MORPHOLOGICAL AND STRATIGRAPHIC RESPONSES OF A TIDAL INLET TO ANTHROPOGENIC ALTERATIONS WITHIN THE ESTUARY

The coast of the Northwestern Gulf of Mexico is dominated by micro-tidal estuaries bounded by barrier island complexes separated by tidal inlets. Human development in the region has led to many of these inlets being modified (e.g. jetty construction) in an effort to maintain them for navigation. Pass Cavallo (PC), located along the central Texas coast, is one of two inlets in the region that remains directly unmodified by human intervention. While it is one of the few “natural” inlets in the region, anthropogenic activity within the adjacent estuary has resulted in dramatic changes to the inlet. This study combined high-resolution CHIRP subbottom data, and sediment cores with historical shoreline data to investigate the morphological and stratigraphic changes of PC that resulted from the anthropogenic alterations within the estuary. The most significant alterations to affect the inlet were the dredging of the Colorado River navigational channel (CRNC) and the creation of the Matagorda Ship Channel (MSC). Both of these alterations primarily affected the tidal prism and tidal prism capture of the estuary. With the dredging of the CRNC the tidal prism was reduced by 13%, resulting in a reduction in cross-sectional area of PC. In 1963 the MSC was opened ~ 10 km from PC capturing ~ 70% of the tidal prism of the bay. Within a few decades spit growth across PC resulting from the reduced tidal exchange decreased the width of the inlet from 5.4 km to 0.7 km. CHIRP data from behind the spit showed two dominant seismic facies separated by prominent reflectors. The lower facies corresponded to sand-dominated sediment, where the upper facies was dominated by mud. These changes in sedimentation correspond to the morphological changes that were observed. The lower facies, interpreted to be an ebb tidal delta facies, reflects environmental conditions prior to any anthropogenic alterations in the bay, and the upper facies is interpreted to be an estuarine facies initiated after the anthropogenic alterations and spit growth across the inlet. This study highlights the effects that anthropogenic alterations within an estuary can have on the “natural” components of the system that were not directly modified. Furthermore, the study shows that these alterations have lasting effects morphologically and preserved within the sediment record.