water from inland direction can result in short periods of flow the south, flow was temporarily westward in the reversal in the GIWW and periods when 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.

SURFACE-WATER HYDROLOGY 23

3.5 1 90°30' 15' 90°00' T Houma Navigation Canal at Dulac ER E RIV

PI NEW ORLEANS E D IP S 3.0 F S Velocity N I

MISS

, Y

2.5 0 E

R E 2.0 O

Stage L P STAGE, IN FEET

ABOVE NGVD29 ABOVE E

B 0.0

a V

y 1.5 -1 o u d Positive velocity indicates southward flow; e 0.0 SEGNETTE s A l WATERWAY negative velocity indicates northward flow lem 0.0 and 3.0 s 2.0 25 29°45' Lake 4.0 4.0 20 Salvador Oct. 9 Oct. Oct. 17 4.0 4.0

a Y 15 y A o

W u Oct. 14 R E P

T e

A r W o B t 10 L A A R T A S T A A R O I C A A W 5 R T A T GU IN E PER THOUSAND PER LF R W

SALINITY, IN PARTS PARTS IN SALINITY, Little A Y B 0 a y o u Lake

L a 29°30' f o u r c

Oct. 3 Oct. h e Oct. 10 Oct. 17 Oct. 23 Sept. 19 Sept. 26 Sept. Oct. 9, 1997

1997 ARKANSAS 0 5 MILES EXPLANATION Gulf Intracoastal Waterway east of Larose 0 510 KILOMETERS

Gulf Intracoastal Waterway east of MISSISSIPPI Houma Navigation Canal at Houma

TEXAS Enlarged Little Lake near Cut Off area 0.6 Salinity, in parts per thousand

Index Map

90°30' 15' 90°00' 90°30' 15' 90°00'

ER RIV IVER I I R PP NEW ORLEANS P NEW ORLEANS I IP S S IS S MISS MISS I

0.6 B SEGNETTE B a

a y WATERWAY y o SEGNETTE o u 8.0 u d WATERWAY e d s e 7.0 s Al A le 0.4 ll m em and a s 3.2 nds 3.2 9.4 5.4 29°45' 5.0 3.4 29°45' 5.2 Lake 9.1 Lake 10.9 5.7 5.3 Salvador Salvador 8.5 5.2

7.5 9.2 5.1 B

B5.0

Y a

A y 4.9 A y W o W o R u 4.2 14.5 R u

E 3.7

T P E T P

A 14.5 e 2.9

A e 11.2 r W 3.3 r

o W 7.9 o

t t B L 5.2 B A A T A A R T S R S A A A A O T T A O A C R R A C I A I R A 6.4 A T R W T W N 5.1 I A GULF A IN T GULF T E E R R W Little W 7.8 Little A B A B Y a Y a y 5.9 o y o u Lake u Lake L L a a f 29°30' o f 29°30' o u u r c r c h h e e Oct. 14, 1997 Oct. 17, 1997 Louisiana Oil Spill Coordinator, Office of the Governor, Louisiana GIS CD: A Digital Map of the State, Version 2.0

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.

24 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Lafourche, and flow was eastward in the GIWW bridge at Amelia preceding each set of measure- from the HNC toward Bayou Lafourche. After ments. The 21-day average discharge at Bayou passage of the weather front, eastward flow along Boeuf at railroad bridge at Amelia was assumed to the entire stretch of the GIWW between the HNC indicate influence of local runoff on flow in the and Bayou Perot resumed. GIWW.

Net Gain or Loss of Water Along Major During slightly more than half the synoptic Segments trips, instantaneous discharge was greater in the GIWW west of the HNC at Houma than near Bay To better understand the surface-water Wallace (segment A, fig. 12). Adjacent wetlands hydrology of the GIWW and its implications for contributed water to the GIWW along this segment regional freshwater availability, it is helpful to con- during high and low stage of the LAR at Morgan sider (1) whether individual segments of the City. On two occasions, water flowed both west GIWW contribute water to the surrounding wet- toward Morgan City and east toward the HNC. lands or drain water from them, and (2) whether When the 21-day average discharge was less than this characteristic changes depending on inflowing 4,500 ft3/s at Bayou Boeuf at railroad bridge at river water or antecedent precipitation. Three seg- Amelia, this segment acted both as a source and ments of the GIWW east of Morgan City between sink of water (fig. 12). The segment tended to Bay Wallace and Bayou Lafourche and three seg- store inflowing water when the LAR at Morgan ments of the GIWW west of Morgan City between City was 3 ft or more above NAVD88. When the the WLO and Cypremort were characterized. Net LAR at Morgan City was less than 3 ft above inflow or outflow can occur in any of these seg- NAVD88, more water tended to flow out of this ments. Net inflow occurs when more water enters segment than entered. When the 21-day average than leaves from one or both ends of a segment and discharge of water from Bayou Boeuf was greater flows into the surrounding wetlands. In this than 6,000 ft3/s, there was a net outflow of water to instance, the GIWW functions as a freshwater adjacent wetlands from the segment of the GIWW diversion, contributing to the water budget of the between Bay Wallace and west of the HNC. wetlands. Net outflow occurs when more water leaves than enters a segment through one or both The GIWW was a source of water to the ends. In this case, the GIWW drains some water HNC in segment B (fig. 12) during almost all the from the wetlands. Water from the surrounding synoptic trips. The volume of water entering the wetlands contributes flow to the GIWW. HNC generally increased as eastward flow in the GIWW near Bay Wallace east of Morgan City For the three segments east of Morgan City increased, but the relation between LAR stage and (segments A, B, C; fig. 12), the difference in discharge from the GIWW to the HNC was vari- instantaneous discharge between segment start and able. For example, when the LAR at Morgan City segment end was plotted with the instantaneous was 4 ft or more above NAVD88, discharge from discharge at GIWW near Bay Wallace east of Mor- the GIWW to the HNC ranged from near zero to gan City. The complex interactions of flow from greater than 8,000 ft3/s. The relation between local Bayou Boeuf, Bayou Chene, and the Avoca Island runoff and the amount of water that left the GIWW Cutoff Channel were the reason the analysis started at the HNC was more direct. As the 21-day aver- at GIWW near Bay Wallace east of Morgan City age discharge at Bayou Boeuf at railroad bridge at and not closer to the LAR. For the three segments Amelia increased, the amount of water leaving the west of Morgan City (segments D, E, F; fig. 13), GIWW through the HNC increased steadily, the difference in instantaneous discharge at the regardless of whether stage of the LAR at Morgan start and end of each segment was plotted with City was high or low. Local runoff appears to con- instantaneous discharge in the GIWW west of tribute to the flow of water in the GIWW near the WLO near Calumet. For all six segments the dif- HNC. ference in instantaneous discharge at the start and end of each segment also was plotted with the 21- The wetlands adjacent to the GIWW day average discharge at Bayou Boeuf at railroad between the HNC and Bayou Lafourche stored and

SURFACE-WATER HYDROLOGY 25 6,000 6,000

4,000 4,000

2,000 2,000

0 0 -2,000 -2,000

-4,000 -4,000

-6,000 -6,000 Navigation at Houma Canal Navigation

City west minus GIWW of Houma -8,000 -8,000 -4,0000 4,000 8,000 12,000 0 4,000 8,000 12,000 GIWW near Bay Wallace east of Morgan 12,000

8,000

4,000

at Houmaat 0

Canal at Houma minus atGIWW minus Houma Canal -4,000 east of Houma Navigation Canal GIWW west of Houma Navigation Navigation Houma of west GIWW -4,0000 4,000 8,000 12,000 0 4,000 8,000 12,000 8,000

6,000

4,000

2,000

0 DIFFERENCE IN INSTANTANEOUS DISCHARGE,IN CUBIC FEETPER SECOND

-2,000 of Bayou Lafourche at at BayouLarose Lafourche of

GIWW east of Houma Navigation Navigation Houma of east GIWW -4,000 Canal at Houma minus GIWW minus at GIWW west Houma Canal -4,0000 4,000 8,000 12,000 0 4,000 8,000 12,000

INSTANTANEOUS DISCHARGE, IN CUBIC FEET 21-DAY AVERAGE DISCHARGE, IN CUBIC FEET PER PER SECOND, OF GIWW NEAR BAY WALLACE SECOND, OF BAYOU BOEUF AT RAILROAD BRIDGE EAST OF MORGAN CITY AT AMELIA (Positive flow is to the east) (Positive flow is to the south)

EXPLANATION

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)

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.

26 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 6,000 6,000 D D 4,000 4,000

2,000 2,000 D

0 0

ECON -2,000 -2,000 S

-4,000 -4,000

-6,000 -6,000 Jaws Bay west of Franklin EET PER -8,000 F -8,000 of Calumet minus GIWW east of east GIWW minus Calumet of

GIWW west of LakeWax Outlet south 5,000 10,000 15,000 20,000 IC 0 0 4,000 8,000 12,000 B 12,000 12,000 E E

8,000 8,000 ARGE, IN CU

H 4,000 4,000 C S I D

S 0 0 of Jaws Bay near Cypremort near Bay Jaws of GIWW eastGIWW of Jaws Bay west of Franklin minus GIWW west

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

TANTANEOU 8,000 S 8,000 F F 6,000 6,000

4,000 4,000

2,000 2,000 ERENCE IN IN F F I 0 0 D

-2,000 -2,000 mort GIWWminus Cypermort at

GIWW GIWW ofwest Jaws Bay near Cypre- -4,000 -4,000 0 5,000 10,000 15,000 20,000 0 4,000 8,000 12,000

INSTANTANEOUS DISCHARGE, IN CUBIC FEET 21-DAY AVERAGE DISCHARGE, IN CUBIC FEET PER SECOND, AT GIWW WEST OF WAX LAKE PER SECOND AT BAYOU BOEUF AT RAILROAD OUTLET SOUTH OF CALUMET BRIDGE AT AMELIA (Positive flow is to the west) (Positive flow is to the south)

EXPLANATION

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

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.

SURFACE-WATER HYDROLOGY 27 drained water (segment C, fig. 12). Typically, above NAVD88, but the volume progressively when the LAR at Morgan City was less than 3 ft decreased as flow into the GIWW from WLO above NAVD88, water was stored in this segment. increased (fig. 13). When the LAR at Morgan City The volume stored did not increase or decrease was 4 ft or more above NAVD88, the volume of depending on the initial amount of water flowing water entering this segment at Jaws Bay generally eastward at GIWW near Bay Wallace east of Mor- was similar to the volume leaving at Cypremort gan City. Less frequently, this segment of the indicating throughflow conditions. The exception GIWW released water to the adjacent parts of the occurred during the high precipitation in January GIWW. When local runoff was low as indicated 1998, when almost 6,000 ft3/s of water was stored by the 21-day average discharge in Bayou Boeuf at along the segment of the GIWW between Jaws Bay railroad bridge at Amelia, the GIWW between the and Cypremort. As the 21-day average discharge HNC and Bayou Lafourche and adjacent wetlands at Bayou Boeuf at railroad bridge at Amelia appeared to store the inflowing water. As local increased from 2,000 ft3/s to more than 8,000 ft3/s, runoff increased, more water appeared to leave this the rate at which water was stored in the GIWW segment by flowing eastward. The data suggest between the Jaws and Cypremort increased by that as local runoff increases, this segment is a about 1,000 to 2,000 ft3/s. Two linear curves with source of water to adjacent parts of the GIWW. the same slope show this trend. The data indicate The relation is reasonably direct for the available that local runoff also accumulates in and contrib- data. utes to flow along this segment of the GIWW. The GIWW between WLO and Jaws Bay The segments of the GIWW east of Morgan shifted from draining water to Jaws Bay to storing City and west of WLO differ hydrologically in water as instantaneous discharge in the GIWW whether they contribute water to, or drain water west of WLO south of Calumet increased in seg- from adjacent wetlands. In the segments closest to ment D (fig. 13). During low discharge, more the source (segments A and D), water entering the water left the GIWW at Jaws Bay than entered the GIWW west of WLO south of Calumet was stored GIWW from the WLO. Water from the Charenton in increasing volumes in the area between WLO Drainage Canal and adjacent wetlands contributed and Jaws Bay as inflow increased. To the east of to flow in the GIWW. Local runoff did not appear Morgan City in segment A, the wetlands between to influence whether more or less water was stored the GIWW near Bay Wallace east of Morgan City along this segment (fig. 13). and west of the HNC at Houma more frequently With one exception, water flowed from the contributed water to the GIWW than stored water GIWW to Jaws Bay in segment E during all synop- from the GIWW. Water left the GIWW both at tic trips (fig. 13). About 2,000 to 3,000 ft3/s or Jaws Bay and at the HNC. The volume of water more water was leaving the GIWW east of Jaws leaving the GIWW at Jaws Bay in segment E Bay west of Franklin than was entering west of increased as the volume of river water entering the WLO south of Calumet when the LAR at Morgan GIWW west of WLO south of Calumet increased. City was 3 ft or less above NAVD88. The volume The volume of water leaving the GIWW through of water leaving increased steadily as flow entering the HNC in segment B increased as discharge at the GIWW west of WLO south of Calumet Bayou Boeuf at railroad bridge at Amelia increased. When the LAR at Morgan City was 4 ft increased. The increase in flow into the HNC was or less above NAVD88, the volume of water leav- influenced less by the volume of water entering the ing the GIWW at Jaws Bay appeared to increase as GIWW near Bay Wallace east of Morgan City (fig. local runoff increased. When the LAR at Morgan 12). The GIWW between Jaws Bay and Cypre- City was 4 ft or more above NAVD88, a relation mort (segment F) rapidly changed from contribut- between local inflow and instantaneous discharge ing water to the wetlands to mainly net across Jaws Bay was not evident. throughflow as the inflow into the GIWW west of WLO near Calumet increased from low to high. At Water entered from the west and was stored low inflow, water entering this segment was from in segment F between Jaws Bay and Cypremort, west of Cypremort and west of Jaws Bay. East of when the LAR at Morgan City was 3 ft or less Morgan City, the volume of water flowing east-

28 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 ward near Bay Wallace east of Morgan City did not averaged from 1989-99, for years when the spring influence storage or release of water in the GIWW flood of the Atchafalaya River was exceptionally in segment C between the HNC and Bayou high or low, and for a period of sustained moderate Lafourche. Local runoff appeared to change stor- flow. age characteristics of the two segments farthest from WLO and the LAR but in opposite ways. Under average conditions, stage of the LAR West of WLO, the volume of water stored in seg- at Morgan City remains 3 ft or more above ment F increased as runoff increased. Increasing NAVD88 from about January to mid-July, and 4 ft local runoff appeared to increase the net volume of or more above NAVD88 for about 6 weeks less. In water leaving segment C of the GIWW between 2000, annual discharges of the Mississippi River Houma and Larose. and Atchafalaya River were among the 10 lowest on record. Stage of the LAR at Morgan City Magnitude, Duration, and Timing of Flow exceeded 4 ft or more above NAVD88 only briefly and was 3 ft or more above NAVD88 from April to The difference in elevation between the mid-May and for about 2 weeks in July. During stage of the LAR or WLO and surrounding waters 1997, which was a record flood year, stage of the controls the magnitude, duration, and timing of the LAR at Morgan City reached 8 ft above NAVD88. flow in the GIWW east and west of Morgan City. For a brief period flow along the GIWW would In an earlier section of the report, it was demon- have been exceptionally high, but because LAR strated that discharge in the GIWW becomes more stage was below the long-term average from July predictable whenever the stage of the LAR at Mor- through December 1997, the total volume of river gan City is 3 to 4 ft or more above NAVD88. The water moving laterally in the GIWW was not much period when this condition is met varies consider- different from an average year. To sustain flow and ably from year to year. An examination of the maximize the volume of freshwater delivered to stage record of the LAR allows inferences to be coastal Louisiana marshes, stage of the LAR at drawn on flow characteristics of the GIWW. Fig- Morgan City should remain 3 ft or more above ure 14 shows stages of the LAR at Morgan City NAVD88 for prolonged periods. This occurred in

7 Break in line indicates missing data N I

, 6 EXPLANATION E Shaded area indicates approximate

TAG 5 stages at or above which flow in the S Gulf Intracoastal Waterway E 4 becomes predictable RAG E 3 CHARACTERISTIC FLOW:

Y-AV Average (1989-98) L I FEET ABOVE NAVD88 2 A High spring flood (1997) D Sustained moderate flow (1993) 1 Low spring flood (2000)

1 1

1 1 1 1 1

1 1 1

. .

......

r y

n n

b g

a a

e a a

J M J

F N

D A S

MONTH

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

SURFACE-WATER HYDROLOGY 29 1993, when the LAR at Morgan City was 4 ft or along the GIWW. The percent of time in the year a more above NAVD88 for almost 11 months in discharge equaled or exceeded indicated levels in 1993 and was below 3 ft above NAVD88 for less the GIWW near Bay Wallace east of Morgan City, than 2 weeks. west of HNC at Houma, east of Jaws Bay west of Franklin, and across Jaws Bay was calculated for The percent of time in a year that stage of the characteristic flows in the LAR when stage in the LAR at Morgan City was between 3 ft above LAR was 3 ft or more above NAVD88. The rela- NAVD88 and the maximum elevation for that time tions in figure 15 suggest the time during a year at period was tabulated in ½-ft increments for the characteristic flows indicated discharge is expected four characteristic flows described above (fig. 15). to occur at the selected sites. The amount of fresh- The regression equations relating discharge at water flowing in the GIWW influences sediment selected sites along the GIWW with stage of the transport and salinity in coastal Louisiana (dis- LAR at Morgan City (see appendix) were then cussed in later sections of this report) and has applied to estimate the discharge at selected sites implications for wetland restoration.

EAST OF MORGAN CITY 100 100 90 Near Bay Wallace east of Morgan City 90 West of Houma Navigation Canal at Houma 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 0 0 0 4,000 8,000 12,000 16,000 20,000 0 4,000 8,000 12,000 16,000 20,000

WEST OF WAX LAKE OUTLET 100 100 90 90 Across Jaws Bay 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 0 0 PERCENT OF TIME IN A YEAR DISCHARGE IS EQUALED OR EXCEEDED 0 4,000 8,000 12,000 16,000 20,000 0 4,000 8,000 12,000 16,000 20,000 CALCULATED DISCHARGE, IN CALCULATED DISCHARGE, IN CUBIC FEET PER SECOND CUBIC FEET PER SECOND EXPLANATION CHARACTERISTIC FLOW: Discharge calculated from regression Average (1989-98) equations relating instantaneous discharge to stage of the Lower Atchafalaya River at High spring flood (1997) Morgan City listed in the appendix Sustained moderate flow(1993) in 0.5-ft (feet) increments when stage 3 ft or more above NAVD88 Low spring flood(2000)

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.

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

N 14,000

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

E above NAVD88 about 80 percent of the time; dis- D

N 12,000

R charge was predictable for less than 20 percent of O

E 10,000

the time. Peak discharge was 25 to 60 percent C

lower than in the other three time periods. For the D 8,000

time periods 1989-98 and 1997, discharge was pre- T

E 6,000

E dictable 55 to 60 percent of the time. In 1997, E

F N 4,000

when the Atchafalaya River experienced a large I

U 2,000 spring flood, calculated discharge was greater than A

T for the three other time periods about 45 percent of S

N I 0 the time. In 1993, discharge was predictable for 012345678 more than 80 percent of the time. For average con- SAME-DAY AVERAGE STAGE, ditions (1989-98) and in 1993, discharge in the IN FEET ABOVE NAVD88 GIWW near Bay Wallace east of Morgan City was EXPLANATION between 9,000 and 11,000 ft3/s for at least 40 per- Measurements by U.S. Army Corps of Engineers (1983-84) Discharge = 1,400 + 900*LAR; r2 =0.62 cent of the time; peak discharge for the two time periods was about 70 percent of that reached in Measurements made by U.S. Geological Survey (1997-98) Discharge = 570 + 1,600*LAR, r2 = 0.73 (LAR is stage of the 1997. For average conditions and in 1993, about Lower Atchafalaya River at Morgan City) 6,000 to 8,000 ft3/s of water discharged across the Figure 16. Relation between instantaneous dis- Jaws Bay opening at least 40 percent of the time 3 charge in the Gulf Intracoastal Waterway west of (fig. 15). In 1997, between 12,000 and 16,000 ft /s Houma Navigation Canal at Houma and same-day of water flowed across the Jaws Bay opening at average stage of the Lower Atchafalaya River at Mor- least 40 percent of the time. In 2000, at most 4,000 gan City, Louisiana, 1983-84 and 1997-98. to 6,000 ft3/s of water flowed across Jaws Bay, and this maximum flow occurred less than 20 percent The distance from the GIWW that coastal of the time. wetlands have been affected by freshwater entering from the GIWW has increased since the 1970’s. Stage in the LAR has increased steadily Vegetation surveys of coastal Louisiana marshes since the 1950's and in 1999 was about 2 to 2.5 ft made at regular intervals indicate less salt-tolerant higher (Harley Winer, USACE, oral commun., species have expanded in a southeasterly direction 1999). The downstream emergence of the Atchafa- in western Terrebonne Basin wetlands (Visser and laya Delta in the late 1970's and the aggradation of others, 1999). Average monthly salinity measured the river bed are the principal reasons for this during 6 consecutive years in the 1970’s, and again increase. As the time when the stage of the LAR is from 1995-99, at Caillou Lake (Sister Lake) south- 3 to 4 ft or more above NAVD88 increases, and the west of Dulac, shows a freshening in the spring duration of stage differences between the LAR and months for the more recent time period (fig. 17). canals and bayous in adjacent waterbodies increases, the volume of water moving laterally in 20 the GIWW also will increase. The percent of time 1972-77 1995-99 when the stage of the LAR is greater than 3 ft 15 above NAVD88 could increase in the future with LY SALINITY, further aggradation of the river bed. H

NT 10 O

Discharge in the GIWW at Bay Wallace M E measured during 1997-99 is higher relative to a G 5 given stage of the LAR at Morgan City than in a PER THOUSAND PARTS IN series of measurements made in 1983 and 1984 by AVERA Brackets show±1 standard error the USACE (Donnel and Lederer, 1992) (fig. 16). 0 The increase is substantial. Some of the increase Apr. Jan. Mar. July May Nov. Feb. Dec. Aug. may be attributed to widening and deepening of the Sept. Avoca Island Cutoff Channel in the early 1980's Figure 17. Average monthly salinity at Caillou (N.J. Powell, USACE, oral commun., 1999). Lake (Sister Lake) southwest of Dulac, Louisiana, from 1972-77 and from 1995-99.

SURFACE-WATER HYDROLOGY 31 In the GIWW at the intersection with the recorded at all sites. Water-level gradients were HNC and in Larose, salinity in the spring averaged steepest closest to the LAR and WLO and about 0.2 ppt from 1957-60, prior to the construc- decreased rapidly with increasing distance from the tion of the HNC, and again more recently, from two sources. Water-level gradients along the 1983-88 (fig. 18). From 1972-77, salinity was GIWW were most pronounced during the spring higher in the spring, almost reaching 1 ppt. The flood and were attenuated in the fall months. The flow of freshwater in the GIWW presumably low- September storms in 1998 raised stage along the ered the salinity in the spring months during 1983- entire study reach. Elevated Gulf of Mexico levels 88. In the fall, this freshwater flow was not avail- contributed to the high stage in the GIWW, by able, and average monthly salinity was greater than direct influence and by impeding flow of accumu- 1 ppt. lated runoff from the heavy precipitation.

4 East of Morgan City, more stage data were Intersection with Houma Navigation Canal available from which to infer water-level gradients 3 in the GIWW and adjacent water bodies. Monthly- 2 average stage at the Avoca Island Cutoff Channel, Bayou Boeuf at railroad bridge at Amelia, and 1 Bayou Penchant south of Morgan City were very 0 similar and tracked stage of the LAR at Morgan Apr.

Jan. Mar. July City (fig. 19). Differences in stage between the May Feb. Nov. Dec. Aug. Sept. LAR at Morgan City and these three sites were greater than 3 ft during the spring flooding, but

MONTHLY 4 Intersection with Bayou Lafourche at Larose decreased to less than 1 ft in the fall low-water sea- 3 son. The limited data available for the GIWW near PARTS PER THOUSAND PER PARTS 2 Bay Wallace east of Morgan City during the spring

AVERAGE IN SALINITY, floods for 1998-99 indicate stage is about 0.3-0.5 ft 1 lower compared to the three sites farther west when 0 the LAR at Morgan City was at flood stage. The difference in stage was greater in 1998, when the Apr. July Jan. Mar. Nov. Feb. May Dec. Aug. Sept. spring flood was greater, than in 1999. Along this EXPLANATION segment of the GIWW, stage is lowest east of the 1957-60 1972-77 1983-88 Brackets show± 1standard error HNC at Houma. There was a steeper rise in stage Data source: U.S. Army Corp of Engineers, New Orleans District in the GIWW near Bay Wallace east of Morgan City in response to rising stage of the LAR than Figure 18. Average monthly salinity in the Gulf east of the HNC at Houma when the LAR at Mor- Intracoastal Waterway at the intersections with gan City was 4 ft or more above NAVD88. The Houma Navigation Canal and Bayou Lafourche at Larose, Louisiana, from 1957-60, 1972-77, and water-level gradient between the two sites became 1983-88. steeper as the stage of the LAR at Morgan City increased. From August to December, water-level Stage and Water-Level Gradients gradients among the sites in the western Terreb- Flow in the GIWW is influenced by the dif- onne Basin were at their lowest. ferences in stage between the LAR or WLO and Monthly-average stage at sites along the the interconnecting network of canals, bayous, and GIWW between Morgan City and Houma other surface-water bodies along and near the responded to those of the LAR at Morgan City GIWW. Differences in stage and water-level gradi- when the LAR at Morgan City was about 2.75 to ents in the GIWW and adjacent waterways are vital 3 ft or more above NAVD88 (fig. 20). In the to understanding flow in the GIWW. GIWW east of HNC at Houma, the response was Stage at all the sites along the GIWW small, with an average increase of about 0.3 to tracked the stage of the LAR at Morgan City 0.5 ft as the LAR at Morgan City increased from 3 (figs. 19-22). The spring flood on the LAR was to 7 ft above NAVD88. In contrast to the sites

32 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 8 88 Lower Atchafalaya River at Morgan City D V Avoca Island Cutoff Channel south of Morgan City 7 NA Bayou Penchant south of Morgan City E V Bayou Boeuf at railroad bridge at Amelia O

B 6 GIWW near Bay Wallace east of Morgan City T A

E GIWW east of Houma Navigation Canal at Houma

E 5 , IN F , IN

E 4 TAG S 3 E AG R

E 2 V -A Y L

H 1

NT Break in line indicates missing data O

M 0 Apr. Apr. Apr. Jan. Mar. July July July Jan. Mar. Jan. Mar. Feb. May Feb. May Feb. May Nov. Nov. Nov. Nov. Dec. Dec. Dec. Dec. Aug. Aug. Aug. Sept. Sept. Sept.

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 Stage also decreased southward from GIWW east of the HNC in Houma was more vari- Houma along the HNC. The gradient in stage in able, especially when the LAR at Morgan City was the GIWW from Bayou Lafourche to Bayou Perot less than 5 ft above NAVD88. Stage in the GIWW was very low. During spring peak flow of the east of HNC at Houma responded in a similar man- LAR, the gradient was highest. High stage in Sep- ner as stage in the HNC at Dulac. There appeared tember 1998 was characteristic of all sites in this to be two relations between stage in the Avoca part of coastal Louisiana. At sites near Morgan Island Cutoff Channel, Bayou Penchant south of City, the spring peak in the LAR influenced stage Morgan City, Bayou Boeuf at railroad bridge at more than at distant sites. When gradients are low, Amelia, and GIWW east of HNC at Houma, for example in the fall, a small error in datum depending on whether the LAR was in flood or not. rather than real differences in stage may be the cause of the observed gradient. Stage in the sites to East of Houma, stage decreased toward the east of Houma appeared to track stage in the Bayou Lafourche, both in Company Canal, and in HNC at Dulac more closely than the LAR at Mor- the GIWW east of Bayou Lafourche (fig. 21). The gan City with the exception of the GIWW-Bayou site at Company Canal is located about a tenth of a Perot intersection (fig. 21). mile from Bayou Lafourche and is about 7 mi upstream of the GIWW-Bayou Lafourche intersec- Stage for sites between the WLO and Ver- tion. Stage in Company Canal was higher than in milion Bay near Cypremort Point are shown in fig- the GIWW at Bayou Lafourche. ure 22. The record at all five sites is incomplete.

SURFACE-WATER HYDROLOGY 33 EAST OF MORGAN CITY WEST OF WAX LAKE OUTLET Between Lower Atchafalaya River East of Houma Navigation Canal and Houma Navigation Canal

4 4 4 88

D 3 3 3

2 2 2

1 1 1

0 0 0 0 1 2345678 0 12345678 0 12345 678 MONTHLY-AVERAGE STAGE, WAX LAKE MONTHLY-AVERAGE STAGE, LOWER ATCHAFALAYA OUTLET AT CALUMET, IN FEET ABOVE NAVD88 RIVER AT MORGAN CITY, IN FEET ABOVE NAVD88 4 4 4

3 3 3

2 2 2

LY-AVERAGE IN FEETSTAGE, ABOVE NAV LY-AVERAGE 1 H 1 1

ONT 0 0 0 M 0 1234 0 1234 0 1234 MONTHLY-AVERAGE STAGE, HOUMA NAVIGATION MONTHLY-AVERAGE STAGE, CANAL AT DULAC, IN FEET ABOVE NGVD29 VERMILION BAY NEAR CYPREMORT POINT, IN FEET ABOVE NAVD88

EXPLANATIONEXPLANATION EXPLANATION

Avoca Island Cutoff Channel Company Canal at Highway 1 GIWW west of Wax Lake Outlet south of Morgan City at Lockport south of Calumet Bayou Penchant south of GIWW east of Bayou Lafourche GIWW east of Jaws Bay west Morgan City of Franklin GIWW intersection with Bayou Perot Bayou Boeuf at railroad GIWW at Cypremort bridge at Amelia GIWW near Bay Wallace east of Morgan City GIWW east of Houma Navigation Canal at Houma

Figure 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.

Moreover, stage in the GIWW west of Wax Lake east of Jaws Bay west of Franklin and Cypremort, Outlet near Calumet is from a gage surveyed to the maximum difference in stage that occurred dur- NAVD88, but not by the USGS. Measured stage ing the annual river flood was about 1 ft. This dif- may deviate from the actual stage by some ference did not occur in the fall and winter months unknown amount. Data from Vermilion Bay near during periods when the river was not in flood. Cypremort Point indicate the Gulf of Mexico lev- Stage in the GIWW at Cypremort and in Vermilion els. Bay was similar. Stage in the GIWW west of WLO near Calu- Monthly-average stages along the GIWW met was between 0.5 and 2 ft higher than east of are plotted with stage for WLO at Calumet and for Jaws Bay west of Franklin. In the GIWW between Vermilion Bay near Cypremort Point (fig. 20).

34 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 8 Lower Atchafalaya River at Morgan City GIWW east of the Houma Navigation Canal at Houma 7 Company Canal at Highway 1 at Lockport GIWW east of Bayou Lafourche 6 GIWW at the intersection with Bayou Perot Houma Navigation Canal at Dulac (NGVD29) 5

0 MONTHLY-AVERAGE STAGE, IN FEET ABOVE NAVD88 MONTHLY-AVERAGE Apr. Apr. Apr. Jan. Mar. July July July Jan. Mar. Jan. Mar. May May May Nov. Nov. Nov. Nov. Feb. Feb. Feb. Dec. Dec. Dec. Dec. Aug. Aug. Aug. Sept. Sept. Sept.

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, Lou- isiana, October 1996 to December 1999.

8 Wax Lake Outlet at Calumet GIWW west of Wax Lake Out south of Calumet 7 GIWW east of Jaws Bay west of Franklin GIWW at Cypermort 6 Vermilion Bay near Cypremort

Break in line indicates missing data 0 MONTHLY-AVERAGE STAGE, STAGE, IN FEET ABOVE NAVD88 MONTHLY-AVERAGE Apr. Apr. Apr. Jan. Mar. July July July Jan. Mar. Jan. Mar. May May May Feb. Nov. Nov. Feb. Nov. Feb. Nov. Dec. Dec. Dec. Dec. Aug. Aug. Aug. Sept. Sept. Sept.

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.

SURFACE-WATER HYDROLOGY 35 Stage increased in the GIWW west of WLO near LAR and WLO, and with increasing distance from Calumet from 2 to over 4 ft as stage at WLO at the GIWW. In rare instances when Gulf of Mexico Calumet increased from 2 to 7 ft. A similar water intrudes inland, the GIWW can also move increase was evident in the GIWW east of Jaws more saline water laterally along the coast. Bay west of Franklin, but the magnitude was smaller. As WLO at Calumet stage increased by Suspended Sediment 4 ft, stage in the GIWW east of Jaws Bay west of Suspended-sediment concentrations and dis- Franklin increased by less than 1 ft. Stage in the charge data are summarized in table 6. On the east GIWW at Cypremort did not track WLO at Calu- side of the LAR, average suspended-sediment con- met stage fluctuations; it tracked stage in Vermilion centrations in the Avoca Island Cutoff Channel and Bay near Cypremort Point. The two sites on either Bayou Penchant south of Morgan City were about side of Jaws Bay also responded to changes in Ver- 50 to 70 mg/L higher than at the next four sites to milion Bay stage. Two separate relations between the east, averaging 162 mg/L (table 6). Suspended- stage in the GIWW east of Jaws Bay west of Fran- sediment discharge ranged from 74 to 7,440 tons/d klin and stage in Vermilion Bay near Cypremort at Bayou Penchant south of Morgan City and was Point were offset by about 0.5 ft, presumably as much as 22,900 tons/d at the Avoca Island Cut- depending on whether the LAR was in flood or not. off Channel. Discharge at the latter site was vari- Stage in the GIWW west of WLO near Calumet able. Bayou Boeuf at railroad bridge at Amelia always increased as stage at Vermilion Bay near consistently had the lowest suspended-sediment Cypremort increased, but stage was often much concentrations among all sites sampled. Concen- higher than would have occurred if Vermilion Bay trations ranged from 47 to 175 mg/L but averaged was the only factor affecting stage in the GIWW. 91 mg/L. Suspended-sediment discharge ranged The differences in stage between the LAR from 2,990 tons/d moving into the Verret Subbasin and WLO and between the GIWW and adjacent to 4,250 tons/d moving out of the basin. In the surface-water bodies, strongly influence the local GIWW near Bay Wallace east of Morgan City, and and regional surface-water hydrology of large parts the GIWW west of the HNC at Houma, suspended- of coastal Louisiana. The stage differences, and sediment concentrations averaged 114 and the resulting flows affect the north-south, and east- 108 mg/L, respectively. In the GIWW near Bay west movement of water, much of it fresh, in major Wallace east of Morgan City, an average of bayous and canals that criss-cross the wetlands. 1,610 tons/d of suspended sediment were trans- Long-term measurements of stage and discharge in ported eastward, with about one-fourth of the sus- the GIWW and adjacent surface-water bodies are pended-sediment discharge entering at the Avoca needed to more fully characterize the surface-water Island Cutoff Channel. hydrology and to better incorporate the flows into In the GIWW west and east of Bayou the ongoing efforts relating to coastal wetland res- Lafourche at Larose, suspended-sediment concen- toration. This is perhaps most easily achieved if trations averaged about 134 and 161 mg/L, respec- stage and discharge measurements in the GIWW tively. The higher average concentrations of are integrated into long-term monitoring programs suspended sediment to the east of Bayou Lafourche of coastal Louisiana waters. Studies are also are likely due to the inflow and eastward move- needed to understand how the water flowing in the ment of water from Bayou Lafourche. Average GIWW may affect the wetlands. To date, the effect concentration is about 45 percent higher than in the of the freshwater flow and associated water-quality GIWW near Bay Wallace east of Morgan City. in the GIWW for the integrity and health of adja- Average suspended-sediment discharge was about cent wetlands is largely unknown. 1,150 tons/d flowing eastward into Barataria Basin, with maximums of 2,500 to 3,240 tons/d. Selected Water-Quality Constituents Suspended-sediment concentrations at the The water flowing in the GIWW carries sed- two sites between WLO and the LAR were similar, iment and generally is fresh. The freshwater mod- about 172 to 183 mg/L. On average, about half the erates salinity in receiving water bodies. This sediment discharge entering the GIWW from the effect decreases with distance from the sources, the LAR left the GIWW through the WLO.

36 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 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 Concentration (mg/L) Discharge (tons per day) Number Map Range Range Site name of Average Average number samples (±standard (±standard (pl. 1) Mini- Maxi- Mini- Maxi- deviation) mum mum deviation) mum mum

East of Lower Atchafalaya River Between Lower Atchafa- laya River and Houma Navi- gation Canal

10 Avoca Island Cutoff Channel 17 164±64 79 273 6,540±6,490 65 22,900 south of Morgan City

11 Bayou Penchant south of 13 161±64 103 270 1,470±1,164 74 7,440 Morgan City

12 Bayou Boeuf at railroad 14 91±35 47 175 143±1,910 -2,990 4,250 bridge at Amelia

13 GIWW near Bay Wallace east 15 114±33 68 180 1,610±1,050 -421 3,050 of Morgan City

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

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

20 GIWW west of Bayou 14 134±62 74 246 1,020±570 306 2,500 Lafourche at Larose

21 GIWW east of Bayou 14 161±62 80 267 1,230±827 22 3,240 Lafourche at Larose

West of Lower Atchafalaya River Between Lower Atchafa- laya River and Wax Lake Outlet

8 GIWW at Lower Atchafalaya 13 183±69 85 286 8,690±6,650 900 19.900 River south of Morgan City

7 GIWW east of Wax Lake Out- 15 172±42 105 248 4,550±2,320 1,840 10,900 let near Calumet

West of Wax Lake Outlet

5 GIWW west of Wax Lake 13 179±66 87 332 5,130±3,530 1,731 11,800 Outlet south of Calumet

4 GIWW east of Jaws Bay west 13 172±62 106 307 4,130±3,420 1,020 14,100 of Franklin

3 GIWW west of Jaws Bay near 13 167±55 110 290 1,670±1,570 572 6,390 Cypremort

2 GIWW at Cypremort 12 191±65 127 325 509±1,440 -2,480 2,660

SURFACE-WATER HYDROLOGY 37 Suspended-sediment concentrations aver- generally was about 0.2 to 0.6 ppt (pl. 1). In the aged 179 to 172 mg/L at sites in the GIWW west of fall, salinity was mostly low, but increased briefly WLO south of Calumet and the GIWW east of on a frequent basis, at times to 10 ppt. In the fall of Jaws Bay west of Franklin, respectively. Concen- 1997 and 1999, the salinity increases were of trations greater than 300 mg/L were measured at longer duration than in the fall of 1998. Salinity in least once at the two sites. The average suspended- the HNC at Dulac had a similar seasonal pattern, sediment discharge was about 15 percent less in the but the brief salinity increases were higher, and GIWW east of Jaws Bay west of Franklin com- more frequent. In 1996 and 1998, salinity did not pared to the GIWW west of WLO south of Calu- decrease as much between the brief increases and met. In the GIWW west of Jaws Bay near the salinity was higher during the increases than in Cypremort, average suspended-sediment concen- the intervening 2 years. Although less frequent, trations were 167 mg/L. In the GIWW at Cypre- salinity increased even during periods of high stage mort, average suspended-sediment concentrations of the LAR at Morgan City, most noticeably in were 191 mg/L. Reversed flow occurred here, with April 1997, when stage of the LAR at Morgan City about 2,500 tons/d of suspended sediment moving was near 8 ft above NAVD88. east on at least one occasion. Salinity was related to stage of the LAR at Suspended sediment is important to the Morgan City at sites adjacent to and east of the maintenance of wetlands. It provides nourishment LAR at Morgan City and distant from the GIWW to wetland plants and building materials for soil. (pl. 1). Salinity at Bayou du Large near Theriot The GIWW carries a substantial amount of sus- decreased to about 1 ppt, when stage of the LAR at pended sediment to points east and west of Morgan Morgan City was elevated. Salinity at Bayou Pen- City that may be of use in wetland restoration chant at Brady Canal had brief increases of less projects. The data collected from this study indi- than 5 ppt occurring in the fall, but otherwise was cate that in addition to sediments from the Atchafa- low. At Caillou Lake (Sister Lake) southwest of laya River, local sources of inflow such as Bayou Dulac, salinity peaked at 15 to 25 ppt in the fall, Boeuf and Bayou Lafourche influence suspended- when the stage of the LAR at Morgan City was sediment concentrations in the GIWW. Resuspen- low. When the LAR at Morgan City was in flood sion of sediments presumably becomes more in the spring of 1997-98, salinity decreased to less important farther away from the LAR and WLO, than 4 ppt for extended periods. but the sustained flow in the GIWW also will help At sites between the HNC and Bayou in moving the resuspended sediment. Lafourche, salinity was similar and responded less Salinity to changes in stage of the LAR at Morgan City than at the previously discussed sites with the exception Sites where salinity was measured in the of Bayou L’Eau Bleu (pl. 1). At this site in 1996- study area were (1) directly on the GIWW, (2) 97, salinity was low when the LAR was in flood hydrologically connected and in close proximity to but increased in the fall. the GIWW, and (3) only indirectly connected to the GIWW. Data were collected at some sites during Two sites, Vermilion Bay near Cypremort most of the study, whereas data collection was dis- Point and Côte Blanche 4, provided the only avail- continued or began operation at other sites during able salinity data from the west side of the study the study. Most sites were operated by the USGS. area (pl. 1). Côte Blanche 4 is located in the wet- Data from sondes installed by DNR in mid-sum- lands east of Jaws Bay and south of the GIWW. mer of 1998 on Bayou Penchant at Brady Canal, in Salinity was low during peak stage of the LAR at the GIWW at a point about 5 mi east of Larose, and Morgan City in the spring. In 1997-98, salinity at the GIWW intersection with Bayou Perot also decreased to almost zero. In 1998, salinity started were used in this study. increasing before the river stage decreased. In mid-summer 1999 when the LAR was in flood, During high stage of LAR at Morgan City, salinity was 2 to 3 ppt. In the fall of 1999, salinity salinity in the GIWW east of HNC at Houma and reached the highest concentration for the study east of Bayou Lafourche intersection at Larose period at the two sites.

38 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Salinity in the study area is affected by rain may be seen by the prolonged high salinity in freshwater inflow from the LAR and WLO through the late fall months of 1999 (fig. 24). Even the GIWW and by local precipitation and runoff. though precipitation in October was high, the The freshwater from these sources mixes with salt- south-central Climate Division was in a moderate water from the Gulf of Mexico. The influence of to extreme drought during the entire fall (see fig. river stage and precipitation on salinity was evalu- 4). Cumulative annual rainfall was substantially ated first by plotting monthly-average values of below average for 1996 and 1999 and above aver- salinity at selected sites in relation to monthly- age for the first few months of 1997 and in the average stage of the LAR at Morgan City and mid-summer of 1998 (fig. 24). WLO and monthly total precipitation. Sites were grouped depending on whether they were (1) on or Only data from the east side of the LAR near the GIWW or (2) at some distance from the were available to evaluate response of salinity to GIWW. Next, pair-wise correlation coefficients freshwater inflows at some distance from the between monthly-average salinity at a given site GIWW (fig. 23). Salinity was similar and and stage of the LAR at Morgan City, monthly total decreased at the three sites during the first six precipitation at Morgan City, and the PHDI value months of 1997. In mid-summer 1997 salinity was were established (table 7). Only sites with the the lowest measured during the study. In 1998-99, most complete data record were used for this anal- salinity increased from a mid-winter low to a high ysis. in May and June. In the fall of 1998 and 1999, salinity increased gradually from September to Salinity at all four sites on or near the December at Bayou Terrebonne at Bush Canal, GIWW, both east and west of the LAR, was less south of Chauvin and at Grand Bayou tributary than 4 ppt whenever the LAR was in flood west of Galliano. This was unusual. Typically, (fig. 23). Water in the GIWW east of HNC at salinity follows a bell-shaped curve for the fall Houma and Côte Blanche 4 remained fresh dur- months, such as occurred in 1996-97 for all three ing high river stage in 1997-1999, but salinity sites. The high rainfall that occurred in September became slightly elevated in September, October, 1998 probably decreased salinity at these sites at and November. Salinity was not as high in the this time. The lack of rain during the year likely fall of 1998 as in the fall of 1997 or 1999. Salin- underlay the atypical pattern of salinity seen at all ity at the HNC at Dulac and at Vermilion Bay near sites in fall 1999. Salinity increased steadily after Cypremort Point ranged from 8 to 15 ppt during August and continued through December at sites the fall of 1998. Salinity at both sites did not distant from the GIWW (fig. 23), or increased rap- decrease as much in 1999 as in 1998. In late fall idly and remained high at sites on or near the 1999, salinity was between 12 and 14 ppt. In Ver- GIWW (fig. 23). Salinity during the first 5 months milion Bay near Cypremort Point, the salinity pat- of 1999 was higher than during comparable times tern appeared to be low in late winter and high in in the 2 previous years. September and October in response to, first, the spring peak in river flow, and then, the annual Caillou Lake (Sister Lake) southwest of peak Gulf of Mexico level in the fall. The pat- Dulac is located between Four League Bay to the tern changed in 1998 when salinity in September west and the HNC to the east. Salinity at this site and October was lower than the salinity in July. ranged from 3 to about 20 ppt and was influenced The relatively low salinity at all four sites in fall by the stage and discharge of the LAR and seasonal 1998 compared to fall of 1997 and 1999 was most variations in Gulf of Mexico levels. Typically likely due to the excess precipitation that fell salinity peaked in the spring and again in the fall. along Louisiana’s coast during three storms in late Annual highs occurred during the fall peak. In September and early October 1998. Salinity 1996 and 1999, the fall peak was 2 to 4 ppt higher increased for June and July, after the spring flood than in the intervening 2 years. Lowest monthly- of the Mississippi and Atchafalaya Rivers average salinity occurred during the mid-summer receded. Salinity reached a maximum during the low in 1997, after the record flood in spring 1997. fall. Salinity increased from 2 ppt to greater than The lowest salinity in 1999 occurred in August and 10 ppt from July to August 1999 at Vermilion Bay was more than twice the lowest salinity in 1997, near Cypremort Point; the sensitivity to lack of which occurred in July.

SURFACE-WATER HYDROLOGY 39 .010 .093 -.53 -.42 -.51 -.61 -.34 0.037 Index <.001 <.001 <.001 logical logical Hydro- -0.30 Palmer Drought Drought .769 .380 .442 .856 .92 .607 City -.05 -.15 -.13 -.03 -.02 0.105 (inches) (inches) at Morgan at Morgan precipitation Monthly total Monthly Vertical Datum of 1988; Datum of Vertical .079 .002 .004 -.30 -.49 -.63 -.79 -.66 <.001 <.001 <.001 NAVD88) -0.49 age stage, (feet above above (feet Morgan City City Morgan Lower Atcha- Lower falaya River at falaya .78 .57 .22 .242 .20 .251 .680 Grand Bayou 0.08 <.001 <.001 west of Galliano tributary tributary rway; NAVD88, North American NAVD88, rway; erence is indicated when the significance level is level significance the when indicated is erence .91 .64 .78 0.67 <.001 <.001 <.001 <.001 Dulac Caillou Lake Caillou (Sister Lake) (Sister southwest of southwest ts per thousand) Monthly-aver- .60 .67 0.62 <.001 <.001 <.001 Bayou at Bush m number. A significant diff A significant m number. of Chauvin Terrebonne Terrebonne Canal, south .73 Dulac 0.87 <.001 <.001 Houma Canal at Navigation Point Monthly-average salinity (par Monthly-average 0.65 <.001 Bay near Bay Vermilion Vermilion and significance level is the botto level and significance Cypremort Houma Canal at of Houma Navigation GIWW east east GIWW ion coefficient is the top number ion coefficient Variable, cated site monthly-average monthly-average thousand, at indi- thousand, salinity, in parts per salinity, GIWW east of HoumaGIWW east of CanalNavigation at Houma near Bay Vermilion Cypremort Point Houma Navigation Canal at Dulac at Terrebonne Bayou Canal,Bush south of Chauvin (Sister Caillou Lake of southwest Lake) Dulac tributary Bayou Grand of Galliano west Correlation matrix forCorrelationsalinity monthly-average matrix at selected monthly-average sites, stageAtchafalaya of the Lower River at 1 17 29 28 30 26 Map (pl. 1) number Table 7. Hydrological and Palmer Drought Index Louisiana, monthly total precipitation at Morgan City, Morgan City, Pearson correlat [Individual <, less than] less than 0.05. Bold result indicates a significant correlation with the variable in column 2. GIWW, Gulf Intracoastal Wate Intracoastal Gulf GIWW, 2. column in variable the with correlation significant a indicates result Bold 0.05. than less

40 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 20

10 LY T OTA L H 5 ONT

M 0 ONDJJFMAMJJASOND J FMAMJ ASONDJ FMAMJJASON D PRECIPITATION, IN INCHES

SITES ON OR NEAR THE GULF INTRACOASTAL WATERWAY

ONDJJFMAMJJASOND J FMAMJ ASONDJ FMAMJJASON D

SITES AT SOME DISTANCE FROM THE GULF INTRACOASTAL WATERWAY MONTHLY-AVERAGE STAGE, STAGE, IN FEET ABOVE NAVD88 MONTHLY-AVERAGE MONTHLY-AVERAGE SALINITY, IN PARTS PER THOUSAND PER PARTS IN SALINITY, MONTHLY-AVERAGE

ONDJFMAMJ JASONDJFMAMJ JASONDJ FMAMJ JASON D

EXPLANATION

Salinity, Gulf Intracoastal Waterway east of Houma Salinity, Grand Bayou tributary west of Galliano Navigation Canal at Houma Salinity, Bayou Terrebonne at Bush Canal, south of Chauvin Salinity, Houma Navigation Canal at Dulac Salinity, Caillou Lake (Sister Lake) southwest of Dulac Salinity, C^ te Blanche 4 Stage, Lower Atchafalaya River at Morgan City Salinity, Vermilion Bay near Cypremort Point

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 Mor- gan City, October 1996 to December 1999.

SURFACE-WATER HYDROLOGY 41 70 milion Bay near Cypremort Point decreased with 1996 increases in stage of the LAR at Morgan City. 60 1997 1998 When the LAR at Morgan City was 2 ft or more 50 1999 above NAVD88, monthly-average salinity in the Average (1961-1990) 40 GIWW east of HNC at Houma did not increase much above 1 ppt. With one exception, monthly- 30 average salinity at HNC at Dulac was 2 ppt or less 20 when the LAR at Morgan City was 3 ft or more above NAVD88. As stage of the LAR at Morgan CUMULATIVE MONTHLY 10 PRECIPITATION, IN INCHES IN PRECIPITATION, City increased to 7 ft above NAVD88, salinity did 0 not exceed progressively smaller values for Vermil- Apr. Feb. Jan. Mar. July May Aug. Nov. Dec. Sept. ion Bay near Cypremort Point. When the LAR at Data from Louisiana State Office of Climatology Morgan City was 5 ft or more above NAVD88, Figure 24. Cumulative monthly precipitation by salinity was never greater than 3 ppt. However, year (1996-99) and average (1961-90) for the salinity was frequently lower than a direct correla- South-Central Climatological Division in Louisiana. tion with stage would predict, indicating that aside from LAR water, freshwater runoff and Gulf of Bayou Terrebonne and Grand Bayou are Mexico stage also influenced salinity in Vermilion located to the east of the HNC and to the south of Bay near Cypremort Point. the GIWW. Grand Bayou tributary west of Gal- liano is mostly cut off from freshwater sources Stage of the LAR at Morgan City affected except for rain and some inflow through Bayou salinity at Caillou Lake (Sister Lake) southwest of L’Eau Bleu through the GIWW. Company Canal Dulac and salinity at Vermilion Bay near Cypre- connects Bayou Terrebonne at Bush Canal, south mort Point in much the same way. As stage of the of Chauvin with the GIWW at Houma. Bayou Ter- LAR at Morgan City increased, maximum salinity rebonne at Bush Canal, south of Chauvin is also decreased. Salinity at the other three sites at some connected indirectly to the HNC near Dulac and distance from GIWW did not respond to changing Bayou Grand Caillou by channels crossing Lake stage of the LAR at Morgan City as directly, Boudreaux. In 1997, salinity at the two sites was increasing and decreasing as stage increased at similar. Salinity in Grand Bayou tributary west of most stages of the LAR at Morgan City (fig. 25). Galliano was slightly higher during early summer; However, salinity at all three sites was consistently salinity in Bayou Terrebonne was higher during the higher when the LAR at Morgan City was 7 ft fall. In May and June 1998, salinity was 2 to 5 ppt above NAVD88 than when the LAR at Morgan higher than the previous year at the same time. At City was 5 ft above NAVD88. More data could aid Bayou Terrebonne at Bush Canal, south of Chau- in verifying this relation. There may be an indirect vin, salinity increased from July to December effect of seasonally higher Gulf of Mexico levels 1998. The fall peak salinity was less than the pre- along the coast of Louisiana. Discharges from the vious year, probably due to local runoff. At Grand Mississippi River and the LAR are highest in April Bayou tributary west of Galliano, salinity increased and May. Onshore winds last for several weeks from the fall of 1998 through April and May 1999 during much of the same time and push Gulf of but then decreased through June and July 1999. Mexico water into estuaries along the coast (Byrne and others, 1976). Freshwater input in the form of Monthly-average salinity at the sites with the precipitation and variations in Gulf of Mexico level most complete record from both sides of the study affected salinity at all four sites. At Grand Bayou area were plotted against monthly-average stage of tributary west of Galliano and at Bayou Terrebonne the LAR at Morgan City and the HNC at Dulac at Bush Canal, south of Chauvin, there was a rela- (fig. 25) to determine the influence of river stage tion below which salinity did not decrease as stage and Gulf of Mexico levels, respectively. Salinity at of the HNC at Dulac increased. Such a relation sites on or near the GIWW, east of the HNC at was less apparent at Bayou Jean Lacroix near Mon- Houma, HNC at Dulac, Côte Blanche 4, and Ver- tegut, La.

42 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 SITES ON OR NEAR THE GULF INTRACOASTAL WATERWAY 20 20

15 15

10 10

5 5 IN PARTS PERIN PARTS THOUSAND MONTHLY-AVERAGE SALINITY, SALINITY, MONTHLY-AVERAGE 0 0 0 123 4 56 7 8 0.0 0.5 1.0 1.5 2.0 2.5 MONTHLY-AVERAGE STAGE, MONTHLY-AVERAGE STAGE, LOWER ATCHAFALAYA RIVER AT HOUMA NAVIGATION CANAL AT MORGAN CITY, IN FEET ABOVE NAVD88 DULAC, IN FEET ABOVE NGVD29 EXPLANATION Gulf Intracoastal Waterway east of Côte Blanche 4 Houma Navigation Canal Houma Navigation Canal at Dulac Vermilion Bay near Cypremort Point

SITES AT SOME DISTANCE FROM THE GULF INTRACOASTAL WATERWAY 20 20

15 15

10 10

5 5 IN PARTS PER THOUSAND PER PARTS IN MONTHLY-AVERAGE SALINITY, SALINITY, MONTHLY-AVERAGE 0 0 012345678 0.0 0.5 1.0 1.5 2.0 2.5 MONTHLY-AVERAGE STAGE, MONTHLY-AVERAGE STAGE, LOWER ATCHAFALAYA RIVER AT HOUMA NAVIGATION CANAL AT MORGAN CITY, IN FEET ABOVE NAVD88 DULAC, IN FEET ABOVE NGVD29

EXPLANATION Grand Bayou tributary west of Galliano Caillou Lake (Sister Lake) southwest of Dulac

Bayou Terrebonne at Bush Canal south Bayou Jean Lacroix near Montegut of Chauvin

Figure 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, Louisi- ana, October 1996 to December 1999.

Table 7 lists pair-wise regression coefficients the significant negative correlation between for monthly-average salinity at selected sites and monthly-average salinity and PHDI. Salinity at monthly-average stage of the LAR at Morgan City, Grand Bayou tributary west of Galliano showed monthly total precipitation at Morgan City, and the the highest correlation with the PHDI and the least PHDI. Pair-wise regression coefficients also are with stage of the LAR at Morgan City, which indi- listed for monthly-average salinity among sites. cates isolation from the GIWW freshwater and Salinity at Vermilion Bay at Cypremort Point and strong climatic influence on salinity. Monthly total Caillou Lake (Sister Lake) southwest of Dulac cor- precipitation and salinity were not significantly related significantly with stage of the LAR at Mor- correlated at any stations. There were significant gan City. Vermilion Bay near Cypremort Point correlations in monthly-average salinity among all was sensitive to drought as well, as indicated by stations.

SURFACE-WATER HYDROLOGY 43 The GIWW influences salinity in a wide part off Channel south of the GIWW, drained on aver- of coastal Louisiana and in many instances in a age 3,250 ft3/s of varying proportions of LAR predictable manner. The significant correlations and Verret Subbasin water in a southeasterly between salinity at selected sites and stage of the direction. A combination of LAR and Verret LAR have important implications for managing Subbasin water flowed east in the GIWW near and restoring coastal wetlands in Louisiana. The Bay Wallace east of Morgan City and west of the relations between salinity and stage of the LAR can HNC (Houma Navigation Canal) at Houma, aver- be used with relations presented previously in this aging about 5,700 ft3/s and frequently reaching at report between stage of the LAR and discharge at least 8,000 ft3/s at both sites. selected sites to better understand availability of Average discharge in the GIWW east of the freshwater inflow or lack of inflow at specific loca- 3 tions along the Louisiana coast. HNC at Houma was about 2,610 ft /s in an eastward direction, and ranged from 1,100 ft3/s flow- 3 SUMMARY AND CONCLUSIONS ing west toward the HNC to 4,640 ft /s flowing east toward the intersection of the GIWW. Dis- charge in the GIWW east of Bayou Lafourche at The flow of freshwater and suspended sedi- Larose averaged 2,220 ft3/s, and peaked at ments from the Lower Atchafalaya River (LAR) 5,320 ft3/s, also flowing to the east. Flow and Wax Lake Outlet (WLO) into and along the towards Houma was observed on two occasions, Gulf Intracoastal Waterway (GIWW) and selected with a maximum discharge of 1,860 ft3/s. At adjacent surface-water bodies between Cypremort sites west of the HNC, discharge in the GIWW and Larose in south-central Louisiana from Octo- increased predictably with increasing LAR stage. ber 1996 to December 1999 was characterized This was not the case to the east of the HNC. using instantaneous and continuous discharge measurements, and measurements of suspended- Flow was always to the west between the sediment concentrations. The GIWW parallels the LAR and WLO. Average discharge was about entire Louisiana coast near the wetland/upland 13,400 ft3/s in the GIWW at LAR south of Mor- interface. Following natural hydraulic gradients, gan City. The highest discharge measurement of the GIWW captures water from the southward any synoptic trip was made here, at 35,200 ft3/s. flowing LAR and the WLO where it crosses those Discharge increased predictably with increasing waterways, and distributes the freshwater and sed- LAR stage. In the GIWW east of WLO near iment to points east and west. Water-level gradi- Calumet, average discharge was 9,460 ft3/s. Dis- ents of surface waters in the study area were charge did not appear to increase with increasing determined and the potential for flow evaluated. LAR stage beyond 5 ft above NAVD88, indicat- The influence of freshwater flow in the GIWW on ing increasing volumes of water flowed south current and historical salinity patterns in the toward Atchafalaya Bay when this condition was GIWW and adjacent surface-waters also was eval- met. uated. Flow was always to the west in the The LAR was the primary source of water GIWW west of WLO south of Calumet, and dis- to the GIWW east of Morgan City. East of Mor- charge ranged from 2,050 to 20,300 ft3/s with an gan City, an average of 12,200 ft3/s (cubic feet average of 9,460 ft3/s. Flow was to the west in per second) of water flowed from the LAR the GIWW east of Jaws Bay west of Franklin, directly into the Avoca Island Cutoff Channel. and average discharge was 8,230 ft3/s, with a Measured discharge ranged from 267 to maximum of 17,000 ft3/s. Discharge at both 31,100 ft3/s entering the Avoca Island Cutoff these locations on the GIWW increased predict- Channel. Drainage from the Verret Subbasin ably with increasing stage of the LAR at Mor- through Bayou Boeuf at railroad bridge at Ame- gan City. In the GIWW west of Jaws Bay near lia contributed an average of 1,000 ft3/s to flow Cypremort, average discharge was 3,310 ft3/s east of Morgan City. Measured discharge ranged and flow was always to the west. In the GIWW from 9,110 ft3/s entering to 15,300 ft3/s draining at Cypremort, average discharge was 1,350 ft3/s the Verret Subbasin. Bayou Penchant south of to the west, but discharge as high as 2,830 ft3/s Morgan City, connecting to the Avoca Island Cut- was measured flowing eastward to Jaws Bay.

44 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Discharge along the GIWW was highest Calumet increased. As local runoff increased, where it crossed the LAR and WLO. Water the GIWW between Houma and Bayou flowed east and west in the GIWW, in decreas- Lafourche shifted from storing to draining water. ing volumes with increasing distance from the In contrast, the GIWW between Jaws Bay and source. Discharge tended to increase as same- Cypremort stored increasing volumes of water as day average stage of the LAR at Morgan City runoff increased. Here, there was a net inflow increased beyond 3 ft (feet) above NAVD88 at of water at low LAR stage which decreased rap- most of the sites between HNC and Jaws Bay. idly and steadily as the volume of river water At sites in the GIWW east of HNC, discharge entering at WLO increased. did not increase predictably. For all measurements made when the LAR at Morgan City was The flow of water from the LAR to the 3 ft or more above NAVD88, average discharge GIWW has changed over time. Aggradation of was about 3,100 ft3/s in the GIWW east of HNC the bed of the LAR since the 1950’s has raised at Houma and 2,880 ft3/s east of Bayou water levels where the river joins the Avoca Lafourche at Larose. When the LAR at Morgan Island Channel Cutoff by about 2 to 2.5 ft. The City reached 3 to 4 ft above NAVD88, dis- aggraded riverbed has increased the time when charge in the GIWW became more predictable. the stage of LAR at Morgan City is at least 3 ft The flow of freshwater along most of the GIWW above NAVD88. Lateral flow in the GIWW studied was enhanced not so much by absolute presently (1999) occurs for longer periods than in high stage of the LAR at Morgan City as by the the 1950's. Infilling of the riverbed also has total time the LAR at Morgan City was 3 ft or increased the absolute difference in stage more above NAVD88. The LAR at Morgan City between the LAR and adjacent waters, so that the is 3 ft or more above NAVD88 for about 7 GIWW predictably can move water to points far- months in a normal year. ther away from the source. During 1997-99, more water was flowing in the GIWW west of The GIWW not only moves freshwater HNC at Houma relative to a given stage of the along the coast, but also carries saltwater when LAR at Morgan City than at the same location in there are several days of reversed flow in north- 1982-83. to-south trending ship channels such as the HNC. Saltwater then can flow laterally in the GIWW Flow in the GIWW results from differ- when normal hydraulic gradients resume and ences in stage along the GIWW and adjacent sur- reach areas which contain marsh plants that have face-water bodies. In the coastal zone, stage a low tolerance for salt. narrowly fluctuates around Gulf of Mexico levels most of the time. Where the GIWW crosses Localized runoff and drainage contributed the LAR and WLO, stage typically reaches 3 to 4 to flow along segments of the GIWW on both ft above NAVD88. Stage differences between sides of Morgan City. When the LAR at Mor- LAR and the rest of the coast induce flow in the gan City was less than 3 ft above NAVD88, GIWW and allow LAR water to penetrate dis- water flowed along the prevailing gradients, to tant reaches of the coast for several months each the east between Bay Wallace and the HNC and year. Monthly-average stage at sites on the to the west between WLO and Jaws Bay. How- GIWW east of Morgan City to the HNC and ever, frequently more water left the GIWW at west of Morgan City to Jaws Bay tracked stage the downstream end of the segments than of the LAR reasonably well. The correlation was entered, indicating that flow was sustained by better and the response greater closer to the LAR. water draining from areas adjacent to the GIWW. Substantial volumes of water left the GIWW The spring flood of the LAR was the domi- through the HNC and at the opening to Jaws nant influence on flow in and near the GIWW, rais- Bay. As local runoff increased, the volume of ing water levels high above levels in surrounding water leaving the GIWW through the HNC waters. Seasonal and event-driven fluctuations in increased, whereas at Jaws Bay, the volume of Gulf of Mexico levels changed stage along the water leaving the GIWW increased steadily as coast and impacted gradients. Stage in and near the water entering the GIWW west of WLO near GIWW also was affected by winds, precipitation,

SUMMARY AND CONCLUSIONS 45 and storms. Together, the factors influenced water- large parts of coastal Louisiana. The stage differ- level gradients along the GIWW and controlled the ences, and the resulting flows affect the north to direction and movement of water from the LAR to south, and east to west movement of water, much the GIWW and from the GIWW to adjacent of it fresh, in major bayous and canals that criss- waters. cross the wetlands. Long-term measurements of stage and discharge in the GIWW and adjacent East of the LAR, suspended-sediment con- surface-water bodies are needed to more fully centrations averaged about 162 mg/L (milligrams characterize the surface-water hydrology and to per liter) at the two sites closest to the LAR, at the better incorporate the flows into the ongoing Avoca Island Cutoff Channel and at Bayou Pen- efforts relating to coastal wetland restoration. chant south of Morgan City. The water entering This is perhaps most easily achieved if stage and the GIWW through Bayou Boeuf at railroad bridge discharge measurements in the GIWW are inte- at Amelia averaged 91 mg/L of suspended sedi- grated into long-term monitoring programs of ments. The water in the GIWW near Bay Wallace coastal Louisiana waters. Studies are also needed east of Morgan City to the HNC was intermediate to understand how the water flowing in the in concentrations (108 to 114 mg/L), reflecting the GIWW may affect the wetlands. To date, the mixture of the two water types. Concentrations of effect of the freshwater flow and associated suspended sediment in the GIWW near Bay Wal- water-quality in the GIWW for the integrity and lace east of Morgan City, and on both sides of the health of adjacent wetlands is largely unknown. HNC were comparable, but in the GIWW east of Larose they increased by about 45 percent. To the When the LAR at Morgan City is 3 or west of WLO, suspended-sediment concentrations more ft above NAVD88, the GIWW effectively averaged between 167 and 191 mg/L. distributes freshwater and sediments from the LAR and WLO to points in coastal Louisiana 30 Freshwater flow affected salinity patterns in to 50 mi east and west of Morgan City. The the GIWW and areas immediately adjacent to it freshwater and sediments, some of which origi- in the study area. Monthly-average salinity at nate indirectly from the Mississippi River, are many coastal sites was correlated with monthly- important building blocks for wetlands and could average stage of the LAR at Morgan City. In the prove valuable in ongoing efforts to restore GIWW east of HNC at Houma and at Côte coastal Louisiana. Blanche 4 salinity was near zero whenever stage of the LAR at Morgan City was 2 ft or more SELECTED REFERENCES above NAVD88. At HNC at Dulac and Vermil- ion Bay near Cypremort Point, maximum Arcement, G.J., Jr., 1988, Discharge and sus- monthly-average salinity progressively decreased pended-sediment data for the lower Atchafa- as stage of the LAR at Morgan City increased. laya River, Atchafalaya Bay, and Wax Lake When the LAR at Morgan City was 5 ft or more Outlet, Louisiana, 1980-82: U.S. Geological above NAVD88, salinity remained below 3 parts Survey Open-File Report 87-553, 30 p. per thousand. At these four sites, salinity during the fall months was appreciably higher than dur- Boesch, D.F., Josselyn, M.N., Ashish, J.M., Mor- ing the rest of the year. Salinity patterns at sites ris, J.R., Nuttle, W.K., Simenstad, C.A., and distant from the GIWW--Caillou Lake (Sister Swift, D.J.P., 1994, Scientific assessment of Lake), HNC at Dulac, and Vermilion Bay near coastal wetland loss, restoration and manage- Cypremort Point--were similar, with maximum ment in Louisiana: Journal of Coastal salinity decreasing with increasing stage of the Research, Special Issue no. 20, 103 p. LAR at Morgan City. Byrne, P., Borengasser, M., Drew, G., Muller, R.A., Smith, B.L., Jr., and Wax, C., 1976, Barataria The differences in stage between the inter- Basin—hydrological and climatological pro- sections of the LAR and WLO with the GIWW cesses: Baton Rouge, La., Louisiana State and the GIWW, and between the GIWW and University Center for Wetland Resources, adjacent surface-water bodies, strongly influence Sea Grant Publication no. LSU-T-76-010, the local and regional surface-water hydrology of 58 p.

46 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 Chabreck, R.H., 1972, Vegetation, water, and soil Kolb, C.R., and Van Lopik, J.R., 1958, Geology of characteristics of the Louisiana Coastal the Mississippi River Deltaic Plain, south- Region: Baton Rouge, La., Louisiana State eastern Louisiana: Vicksburg, Ms., U.S. University Agricultural Experimental Sta- Army Corps of Engineers, Technical Reports tion, Bulletin 664, 72 p. 3-483 and 3-484. Coleman, J.M., 1988, Dynamic changes and pro- Mendelssohn, I.A., Turner, R.E., and McKee, K.L., cesses in the Mississippi River delta: Geo- 1984, Louisiana’s eroding coastal zone— logical Society of America Bulletin, v. 100, management alternatives: Journal of the p. 999-1015. Limnological Society of South Africa, v. 9, Craig, N.J., Turner, R.E., and Day, J.W., Jr., 1980, no. 2, p. 63-75. Wetland losses and their consequences in Morlock, S.E., Nguyen, N.T., and Ross, J.H., 2001, coastal Louisiana: Zeitschrift zur Geomor- Feasibility of acoustic doppler velocity phologie Neue Folge, v. 34, p. 225-241. meters for the production of discharge Donnel, B.P., and Lederer, J.V., Jr., 1992, The records from U.S. Geological Survey stream- Atchafalaya River Delta, Report 12, Two- flow-gaging stations: U.S. Geological Sur- dimensional modeling of alternative plans vey Water-Resources Investigations Report and impacts on the Atchafalaya Bay and Ter- 01-4157, 56 p. rebonne marshes: Vicksburg, Ms., U.S. National Oceanic and Atmospheric Administration Army Corps of Engineers, Waterways Exper- National Climatic Data Center, 2000, Cli- iment Station, Hydraulics Laboratory, Tech- mate Division Drought Data-Graphing nical Report HL-82-15. Options, monthly Palmer Hydrologic Edwards, T.K., and Glysson, G.D., 1999, Field Drought Severity Index maps data for the methods for measurement of fluvial sedi- south Central and South-East Divisions in ment: U.S. Geological Survey Techniques of Louisiana from October 1996 to December Water-Resources Investigations, book 3, 1999: accessed June 19, 2000, at URL chap. C2, 89 p. http://lwf.ncdc.noaa.gov/oa/cli- Evers, D.E., Gosselink, J.G., Sasser, C.E., and Hill, mate/onlineprod/drought/main.html J.M., 1992, Wetland loss dynamics in south- Paille, R.F., 1997, Lower Atchafalaya Basin western Barataria basin, Louisiana, 1945- reevaluation study—planning aid report on 1985: Wetlands Ecology and Management, freshwater inflows to the Terrebonne Basin: v. 2, p. 103-118. Lafayette, La., U.S. Fish and Wildlife Ser- Frazier, D.E., 1967, Recent deltaic deposits of the vice, Ecological Services, 28 p. Mississippi River; their development and chronology: Transactions of the Gulf Coast Reed, D.J., ed., 1995, Status and historical trends Association of Geological Sciences, v. 17, of hydrologic modification, reduction in sed- p. 287-315. iment availability, and habitat loss/modification in the Barataria and Terrebonne Gagliano, S.M., Meyer-Arndt, K.J., and Wicker, estuarine system: Thibodaux, La., Barataria- K.M., 1981, Land loss in the Mississippi Terrebonne National Estuary Program, River Deltaic Plain: Transactions of the Gulf BTNEP Publication no. 20, 338 p. Coast Association of Geological Societies, v. 31, p. 295-300. Simpson, M.R., and Oltmann, R.N., 1993, Dis- charge-measurement system using an acous- Garrison, C.R., Lovelace, W.M., and Montgomery, tic Doppler current profiler with P.A., 1996, Water resources data—Louisiana, applications to large rivers and estuaries: water year 1996: U.S. Geological Survey Water-Data Report LA-96-1, 500 p. U.S. Geological Survey Water-Supply Paper 2395, 32 p. Gosselink, J.G., 1984, The ecology of delta marshes of coastal Louisiana—a community Swarzenski, C.M., Swenson, E.M., Sasser, C.E., profile: Washington, D.C., U.S. Fish and and Gosselink, J.G., 1991, Marsh mat flota- Wildlife Service, Biological Services Pro- tion in the Louisiana delta plain: Journal of gram, FWS/OBS-84/09, 134 p. Ecology, v. 71, p. 999-1011.

SELECTED REFERENCES 47 Turner, R.E., and Cahoon, D.R., eds., 1988, Wells, F.C., and Demas, C.R., 1977, Hydrology Causes of wetland loss in the coastal cen- and water quality of the Atchafalaya River tral Gulf of Mexico, v. 1: New Orleans, La., basin: Louisiana Department of Transporta- Minerals Management Service, Executive tion and Development, Office of Public Summary OCS Study. Works Water Resources Technical Report Visser, J.M., Sasser, C.E., Chabreck, R.H., and no. 14, 53 p. Linscombe, R.G., 1999, Long-term vegetation changes in Louisiana tidal marshes, 1968-92: Wetlands, v. 19, no. 1, p. 168-175. Wells, F.C., 1980, Hydrology and water quality of the lower Mississippi River: Louisiana Department of Transportation and Develop- ment, Office of Public Works Water Resources Technical Report no. 21, 83 p.

48 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 APPENDIX: Regression Equations

APPENDIX 49 Gulf Intracoastal Waterway west of Houma Navigation Canal at Houma 14,000

12,000

10,000

8,000 ND O C 6,000

4,000

2,000

FEET FEET SE PER Discharge = 1570* LAR + 727; r2 =0.64, p< 0.001 C

BI 0

U 01 2 345 678 C

E, IN IN E, Gulf Intracoastal Waterway near G

R Bay Wallace east of Morgan City A 12,000 H C

10,000 S DIS U O 8,000

NE Bay Wallace A

12000 NT

A 6,000 10000

INST 4,000 8000 a t a 6000 D

2,000 4000

Discharge2000 = 6110*ln (LAR)-393; r2 =0.90, p< 0.001 0 0 12345 6 78 DAILY-AVERAGE STAGE OF LOWER ATCHAFALAYA RIVER AT MORGAN CITY, IN FEET ABOVE NAVD88

EXPLANATION LAR is daily-average stage of the Lower Atchafalaya River at Morgan City r2 is correlation coefficient, p is level of significance

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.

50 SURFACE-WATER HYDROLOGY OF THE GULF INTRACOASTAL WATERWAY IN SOUTH-CENTRAL LOUISIANA, 1996-99 24,000 Gulf Intracoastal Waterway west of 20,000 Wax Lake Outlet south of Calumet

16,000

12,000

ND 8,000 O 4,000 Discharge = 3440*LAR-3540; r2 =0.96, p<0.001 0 067812345 16,000 Gulf Intracoastal Waterway east of Jaws Bay west of Franklin 12,000

E, IN CUBIC FEET PER SEC PER FEET CUBIC IN E, 8,000 G

4,000

Discharge = 8870*ln(LAR)-2640; r2 =0.91, p<0.001 US DISCHAR US 0 O 067812345 12,000 Across Jaws Bay 10,000

INSTANTANE 8,000

6,000

4,000

2,000 Discharge = 6510*ln(LAR)-2450; r2 =0.84 p<0.001 0 0612345 7 8 DAILY-AVERAGE STAGE OF LOWER ATCHAFALAYA RIVER AT MORGAN CITY, IN FEET ABOVE NAVD88

EXPLANATION

LAR is daily-average stage of the Lower Atchafalaya River at Morgan City; r2 is correlation coefficient, p is level of significance

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 Intra- coastal Waterway west of Wax Lake Outlet.

APPENDIX 51