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In cooperation with the Texas Water Development Board Discharge Between San Antonio Bay and Aransas Bay, Southern Gulf Coast, Texas, May–September 1999

Along the Gulf Coast of Texas, many estuaries and bays are Madre, an estuary that begins just south of Corpus Christi Bay important habitat and nurseries for aquatic life. San Antonio Bay and runs southward 140 miles to South Padre Island. Most of and Aransas Bay, located about 50 and 30 miles northeast, respec- the remaining seagrasses, about 45,000 acres, are located in the tively, of Corpus Christi, are two important estuarine nurseries on heavily traveled San Antonio, Aransas and Corpus Christi Bay the southern Gulf Coast of Texas (fig. 1). According to the Texas areas” (Shook, 2000). Parks and Wildlife Department, “Almost 80 percent of the sea- grasses [along the Texas Gulf Coast] are located in the Laguna Population growth has led to greater demands on water sup- plies in Texas. The Texas Water Development Board, the Texas Parks and Wildlife Department, 97o30' DEWITT o o and the Texas Natural Resource GUADALUPE 97 96 30' o JACKSON 96 Conservation Commission have 08176500 o 08176500 the cooperative task of deter- 29 VICTORIA MATAGORDA RIVER mining inflows required to

COLORADO maintain the ecological health of Victoria RIVER COLETO the State’s streams, rivers, bays, CREEK o RESERVOIR 98 SAN and estuaries. To determine BEE ANTONIO these inflow requirements, the GOLIAD 08188500 Port Lavaca LAVACA BAY three agencies collect data and RIVER 28o30' CALHOUN MATAGORDA BAY GULF OF MEXICO conduct studies on the need for REFUGIO Copano ARANSAS Gulf Intracoastal instream flows and freshwater/ MISSION SAN ANTONIO031 BAY Waterway saline water inflows to Texas RIVER 08189200 PASS CAVALLO ND estuaries. Creek ARANSAS ESPIRITULA SANTO BAY 08189700 08189500 IS RIVER A RD O AG LAKE AT CORPUS SAN M To assist in the determination CHRISTI PATRICIO COPANO BAY ST. CHARLES of freshwater inflow require- o ARANSAS BAY BAY 28 Rockport ST. JOSEPH ments, the U.S. Geological Sur- NUECES ISLAND CEDAR BAYOU RIVER EXPLANATION vey (USGS), in cooperation Ptat2 CORPUS 08188500 Streamflow station and number with the Texas Water Develop- CHRISTI Corpus BAY ARANSAS PASS 08176500 ment Board, conducted a hydro- Christi Port Aransas Rainfall station and number NUECES graphic survey of discharge 031 TCOON1 wind gage and number (flow) between San Antonio

Ptat2 2 Bay and Aransas Bay during the o KLEBERG NOAA tide gage and number 27 30' period May–September 1999. Kingsville Automated instrumentation and acoustic technology were used

PADRE ISLAND BAFFIN BAY LAGUNA MADRE GULF OF MEXICO to maximize the amount and KENEDY quality of data that were col- TEXAS lected, while minimizing per- Gulf Intracoastal Area enlarged sonnel requirements. This report 27o Waterway 0 10 20 30 40 50 MILES documents the discharge mea-

LAGUNA MADRE sured at two sites between the bays during May–September 1999 and describes the influ- 1 Texas Coastal Ocean Observation Network ences of meteorologic (wind and 2 National Oceanic and Atmospheric Administration tidal) and hydrologic (fresh- water inflow) conditions on Figure 1. Southern Gulf Coast of Texas.

U.S. Department of the Interior USGS Fact Sheet 082–01 U.S. Geological Survey November 2001 FS_082-01.fm Page 2 Thursday, June 3, 2004 3:47 PM

discharge between the two bays. The movement of water between covers an area of about 70 square miles. The bays are “generally the bays is controlled primarily by prevailing winds, tidal fluctua- less than 6 feet deep” (McGowen and others, 1976, p. 10). Espir- tions, and freshwater inflows. An adequate understanding of mix- itu Santo Bay and Matagorda Bay are located just to the north of ing and physical exchange in the estuarine waters is fundamental San Antonio Bay (fig. 1); and Ayres Bay, Mesquite Bay, Carlos to the assessment of the physical, chemical, and biological pro- Bay, and Aransas Bay are located to the south of San Antonio Bay cesses governing the aquatic system. (fig. 2).

Physical Setting Freshwater inflow to San Antonio Bay is predominantly from the Guadalupe and San Antonio Rivers (fig. 1). No major fresh- San Antonio Bay and Aransas Bay are separated from the water tributaries flow into Aransas Bay although the Aransas Gulf of Mexico by barrier islands—San Antonio Bay by Mat- River, Mission River, and Copano Creek flow into Copano Bay, agorda Island and Aransas Bay by St. Joseph Island. San Antonio which flows into Aransas Bay (fig. 1). The main connection to Bay covers an area of about 100 square miles, and Aransas Bay the Gulf of Mexico for San Antonio Bay is through Pass Cavallo at the southern end of 96o57' 96o52' 96o47' Matagorda Bay (fig. 1). 28o20' For Aransas Bay, the main connection to the Gulf is through Aransas Pass at the southern end SAN ANTONIO of Aransas Bay. Cedar BAY Bayou, a natural pass that connects Mesquite Bay and the Gulf of

ARANSAS COUNTY

CALHOUN COUNTY Mexico (fig. 2), is the only direct connection to the Gulf between San Antonio Bay and Aransas Bay.

AYRES BAY San Antonio Bay and ST. CHARLES BAY Aransas Bay are hydrau- 28o15' lically connected by the Cedar Gulf Intracoastal Water- Point way (GIWW) (fig. 1), a MESQUITE BAY dredged channel that ISLAND Cedar Dugout runs along the entire UNNAMED ISLAND Gulf Coast. The GIWW Cedar is a maintained naviga- Reef tion channel that is more BLUDWORTH than 300 feet wide and CARLOS BAY about 15 feet deep.

ST. JOSEPH Cedar Dugout, a channel ISLAND between Bludworth

MATAGORDA ISLAND Island and a small unnamed island (fig. 2), is about 250 feet wide ARANSAS BAY and 13 feet deep. Cedar 28o10' Gulf Intracoastal Waterway Reef is an oyster reef that extends about 1 mile CEDAR BAYOU from the same small GULF OF MEXICO unnamed island to St. Joseph Island. During the study period, there were several instances when water did not flow 0 1234 MILES EXPLANATION over Cedar Reef, as the tide level was below the USGS gaging station crest of the reef. Figure 2. Study area, San Antonio Bay and Aransas Bay. 2 FS_082-01.fm Page 3 Thursday, June 3, 2004 3:47 PM

Table 1. Estimated net monthly discharge volumes at three sites between San Antonio Bay and Aransas Bay, Texas, May–September, 1999

Net discharge volume (acre-feet) Site number Site name May June July August September 1 Gulf Intracoastal Waterway at Bludworth Island 122,600 19,000 2,300 -100,600 112,200 2a Cedar Dugout near Cedar Point at Bludworth Island 56,000 3,380 90 -29,700 18,800 2b Cedar Reef near Cedar Point at Bludworth Island 50,500 -16,700 -4,630 -8,980 10,000

Data Collection and Discharge Computation During the period May 1–September 30, 1999, the USGS oper- ated gaging stations at the GIWW at Bludworth Island and Hourly wind data (velocity and direction) were obtained for a at Cedar Dugout near Cedar Point (fig. 2). Equipment at these sta- Texas Coastal Ocean Observation Network (TCOON) station on tions included acoustic velocity meters, data-collection platforms San Antonio Bay (fig. 1) (Texas A&M University-Corpus Christi (DCP), and a submersible pressure transducer. Instantaneous Conrad Blucher Institute, 2000); and Gulf of Mexico hourly tide measurements of flow direction and water velocity were made at data (water-surface altitude [stage] relative to sea level) were both stations using the acoustic velocity meters. Instantaneous obtained from a National Oceanic and Atmospheric Administra- measurements of water stage were made only at the GIWW sta- tion (NOAA) tidal station at Port Aransas (National Oceanic and tion because of equipment availability. Water-stage data recorded Atmospheric Administration, 2000). Streamflow data for inflow at the GIWW station were assumed representative of conditions tributaries (San Antonio River, Guadalupe River, Aransas River, at all locations in the study area. Data were recorded at 15-minute Mission River, and Copano Creek) were available from existing intervals and transmitted by the DCPs to the USGS database at 4-hour intervals. USGS streamflow stations (08188500, 08176500, 08189700, 08189500, and 08189200, respectively). Rainfall data were avail- Discharge measurements were made at the GIWW and Cedar able from USGS rainfall station 08176500 on the Guadalupe Dugout stations at various velocities and stages using a boat- River in Dewitt County. mounted acoustic Doppler current profiler (ADCP). For each

140,000 To Aransas Bay 120,000

100,000 Gulf Intracoastal Waterway 80,000 Cedar Dugout and 60,000 Cedar Reef

40,000

20,000

0

-20,000

-40,000

-60,000

DISCHARGE VOLUME, IN ACRE-FEET VOLUME, DISCHARGE -80,000

-100,000

-120,000 To San Antonio Bay

-140,000 MAY JUNE JULY AUGUST SEPTEMBER 1999

Figure 3. Net monthly discharge volumes for two stations between San Antonio Bay and Aransas Bay, May–September 1999. Positive indicates direction is from San Antonio Bay toward Aransas Bay; negative indicates reverse direction. 3 FS_082-01.fm Page 4 Thursday, June 3, 2004 3:47 PM

20,000 16 (a) Discharge volume1 1 14 Wind velocity (Components of velocity vectors along 15,000 azimuth of Gulf Intracoastal Waterway [220 degrees]) 12

10

10,000 8

6

5,000 4

2

0 0

-2 VELOCITY, IN MILES PER HOUR VELOCITY, DISCHARGE VOLUME, IN ACRE-FEET VOLUME, DISCHARGE -5,000 -4

-6

-10,000 -8 20,000 2.4 (b) Discharge volume1 Tidal stage 2.0 15,000 1.6

10,000 1.2

0.8 5,000 0.4

0 0

-0.4 DISCHARGE VOLUME, IN ACRE-FEET VOLUME, DISCHARGE -5,000 SEA LEVEL IN FEET ABOVE STAGE, TIDAL -0.8

-10,000 -1.2 MAY JUNE JULY AUGUST SEPTEMBER 1999 1 Positive indicates direction is from San Antonio Bay toward Aransas Bay; negative indicates reverse direction.

Figure 4. Daily discharge volume between San Antonio Bay and Aransas Bay and (a) wind velocity and (b) Gulf of Mexico tidal stage, May–September 1999. The graphs have been smoothed using the Friedman Supersmoothing technique (MathSoft Inc., 1999).

discharge measurement, as the boat traversed the flow transect, data were used to compute a total discharge for the transect. the ADCP was used to measure water depth, water velocity, and Discharge across Cedar Reef (fig. 2) was intermittent. When tides distance at about 20 locations along the transect; velocity was were sufficiently high, there was some shallow (less than 1 foot measured at two or three different depths at each location. The deep) discharge across Cedar Reef. Because the ADCP equipment

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26,000 10

24,000 9

22,000

8 20,000

18,000 7

16,000 6

14,000

5 12,000

4 10,000

Rainfall IN INCHES RAINFALL, Discharge volume 8,000 3 DISCHARGE VOLUME, IN ACRE-FEET VOLUME, DISCHARGE

6,000 2 4,000

1 2,000

0 0 510152025315 10 15 20 25 30 5 10 15 20 25 31 5101520253151015202530 MAY JUNE JULY AUGUST SEPTEMBER 1999 Figure 5. Daily rainfall at USGS station 08176500 and total daily discharge volume from five USGS streamflow stations into San Antonio Bay and Aransas Bay.

cannot be used for shallow depths, the additional discharge across charge at the GIWW site was the same as that at Cedar Dugout for Cedar Reef was measured using wading and current meter tech- all months although the magnitude of flow was less in Cedar Dug- niques (Rantz and others, 1982). out. However, the direction of net discharge at Cedar Reef was not the same as that at the other two sites during the months of June For both stations, corresponding discharge and instantaneous and July. Intermittent periods of flow over Cedar Reef during June velocity and stage measurements were used to develop velocity- and July typically corresponded to periods when the wind was stage-discharge relations using regression methods described by blowing from Aransas Bay toward San Antonio Bay. Therefore, Dunn and others (1997) and East and others (1998). The regres- sion equations allow computation of discharge from single veloc- during those periods when the tide was above the crest of the reef, ity or single velocity and stage measurements. A third regression there were more instances of negative discharge (flow direction equation was developed for Cedar Reef. Velocity and stage data from San Antonio Bay to Aransas Bay) than positive used to develop the regression for Cedar Dugout provided a suit- discharge. able index of flow over Cedar Reef. These data thus were related The monthly discharge volumes for Cedar Dugout and Cedar to discharge measurements made at Cedar Reef to develop the Reef were then summed and graphed with the concurrent monthly third regression equation. data for the GIWW site (fig. 3). Figure 3 indicates that the net dis- The discharge data for the three sites were used to compute the charge in the GIWW almost always was greater than the net dis- daily volume of discharge between San Antonio Bay and Aransas charge in Cedar Dugout and Cedar Reef combined. This appears Bay. The daily volumes were summed for each month (May, June, reasonable, as the GIWW is a well-maintained channel that is July, August, and September) (table 1). The direction of net dis- larger than Cedar Dugout/Cedar Reef. In late spring (May) and

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early fall (September), the net discharge was from San Antonio wind and tide data not significantly contributing to the estimation Bay to Aransas Bay. During the summer months of June and July of discharge is that discharge is lagged in time relative to wind when the net discharge between the two bays was much smaller, velocity and tidal stage. the directions of net discharge were opposite in the GIWW and Cedar Dugout/Cedar Reef because of flow over Cedar Reef References toward San Antonio Bay. In August, the net discharge increased Dunn, D.D., Solis, R.S., and Ockerman, D.J., 1997, Discharge and was from Aransas Bay to San Antonio Bay. measurement in tidally affected channels during a hydrographic estuarine survey of Sabine Lake, Texas: U.S. Relation of Discharge to Meteorologic and Geological Survey Fact Sheet FS–157–97, 6 p. Hydrologic Conditions East, J.W., Solis, R.S., and Ockerman, D.J., 1998, Computed Discharge in coastal environments commonly is affected by discharges at five sites in lower Laguna Madre near Port meteorologic conditions such as wind and tidal fluctuations. Isabel, Texas, June 1997: U.S. Geological Survey Fact Sheet Because water in the relatively shallow bays and estuaries is easily FS–167–98, 6 p. driven by wind, the direction of flow often is influenced by the McGowen, J.H., Proctor, C.V., Jr., Brown, L.F., Jr., Evans, T.J., prevailing wind direction. Similarly, the magnitude and direction Fisher, W.L., and Groat, C.G., 1976, Environmental geologic of discharge change with the ebb and flow of tides. The influence atlas of the Texas coastal zone—Port Lavaca area: University of wind and tides on the discharge between San Antonio and of Texas, Bureau of Economic Geology, 107 p. Aransas Bays is apparent when graphs of discharge volume and National Oceanic and Atmospheric Administration, 2000, wind velocity and of discharge volume and tidal stage during National Data Buoy Center, station information PTAT2—Port the study period (fig. 4) are compared. There is a close relation Aransas, Texas: Accessed May 3, 2000, at URL between discharge volume and wind velocity during May–Sep- http://www.ndbc.noaa.gov/station_page.phtml?$station=ptat2 tember 1999 (fig. 4a); and there was a noticeable relation between Rantz, S.E., and others, 1982, Measurement and computation of discharge volume and tidal stage (fig. 4b), although the relation is streamflow—Volume 1. Measurement of stage and discharge: not as close as that between discharge volume and wind velocity. U.S. Geological Survey Water-Supply Paper 2175, 284 p. Total gaged freshwater inflow to the two bays during the study Shook, P.,H., 2000, Marine meadows: Texas Parks and Wildlife period (sum of flows from USGS stations 08188500, 08176500, magazine, January 2000, accessed June 1, 2000, at URL 08189700, 08189500, 08189200) (fig. 5) was about 515,000 acre- http://www.tpwd.state.tx.us/news/magazine/meadow.htm ft, of which about 97 percent flowed into San Antonio Bay from Texas A&M University-Corpus Christi Conrad Blucher Institute, the San Antonio and Guadalupe Rivers. About 23 percent of this 2000, CBI-DNR data query: Accessed June 12, 2000, at URL inflow resulted from a series of rainstorms in the Guadalupe and http://dco.cbi.tamucc.edu/pquery/031 San Antonio River Basins during June 13–25. Comparison of daily discharge volume entering San Antonio Bay during the — Jeffery W. East study period (fig. 5) with daily discharge between the bays (fig. 4) shows no discernible relation. The influence of wind and tides on Any use of trade, product, or firm names is for descriptive discharge between the bays seemed to be much more substantial purposes only and does not imply endorsement by the than the influence of freshwater flow into San Antonio Bay during U.S. Government. the study. A final note—in the development of the regression equations to Information on technical reports and hydrologic data related relate measured discharge to instantaneous flow velocity and to this study can be obtained from: stage, wind velocity and tidal stage were used as explanatory vari- ables. Despite the apparent relations between discharge and wind Subdistrict Chief Phone: (713) 718–3655 velocity and discharge and tidal stage, neither wind velocity nor U.S. Geological Survey FAX: (713) 718–3661 tidal stage significantly improved the regressions; thus they were 2320 LaBranch St., Rm. 1112 E-mail: [email protected] not included in the final equations to estimate discharge from Houston, TX 77004 World Wide Web: instantaneous flow velocity and stage. A possible explanation for http://tx.usgs.gov/

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