1

26

o

26 30’

27 30’

o

o

27

o

CAMERON

WILLACY

KENEDY

KLEBERG

NUECES

GRANDE

RIO

Corpus

 Christi

Harlingen

Brownsville

Port

 Isabel

Port

 Mansfield

Land Cut

Port Mansfield

 Channel

Padre Island

Brazos

 Santiago

 Pass

BAFFIN

BAY

GULF OF MEXICO

CORPUS

 CHRISTI

 BAY

UPPER

LAGUNA

LOWER

LAGUNA

MADRE

MADRE

SOUTH BAY

GULF OF MEXICO

Arroyo

Colorado

0 10 20 30 40 MILES

Figure 1. Southern Gulf Coast of Texas.

National Weather 

Service rain gage

NWS 722505

Gulf Intracoastal 

Waterway

Harlingen International 

Airport

TEXA

Area e

at left

LOCATION 

and Wildlife Department (TPWD), and Texas Natural Resource 

Conservation Commission (TNRCC) are charged by the Texas 

Legislature with determining freshwater inflows required to maintain the ecological health of streams, bays, and estuaries in Texas. 

To determine required inflows, the three agencies collect data and 

conduct studies on the needs for freshwater inflows to estuaries. 

The TWDB uses estuarine hydrodynamic and conservativetransport computer models to predict the effects of altering freshwater inflows on estuarine circulation and salinity. To calibrate 

these models, a variety of water-quality and discharge data are 

needed. 

The U.S. Geological Survey (USGS), in cooperation with 

the TWDB, conducted an intensive hydrographic survey during 

June 19–22, 1997, to provide calibration data for a TWDB model 

of lower Laguna Madre near Port Isabel, Tex. The TPWD, 

U.S. Army Corps of Engineers Waterways Experiment Station, 

and Texas A&M University-Corpus Christi Conrad Blucher 

Institute also participated in the study. Lower Laguna Madre is a 

shallow estuary between Padre Island, a barrier island along the 

southernmost Texas Gulf Coast, and the Texas mainland (fig. 1). 

Automated instrumentation and acoustic discharge-measuring 

technology were used to maximize the amount and quality of data 

that were collected and to minimize personnel requirements.

This fact sheet describes the hydrographic survey and the methods of discharge measurement and analysis and presents discharges computed for five sites to account for flow into, within, 

and out of the southern end of lower Laguna Madre during the 

approximately 74-hour survey period (three tidal cycles) of June 

19–22, 1997.

Lower Laguna Madre

Lower Laguna Madre encompasses about 270 square miles 

(Hedgpeth, 1967, p. 409) and extends northward about 50 miles 

from Port Isabel, Tex. (fig. 1). The upper and lower bodies of 

Laguna Madre are connected by the Land Cut, a 26-mile reach of 

the Gulf Intracoastal Waterway (GIW) that breaches a broad area 

of sand dunes that, prior to dredging of the GIW, physically 

separated the two bodies of water. The average depth in Laguna 

Madre is less than 4 feet except in the GIW, in areas of the Arroyo 

Colorado, and in the main channels connecting Laguna Madre to 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S
2

the Gulf of Mexico—Brazos Santiago Pass and Port Mansfield 

Channel (Texas Department of Water Resources, 1983, p. III–1).

Freshwater inflows to lower Laguna Madre average less 

than 530,000 acre-feet per year (Texas Department of Water 

Resources, 1983, p. IV–2) and are concentrated primarily at 

Brownsville Ship Channel (fig. 2) and Arroyo Colorado (fig. 1). 

Because evaporation exceeds the sum of freshwater inflows to 

and precipitation on lower Laguna Madre, salinities in some areas 

at times exceed that of seawater (about 35,000 milligrams per liter 

dissolved solids) (Texas Department of Water Resources, 1983, 

p. V–11). Net flow in lower Laguna Madre generally is assumed 

to be in the direction of prevailing winds, which during the summer months predominantly is toward the northwest. During 

the summer months, Gulf waters are presumed to enter from the 

south at Brazos Santiago Pass, flow northward, and exit at Port 

Mansfield Channel or the Land Cut (Hedgpeth, 1967, p. 410), 

though little physical data exist to confirm the freque

tion of this pattern. The flow pattern presumably reve

the influence of winds from the north during winter m

Data-Collection Sites

Rainfall data from the National Weather Service (

rain gage at Harlingen International Airport (fig. 1) f

approximately 74-hour period of the hydrographic sur

Laguna Madre were obtained from the NWS. Wind v

and direction and tide data were collected at a Texas 

Ocean Observation Network (TCOON) station (fig. 2

series of observation stations along the southern Texa

maintained in part by the Texas A&M University-Co

Conrad Blucher Institute. 

The USGS and the TWDB collected velocity, stag

charge data at two sites—Laguna Madre at Brazos Sa

26 02’30" o

0 1 2 MILES

Site 1

Site 3

051

Measurement site for velocity, stage, and discharge—

Site number corresponds to site number in tables 

Measurement site for velocity and discharge—Site 

number corresponds to site number in tables

Texas Coastal Ocean Observation Network 

(TCOON) station

EXPLANATION

Net discharge volume in acre-feet and flow direction 

during 74-hour survey—Scale 1 inch = 50,000 acre-feet 8,880

Site 4

Site 2

SOUTH BAY

Site 3

Brownsville

Ship Channel

South Bay

Pass

8,880

27,000

2,190

Figure 2. Measurement sites, net discharge volumes, and flow directions in lower Laguna Madre near Port Isabel, Texas, J

1997.

Agency responsible for data collected at sites 1–5 

Site no. Velocity Stage Disc

1 USGS USGS US

2 TWDB TWDB TW

3 USGS -- TW

4 USGS -- TW

5 USGS -- US
3

(site 1) and South Bay Pass (site 2)—and velocity and discharge 

data at three sites—Brownsville Ship Channel (site 3), Port Isabel 

Channel (site 4), and Laguna Madre at Queen Isabella Causeway 

(site 5) (fig. 2).

Meteorological Conditions

During June 1997, about 1 inch of rain was recorded by the 

gage at Harlingen International Airport; only 0.05 inch of rain was 

recorded during the June 19–22 survey, which is consistent with 

the observation of trace amounts of rainfall during the approximately 74-hour survey. Therefore, appreciable inflows of storm 

runoff to the southern end of lower Laguna Madre during the survey were unlikely.

Wind vectors—arrows in which the length is proportional to 

the wind speed and the orientation indicates the direction that 

the wind was blowing—show that during the survey the wind 

typically originated from the southeast and blew to th

and that maximum wind velocity was about 30 miles 

(fig. 3a). Wind vectors from TCOON station 051 for 

periods indicate that the June 19–22 data are typical o

terns in lower Laguna Madre near Port Isabel.

The survey period was chosen to coincide with the 

annual maximum tidal fluctuation so that annual max

inflows to and outflows from the southern end of low

Madre could be measured. The daily tidal fluctuation

the TCOON station during June 1997 ranged from les

to about 2 feet (fig. 3b).

Velocity and Stage Measurements

Instantaneous measurements of flow velocity were 

by electronic data loggers at the five USGS and TWD

Two types of velocity meters were used at the sites—

31 5 10 15 20 25 30

June

1997

2.5

5.5

3.0

3.5

4.0

4.5

5.0

Survey

period

May

1997

July

1997

1 5 10 15 20 25 30

JUNE 1997

Scale for wind speed and direction

East

per hour

(b)

ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

Figure 3. (a) Wind speeds and directions and (b) tidal fluctuations from Texas Coastal Ocean Observation Network station 

Laguna Madre near Port Isabel, Texas, June 1997.
4

point-velocity meters (APVM) and acoustic velocity meters 

(AVM)—to record velocity at intervals of 15 minutes or less (table 

1). An APVM measures water velocity at a single point using a 

Doppler phase-shift technique (Dunn and others, 1997). An AVM 

measures the velocity along an acoustic path set diagonal to flow 

(Laenen, 1985). Project personnel attempted to place the meters at 

locations in a channel where the meters would record velocities 

that were representative of overall flow conditions in the entire 

channel.

Stage was measured by sensors connected to submersible pressure transducers.

Discharge Measurements

A boat-mounted broadband acoustic Doppler current profiler 

(ADCP) was used to measure the tidally affected discharge at each 

of the sites. The ADCP uses acoustic signals to measure water 

velocity, water depth, and boat location at multiple depths and 

locations as the ADCP traverses a channel. These data are composited and used to compute a discharge for a given transect. Discharge measurements were made hourly, coincident with velocity 

measurements that were recorded at the five sites. 

Discharge Ratings and Computation

Velocity and stage measurements in tidally affecte

can be related to corresponding discharges measured 

ADCP using regression methods as described by Dun

(1997). At three of the five sites (sites 1, 4, and 5), ve

measurements were related to ADCP discharge meas

through regression analysis. At South Bay Pass (site 

and stage measurements were related to ADCP disch

measurements. Discharges at Brownsville Ship Chan

were computed on the basis of discharges at sites 1, 2

Regression equations of the following form were u

Q = B1 + B2V + B3S,

where Q is the estimated discharge in cubic feet per s

B2, and B3 are the regression coefficients, V is the m

velocity in feet per second, and S is the measured sta

above (+) or below (-) the mean stage measured durin

graphic survey. The regression equations allow the co

discharge from a single velocity (and possibly stage) 

ment. The regression equation coefficients and coeffi

determination are listed in table 2.

Table 2. Regression coefficients and coefficients of determination for discharge ratings at lower Laguna Madre 

measurement sites 

[equation: Q = B1 + B2V + B3S, where Q is estimated discharge in cubic feet per second, B1, B2, and B3 are regression co

V is measured velocity in feet per second, and S is measured stage in feet above (+) or below (-) the mean stage measured 

hydrographic survey. R2

, coefficient of determination (variability in the measured discharges explained by the equation); n

applicable]

Site

number Site name B1 B2 B3 R

1 Laguna Madre at Brazos Santiago Pass -348 50,739 0 0

2 South Bay Pass 147 5,474 -325 

3 Brownsville Ship Channel n/a n/a n/a n

4 Port Isabel Channel -4,446 16,903 0 

5 Laguna Madre at Queen Isabella Causeway -1,560 36,356 0 
5

2400 0600 1200 1800 2400 0600 1200 1800 2400 0600 1200 1800 2400

19 20 21 2

June 1997

-150,000

-100,000

-50,000

DISCHAR

E

Review of velocity data collected at Brazos Santiago Pass 

(site 1) indicated that the data were not representative of the entire 

channel, probably because of the near-shore location of the 

velocity meter. U.S. Coast Guard regulations prevented location of 

the velocity-measuring equipment in the open channel of 

Brazos Santiago Pass. Because of this, useful regression equations 

could not be developed from velocity data and velocity/stage data 

collected at the site. Therefore, discharge measurements made at 

Brazos Santiago Pass were regressed with concurrent velocity 

data that were collected at Queen Isabella Causeway (site 5).

Efforts to develop regression equations for Brownsville Ship 

Channel (site 3) also were unsuccessful. Estimated discharges 

at three of the other sites were used to compute discharge at this 

site. The authors assumed that flow into and out of storage in the 

estuary was negligible during the 74-hour survey period. Under 

this assumption, all flow through Brazos Santiago Pass (site 1) 

would have to pass one of three other sites, South Bay Pass 

(site 2), Brownsville Ship Channel (site 3), and Queen Isabella 

Causeway (site 5). Therefore, the sum of discharges at South Bay 

Pass, Brownsville Ship Channel, and Queen Isabella Causeway 

were set equal to the discharge at Brazos Santiago Pass. Flow at 

the Brownsville Ship Channel site was then computed as the discharge at Brazos Santiago Pass minus the sum of discharges at 

South Bay Pass and Queen Isabella Causeway1.

Discharges

Hydrographs of discharges estimated from the ratings at sites 1, 

2, 4, and 5 and computed as described for site 3 were superimposed for comparison of flows at the sites during the survey 

(fig. 4). Flood tide flows were considered positive discharges, and 

ebb tide flows were considered negative discharges. F

shows that inflow (flood tide flow) and outflow (ebb 

through Brazos Santiago Pass coincided with flow pa

Isabella Causeway, which was in a northerly directio

flood tide and a southerly direction during ebb tide. T

lesser magnitude cycled into and out of Brownsville S

and Port Isabel Channel. Relatively small rates of inf

outflow occurred in South Bay Pass. Inflow to lower 

Madre from South Bay occurred during ebb tide and o

Laguna occurred during flood tide.

The discharge data were used to compute the cum

discharge volume for the three diurnal tidal cycles of 

period for each site (fig. 2; table 3). During the appro

74-hour period, there was a net inflow to lower Lagu

through Brazos Santiago Pass of 59,300 acre-feet. Th

(plus the relatively small net inflow through South B

1 To check the reasonableness of this computation, the es

discharges at the Brownsville Ship Channel site were comp

discharges measured at the same site by an ADCP during Ju

The estimated discharges for the survey period ranged from 

34,700 cubic feet per second. (Flood tide flows were consi

discharges, and ebb tide flows were considered negative di

June 6–7 ADCP discharges ranged from -17,400 to 22,200 

second. This comparison shows that the estimated discharg

same order of magnitude as the discharges measured on Ju

because the 74-hour survey period coincided with the perio

maximum tidal fluctuation, discharges would be expected t

greater during the survey than during June 6–7. Therefore, 

believe the estimated discharges for Brownsville Ship Cha

reasonable, although the assumption of negligible storage w

Figure 4. Estimated discharges at five sites in lower Laguna Madre near Port Isabel, Texas, June 19–22, 1997.
6

2,190 acre-feet) resulted in a net flow north in lower Laguna 

Madre of 43,400 acre-feet, which is obtained by summing the net 

flow at Queen Isabella Causeway (34,500 acre-feet) and the 

net flow in Port Isabel Channel (8,880 acre-feet). About 71 percent of the net inflow to the estuary through Brazos Santiago Pass 

and South Bay Pass flowed north in lower Laguna Madre, which 

supports the hypothesis of Hedgpeth (1967, p. 410) that water 

generally enters lower Laguna Madre through Brazos Santiago 

Pass and flows northward toward Port Mansfield Channel. There 

was a net flow up Brownsville Ship Channel of 18,100 acre-feet, 

which is obtained by subtracting the net flow in Port Isabel 

Channel (8,880 acre-feet) from the net inflow to Brownsville 

Ship Channel estimated at the entrance to the channel (site 3) 

(27,000 acre-feet). 

The net inflow to lower Laguna Madre from South Bay during 

the survey probably is the result of wind-driven flow. South Bay is 

a shallow estuary, and the prevailing winds during the survey were 

from the southeast to the northwest (fig. 3). 

References

Dunn, D.D., Solis, R.S., and Ockerman, D.J., 1997, Discharge 

measurements in tidally affected channels during a 

hydrographic estuarine survey of Sabine Lake, Texas: 

U.S. Geological Survey Fact Sheet FS–157–97, 6 p.

Hedgpeth, J.W., 1967, Ecological aspects of the Laguna Madre, 

a hypersaline estuary, in Lauff, G.H., ed., Estuaries: 

Washington, D.C., American Association for the 

Advancement of Science Publication 83, p. 408–

Laenen, Antonius, 1985, Acoustic velocity meter sys

U.S. Geological Survey Techniques of Water-Re

Investigations, book 3, chap. A17, 38 p.

Texas Department of Water Resources, 1983, Laguna 

Estuary—A study of the influence of freshwater 

Texas Department of Water Resources LP–182 [

paged].

— J.W. East2

, R.S. Solis3, and D.J. Ockerman2

Any use of trade, product, or firm names is for descri

purposes only and does not imply endorsement by the 

Government.

Information on technical reports and hydrologic d

to this study can be obtained from

Subdistrict Chief

U.S. Geological Survey

2320 La Branch Street

Houston, TX 77004

Phone: (713) 718–3655

FAX: (713) 718–3661

World Wide Web: http://tx.usgs.gov

2 U.S. Geological Survey. 

3 Texas Water Development Board. 

1

 Velocity data from site 5. 

2 Computed as the discharge at site 1 minus the sum of discharges at sites 2 and 5. 

4 Port Isabel Channel 15,000 1.15 13,100 .51 

5 Laguna Madre at Queen Isabella Causeway 97,300 2.72 -64,500 -1.73