Delta of the Guadalupe River,

¶ … Delta of the Guadalupe River, Texas

Summary of "The Recent Delta of the Guadalupe River, Texas"

This study comprehensively describes the processes and features of the delta formed by silt depositions from the Guadalupe and San Antonio rivers as they empty into the Gulf of Mexico. After descriptions of these rivers and delta are presented, the topics of the initiation of delta growth, the seven distinct delta plain environments (distributor-channel, natural-levee, marsh, lake, interdistributary-bay, delta-front, and prodelta) are presented and distinguished from one another. This is followed by analysis of the delta's subdivisions, or sub-deltas, based upon age and macrofauna differences. Next, the sedimentary facies in each of the above seven listed environments are differentiated by location and composition. Finally, the four identified regional sub-deltas (Guadalupe River-Big Bayou, Sommerville Bayou-Plank Bridge Bayou, North Guadalupe River-South Guadalupe River, and Traylor Cut) are described and geologically examined.

Lastly, the paper briefly covers the issues of climatic influence, development of bayous, erosion preservation and the abandoned beach-ridge plan, before a final discussion of the four distinct depositional environments located at the beach-ridge, tidal-mud-flat, marsh and lake sites.

Summary of delta geology and creation

To understand the Texas coastal plain's configuration and geology, one must understand that the lower Guadalupe River/San Antonio Bay region were carved out by the ebb and flow of continental glaciers (Pleistocene-age: 1.8 million to 10,000 years ago). In the course of geological time, the sea level fell as these glaciers advanced and rivers dropped to a lower base level and left shoreline father away. Across the face of the land, two rivers formed steep, narrow valleys out of these broad floodplains, and flowed steadily out onto the continental shelf of the Gulf of Mexico. When the sea level rose during the melting retreat of the glaciers, these late Pleistocene canyons filled with Gulf waters, forming extensive bay regions. The eventual rise in sea level over the last 10,000 years yielded the present-day sea level between 2,000 and 3,000 years ago.

The rivers once again adjusted their levels and carved new bends and channels within the old valleys. Over time, both rivers deposited material into the bays that made new floodplains and deltas, filling the bays with sediment. The modern configuration of San Antonio Bay represents the intermediate stage of this sedimentary process. The growing or "prograding" delta left by the combination of the Guadalupe and San Antonio rivers has presently filled in just the upper portion of the bay.

The Guadalupe River delta is found immediately across from the Guadalupe Bay site. Overall, geologists have divided this complex delta into two major parts: an older, "Old River" delta whose formation surrounded Green Lake, and a younger "Guadalupe" delta, that partly encloses Mission Bay and remains under construction. Regrettably, there is no radiocarbon estimate to accurately determine the ages of the two main deltas or that of the incorporated subdeltas. Researchers think the delta began to build upon San Antonio Bay some 2,000 years ago. Absent such radiocarbon dates for the delta proper, they must rely upon archeological finds from local digs to calculate the history of the various subdeltas.

It is generally accepted that the oldest delta (formed by the Guadalupe) first prograded to the southeast, and in later periods changed and prograded to the northeast, which isolated the upper part of San Antonio Bay, forming Green Lake. The younger delta, to which this study is devoted, prograded southeastwardly into San Antonio Bay, nearly parallel to the axis of the bay.

In a still later time in its history, the Guadalupe River once more flowed to the northeast and eventually would have isolated another part of San Antonio Bay. An artificial cut was made in 1935; however, that diverted almost half the flow of the river into the upper part of the bay, Mission Lake. Thus another delta is now forming at the mouth of the artificial cut in the protected waters of Mission Lake.