RAPID: Collaborative Research: Tracking the Flood Pulse of a Record Discharge of the Brazos River in the Gulf of Mexico

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The transport and fate of sediment carried by rivers to the coastal ocean is of great ecological and societal importance. River input is the primary source of land-derived sediment to the marine environment. These particles have significant impacts on the health of coastal ecosystems and the geology and bathymetry of coastal oceans and shipping channels. They directly and indirectly affect fisheries and navigation. This research focuses on the movement of sediments carried into the Gulf of Mexico, via the Brazos River, during the unprecedented rainfall and flooding event that occurred during Hurricane Harvey. The Brazos River ranks as the second largest contributor of nutrients and organic matter to the Gulf of Mexico, after the Mississippi River. Unlike the Mississippi, however, most sediment delivered to the Gulf of Mexico from the Brazos is carried during flooding events like that which accompanied Hurricane Harvey. Little is known about the fate of this sediment. It is not clear if it is retained in the coastal zone, where the nutrients and chemicals it carries affect the coastal environment, or is carried offshore and deposited in the deep waters of the Gulf of Mexico. Through intensive sampling of sediments just offshore the mouth of the Brazos River and further offshore in the Gulf of Mexico in what is known as the Texas Mud Blanket, this project will determine the principal repository of sediments delivered to the western Gulf of Mexico by a major Texas river. The overall goal of this research is to investigate the hypothesis that flood-borne sediment from the Brazos River is initially deposited in the coastal zone and subsequently mobilized and carried offshore with a large fraction of it being deposited in the Texas Mud Blanket. The work builds on the analysis of sea-bottom sediment samples collected in 2017 during flooding associated with Hurricane Harvey. This new program will include two sampling cruises to augment already collected data. The first cruise will entail acquisition of ~25 cores that will indicate sediment deposition and transport in the near-shore region near the Brazos River mouth. Preliminary work indicates that much of the initial deposition of Brazos River sediment occurred east of the original sampling region. The second cruise in will collect additional Brazos region cores and expand sampling into the Texas Mud Blanket, with the goal of documenting changes in Brazos sediment deposition from the Hurricane Harvey flooding. During both cruises, complementary hydrography data will be collected. This includes water column velocities and suspended sediment loads. Provenance of individual layers, within the sediment samples and box cores, will be determined by digital photos and x-radiographs of intact core slabs that show sediment fabric and structures. To differentiate Gulf of Mexico from Brazos River sediments, sub-samples of collected cores will be geochemically analyzed and profiled for short-lived-radioisotope geochronology, water content, grainsize distribution, and mercury and carbonate content. These indicators will help identify the origin of the sediments deposited in the system.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.