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Water Budget in the Laguna Madre Watershed
Lower Laguna Madre encompasses about 270 square miles
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
the Gulf of Mexico—Brazos Santiago Pass and Port Mansfield Channel.
Freshwater inflows to lower Laguna Madre average less than 530,000
acre-feet per year and are concentrated primarily at Brownsville Ship
Channel (fig. 2) and Arroyo Colorado (fig. 1). Because evaporation
exceeds the sum of freshwater inflows toand precipitation on lower
Laguna Madre, salinities in some areas at times exceed that of seawater
(about 35,000 milligrams per liter dissolved solids). 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, though little physical data
exist to confirm the frequency or duration of this pattern. The flow
pattern presumably reverses under the influence of winds from the north
during winter months.
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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. 408–419.
Laenen, Antonius, 1985, Acoustic velocity meter systems:
U.S. Geological Survey Techniques of Water-Resources
Investigations, book 3, chap. A17, 38 p.
Texas Department of Water Resources, 1983, Laguna Madre
Estuary—A study of the influence of freshwater inflows:
Texas Department of Water Resources LP–182 [variously
paged].
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