THE GEOLOGICAL EVOLUTION OF A NATURAL TIDAL INLET: PASS CAVALLO, TEXAS

Pass Cavallo separates Matagorda Peninsula from Matagorda Island as a natural tidal inlet at the southwest extremity of Matagorda Bay. Historically, the pass was one of the widest on the Gulf Coast (approximately 2.9km). In the last century, the anthropogenic influences have compromised the stability of the pass. In 1929, a log jam in the Colorado River was removed, allowing the river to flow directly into Matagorda Bay. Between 1929 and 1935, the river discharge built a large delta in the bay, decreasing the tidal area available to Pass Cavallo. The stability of the pass was further compromised by the construction of the Matagorda Ship Channel through Matagorda Peninsula in 1965. A majority of the tidal prism once belonging to Pass Cavallo has since been captured by the ship channel, decreasing the flow through the pass. In response to a decreased tidal flow, the cross sectional area is decreasing and shoaling is occurring in the pass. Since 1965, Pass Cavallo has continued to decrease in size with spits accreting on both sides of the pass. Historical maps and photographs, ranging from 1685 to present, have been collected to show the historical shoreline changes of Pass Cavallo. Geophysical studies were conducted in October of 2004 to study the geological evolution of the pass. A magnetometer and a single beam bathymetric echosounder were used to survey the pass and areas proximal to the pass. Several grab samples were collected in the pass and five vibracores were collected on the accreting spit of Matagorda Island to perform grain size analyses. In the future, CHIRP sub-bottom profiles and side scan surveys will be completed to further study the morphology of Pass Cavallo.