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Rio Grande Delta in Texas—Sea-Level, Climate, Neotectonic and Anthropogenic Effects

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delta airphoto
Aerial view of the fluvial sediment-depleted mouth of the Rio Grande at the Rio Grande delta (March, 1999).
The major Holocene coastal depocenter west of the Mississippi delta in the northwestern Gulf of Mexico is the Rio Grande delta at the Texas-State of Tamaulipas, Mexico, border, a system that began to form about 7,000 years ago. A project to define the origin of this low-lying and vulnerable delta, and most specifically to measure effects of sea level, land motion, and paleoclimate changes—and the more recent anthropogenic influences —has recently been initiated by Daniel Stanley of the Smithsonian Institution.

This is clearly not a delta in its constructional phase. Rather, the coastline configuration is presently being altered and cut back by strong wave-driven energy and hurricanes (average rate of one every five years). Increased coastal erosion is due, at least in part, to damming of the Rio Grande and creation of large reservoirs behind them, consequent decrease of sediment carried to the mouth of the river, and enlarged irrigation waterway development during this century.

Dan, who directs the Deltas-Global Change Program at the National Museum of Natural History, is presently spending a sabbatical at the Woods Hole Field Center, where he is coordinating this delta sea-level and paleoclimate-change research with Rob Thieler and other USGS staff.

The Rio Grande system, formed in a semi-arid region, displays the typical delta shape and is arcuate in response to strong coastal currents. The delta's apex is positioned 75 km from the coast, and Texas and Tamaulipas almost equally share its shoreline length of about 140 km.

Rio Grande Basin
The drainage basin of the Rio Grande/Rio Bravo lies mostly in the U.S. and extends as far north as central Colorado.
Unlike many deltas, however, the thickness of the Rio Grande's Holocene section is rather modest, as recorded by some Pleistocene exposures at and near the coast. Also, there is an unusually large number of distributary channels that are well preserved on the Holocene delta plain, and all of these display remarkably well-developed meander forms with relatively high levees.

Together, these attributes suggest that there is a close relation to flood discharge on relatively low delta-plain slopes, and that a substantial amount of Rio Grande sediment previously bypassed the delta coast seaward to the Gulf of Mexico. It would appear that just seaward of the delta, Padre Island, the world's longest barrier island, was influenced by interaction of low land-subsidence rates, formerly rapid progradation and high sediment input at the coast, and strong coastal current activity.

Using the results of his previous delta studies in the Nile, Yangtze, Ganges-Brahmaputra and other deltas as a base for comparison, Dan would like to focus on several problems of the Rio Grande delta that have direct human impact.

delta beach
Ground view of the Rio Grande rivermouth (March, 1999).
Pertinent, for example, are measurements of concurrent changes of sea level and land subsidence that may lead to measurement of relative sea-level motion.

He is seeking project funding and collaboration with partners to help continue field and laboratory research. The study would involve coring across selected Rio Grande delta-plain and adjacent Laguna Madre and Padre Island sectors to the inner shelf.

Recovery of cores and dating sedimentary sections are essential to detail the Holocene stratigraphy with which to define the delta-to-inner-shelf architecture, measure changes of sea level and land subsidence, and survey the effects of Holocene paleoclimate. Short cores that retrieve sections preserving history of the past several centuries are also needed to understand what has led to changes of sediment accumulation versus erosion induced by human activity.

Interest in this project is welcomed, and Dan can be contacted at Woods Hole and at the Smithsonian at stanley.daniel@NMNH.SI.edu (phone: 508-457-2248; fax: 508-457-2309).

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