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Coastal & Marine Geology Program > Coastal Classification Mapping Project > Open File Report 2006-1133

Coastal Classification Atlas

South Texas Coastal Classification Maps - Mansfield Channel to the Rio Grande

USGS Open File Report 2006-1133

Robert A. Morton, Russell L. Peterson

Report Home Maps Overview Mapping Methods Coastal Classifications Geologic Setting Coastal Processes Coastal Vulnerability Classification Summary References

Geologic Setting

The South Texas coast between Mansfield Channel and the Rio Grande consists of a long, narrow, sandy barrier island (South Padre Island) and a deltaic headland with a sandy beach (Brazos Island). Most of the shore is undeveloped. The barrier island and Rio Grande delta formed several thousand years ago as sea level rose, and they have narrowed and retreated landward as sand supplied by the Rio Grande decreased (Morton, 1994). The barrier is characterized by wide beaches and relatively high but discontinuous dunes. The dunes are sparsely vegetated and cut by numerous washover channels. In some areas, such as south of Mansfield Channel, sand dunes have migrated across the barrier and into adjacent Laguna Madre during droughts.

Because the tidal range in the western Gulf of Mexico is so low and long-term evaporation commonly exceeds precipitation, there is only one natural tidal inlet (Brazos Santiago Pass) serving southern Laguna Madre. Mansfield Channel was artificially dredged across central Padre Island and Laguna Madre, and jetties were constructed in 1957 to provide a navigation channel to Port Mansfield on the mainland.

Beaches of the South Texas coast change orientation following an arc that is concave toward the Gulf of Mexico. The beach, which is composed of fine sand, typically contains some broken shells that are eroded from the underlying deltaic sediments.

Coastal & Marine Geology Program > Coastal Classification Mapping Project > Open File Report 2006-1133

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