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

USGS Fact Sheet FS–025–00February 2000

INTRODUCTION

Lake Pontchartrain and adjacent lakes inLouisiana form one of the larger estuaries inthe Gulf Coast region. The estuary drains thePontchartrain Basin (fig. 1), an area of over12,000 square kilometers situated on the easternside of the Mississippi River delta plain. InLouisiana, nearly one-third of the State populationlives within the 14 parishes of the basin.

Over the past 60 years, rapid growth anddevelopment within the basin, along withnatural processes, have resulted in significantenvironmental degradation and loss of criticalhabitat in and around Lake Pontchartrain.Human activities associated with pollutantdischarge and surface drainage have greatlyaffected the water quality in the lake. Thischange is evident in the bottom sediments,which record the historic changes of the lake.Also, land-altering activities such as logging,dredging, and flood control in and around thelake have led to shoreline erosion and loss ofwetlands. The effects of pollution, shorelineerosion, and wetland loss on the lake andsurrounding areas have become a majorpublic concern.

To better understand the basin’s originand the processes driving its developmentand degradation requires a wide-rangingstudy involving many organizations andpersonnel. When the U.S. GeologicalSurvey (USGS) began the study of LakePontchartrain in 1994, information on fourtopics was needed: (1) geologic framework,(2) sediment characterization, (3) shorelineand surrounding wetland change over time,and (4) water circulation.

Geologic framework.—The geologicframework is an assessment of how thesediments accumulated in the area during the

geologic past. The sedimentary layers belowthe lake bottom were imaged using a radar-like technique called high-resolution seismicprofiling. Sidescan-sonar images of the lakefloor also were collected. These images aresimilar to aerial photographs of the land.Sediment samples were collected and theircompositions were analyzed to guide interpre-tations of seismic profiles.

Sediment characterization.—The sedi-ment samples were also used to measure thenatural concentration of chemical elementsknown as trace metals. Trace metals today areassociated with pollution; knowing the amountof trace metals present before humans affectedthe area allows the level of pollution to bedetermined. From surface sediments, scientistscan also locate modern areas of contamination.

Shoreline and wetland change.—Todetermine shoreline and wetland change, his-torical charts and aerial photographs were ana-lyzed digitally. These analyses were comparedwith recent surveys and charts. Because of thiswork, scientists can use information on shore-line change over the past 100 years to predictareas of future change.

Water circulation.—Water circulation inLake Pontchartrain was characterized by wind,current, tide, turbidity, salinity, and temperaturemeasurements. Satellite images were studied toobserve water quality over time. Water circula-tion, especially waves, resuspends polluted bot-tom sediments, and knowledge of circulation iskey to predicting where the contaminants are like-ly to be concentrated. The objective was to con-struct a computer model of circulation patterns.

100 KILOMETERS

Louis iana

Mississippi

BatonRouge

NewOrleans

Jackson

Gulf of Mexico

Mis

siss

ippi

Riv

er

Mis

siss

ippi

Riv

er

PontchartrainPontchartrainBasinBasin

PontchartrainBasin

LakePontchartrainLakePontchartrain

Arkansas

Texas

Ala

ba

ma

Geologic Framework and Processes ofLake Pontchartrain, Louisiana

Wetland loss and pollution arecritical environmental concernswithin the Pontchartrain Basin. U.S.Geological Survey scientists andcollaborators have conducted amultidisciplinary study of lake-bottomsediments, sediment transport, anddistribution of contaminants to assistfuture remediation efforts.

Figure 1. Lake Pontchartrain and the Pontchartrain Basin are located in Louisiana and Mississippi.

N

94° 93° 92° 91° 90° 89°

33°

32°

31°

30°

29°

Mississippi River

Lake Maurepas

Lake B

orgne

a in

PearlRiver

Explanation

Bathymetric surveySidescan-sonar surveySeismic survey

Bottom sediment sampleBox core surveyVibracore survey

MILES

L a k e P o n t c h a r t r a i n

0 10

100KILOMETERS

90° 35' 90° 25' 90° 15' 90° 05' 89° 55' 89° 45' 89° 35' 89° 25'

30° 2

0'30

° 10'

30° 0

0'29

° 50'

NewOrleans

TheRigolets

INVESTIGATIONS

Between 1994 and 1998, surveys wereconducted in Lakes Pontchartrain, Maurepas,and Borgne by the USGS, in cooperationwith private, State, and Federal institutions(see box, figs. 2 and 3). The acquired data arecritical in characterizing the lake system andin providing information for environmentalmanagement of the basin.

Studies include the following:

Sediment Sample Data • 100+ Vibracores in lakes• Data from 100+ U.S. Army Corps of

Engineers boreholes across lake• 80+ box cores in lakes• 1,700 bottom sediment samples

Geologic Analyses• Sediment composition• Stratigraphy (layering)• Palynology (pollen and spores)• Sediment ages • Trace-metal concentrations• Models of water and atmospheric

circulation

Remote Sensing• 650 line km of single-channel

seismic profiling• Five sidescan-sonar grids• Four bathymetric surveys (>200 km)• Continuous wave, turbidity, salinity, and

temperature measurements (five sites up to five months each)

• AVHRR (advanced very high resolution radiometer) satellite imagery

Figure 2. Location of remote-sensing and sediment sampling surveys of Lakes Maurepas, Pontchartrain, and Borgne.

Figure 3. Most of the surveys were conductedfrom the USGS Research Vessel G.K. Gilbert, depicted here acquiring a Vibracore.

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COLLABORATORS USGS

J. Kindinger (project leader)C. Demas (water quality)J. Flocks (geochemistry)C. Holmes (age determination)J. List (modeling)F. Manheim (geochemistry)M. Marot (age determination)C. Polloni (data base, visuals)R. Signell (modeling)R. Stumpf (remote sensing)D. Willard (palynology)S. Williams (framework)

EXTERNALCoalition to Restore Coastal Louisiana (CRCL)

M. Davies (environmental)Environmental Protection Agency (EPA)

N. Thomas (biology)Lake Pontchartrain Basin Foundation (LPBF)

C. Dufrechou (director)J. Waters (project manager)

National Oceanic and AtmosphericAdministration (NOAA)

K. Clemens Dennis (environmental)U.S. Army Corps of Engineers (USACE)

D. Britch (geology)University of Georgia

J. Noakes (geochemistry)University of New Orleans (UNO)

S. Penland (geology)M. Porrier (biology)M. Roth (computers)

GEOLOGIC FRAMEWORK

An evolutionary model (fig. 4A–E) forthe Lake Pontchartrain area was developedby using seismic and sedimentologic data.Following the late Wisconsinan sea-level low-stand (~18,000 years ago), the sea rose andflooded the area (fig. 4A, B). By 6,000 yearsago, sea-level rise had slowed, and longshore

sediment transport began developing a barrier-beach system (fig. 4B). As sea level continuedto rise, the barrier system was bypassed, form-ing the Pine Island Barrier trend, while flood-ing created the Pontchartrain Basin embayment(fig. 4C). By 4,500 years ago, the MississippiRiver moved into the area and began deposit-

ing the St. Bernard Delta lobe (on which thecity of New Orleans is located), enclosing theembayment (fig. 4D). Modern stabilization andsubsidence of the delta lobe began about 2,000years ago, developing Lakes Maurepas,Pontchartrain, and Borgne and creating theland surface seen today (fig. 4E).

3

SEDIMENT CHARACTERIZATIONAND CIRCULATION MODELING

Evidence for environmental change canbe seen in a Lake Pontchartrain sediment core(fig. 5). Percent abundances of selected pollentaxa and trace-metal concentrations show thatabout 120 centimeters (4 feet) beneath thelake bottom, pollen assemblages changed frompine/oak (common Gulf Coast) to herbaceousspecies that indicate human colonizationand land clearing. At the same level, theconcentrations of trace metals increased, alsoan indication of human activity.

Depositional patterns in Lake Pontchartrainwere determined from current and circulationmodeling and from rates of sediment deposition(fig. 6). The current model was based onprevailing wind speed and direction andbathymetry. Current direction and magnitude(size of arrows) show a predominantly northwestflow from the eastern side of the lake(Rigolets) during a prevailing northwest wind,which sets up gyres of circulation in the centralpart of the lake. Recent sediment deposition(darker red areas) appears to coincide withareas of decreased circulation (short arrows).

0 10 0 0 00

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Depth(cm) Lithology

claysandsilts

clays

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Depth(cm)

0 20 40 60Concentration ofpollen/gram x 103

Percent abundanceof pollen

15 25 35 16 24 32 2 3 4Copper (ppm) Lead (ppm) Iron (wt%) Cadmium (ppm)

Colonization and land clearing began

Higher Higher Higher Higher

0.1 0.5 0.9

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mar

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Figure 5. Pollen and trace-metal abundances in (Vibracore) Ipon 97–1. Note: ppm=parts per million, wt%=weight percent.

Prairie TerracesComplex

Wisconsinanexposed surface

PearlRiver

A

Gulf of Mexico

PineIsland

B

LakePontchartrain

New Orleans

EC

PontchartrainEmbayment

D

St. Bernard Delta

Figure 4. Evolutionary model for basin development from 18,000 years ago (A) to today (E).

18,000 years ago 6,000 years ago 5,000 years ago 4,500-2,000 years ago Present day

Wind directionRigolets

N

EW

S

Figure 6. Circulation and sediment accumulationpatterns in Lake Pontchartrain. Current direction andmagnitude (size of arrows) were generated from acomputer model based on bathymetry and windspeed and direction. Areas filled by shades of redshow beryllium-7 (Be7) isotope activity obtained fromlake-bottom sediment samples. Darker red areas havehigher concentrations of Be7, which indicate recentsediment deposition because Be7 forms in the atmo-sphere and dissipates rapidly with a half-life of 53days. Light-blue areas lack Be7.

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AVHRR IMAGERY

Images of Lakes Pontchartrain, Maurepas,and Borgne were derived from data collectedby the advanced very high resolution radiome-ter (AVHRR) onboard polar-orbiting weathersatellites. In March 1997, the Bonnet-Carré

flood-control structure was opened, and thewater from the Mississippi River was divertedthrough Lake Pontchartrain to avoid potentialflooding of New Orleans. The images in figure7 illustrate the increase in suspended material in

the lake as a result of the diversion of floodwa-ters. The sediment-laden water enters from theBonnet-Carré Spillway in the west, travelsthrough the lake, and exits via the Rigolets intoLake Borgne and the Mississippi Sound.

WAVE MODELING

Resuspension and transport of bottomsediments due to wave action are importantphysical components of sediment distributionin Lake Pontchartrain. Wave height andassociated bottom orbital velocity (currentmotion) are functions of wind stress at the

surface and of bathymetry. This action can besimulated by using the numerical wave-pre-diction model HISWA (Hindcasting ShallowWater Waves). Figure 8 shows the HISWApredictions for significant wave height(fig. 8A, B) and maximum orbital velocity

(fig. 8C, D). Results indicate that althoughwaves may be higher in the central portionsof the lake, increased bottom orbital veloci-ties at the shorelines drive resuspension andtransport of fine-grained sediment.

BA C D

Inc

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SHORELINE/WETLAND CHANGE

Having information about shoreline erosionand wetland change is crucial for improvedmanagement in the Pontchartrain Basin. In thepast 60 years, more than 76,000 hectares(188,000 acres) of land in the basin have beenlost due to a complex suite of causes (fig. 9).

Erosion rates are derived from comparison ofrecent measurements with historic data.Where the shoreline has not been stabilized bysea walls or other structures, erosion averages2 to 4 meters per year.

Wetlands have been lost to urbanizationand natural submergence. Changes includecypress swamps converting to freshwatermarsh, freshwater marshes converting to highersalinity marsh, and complete submergence.

Figure 8. Wave modeling for Lake Pontchartrain shows the increase in wave height and lake bottom orbital velocity (turbulence) generated as south winds increasefrom 23 to 39 knots.

Figure 9. Shoreline erosionrates around Lake Pontchartrain.

0-1Erosion Rate (m/yr)

4+

1-2

Stabilized

2-4

March 6, 1997

wave height 23 kts. wave height 39 kts. orbital velocity 23 kts. orbital velocity 39 kts.

Figure 7. Turbidity inLake Pontchartrain afterthe Bonnet-Carré Spillwayopening. The darker redhas more suspendedsediment.

For further information, please contact:

Jack L. KindingerJames G. FlocksU.S. Geological Survey600 4th Street SouthSt. Petersburg, FL 33701Telephone: 727–803–8747 x3018Fax: 727–803–2032E-mail: jkindinger@usgs.gov

March 23, 1997 April 7, 1997

Bonnet-CarréSpillway

Bonnet-CarréSpillway

Bonnet-CarréSpillway

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