Hyperpycnal Flow within Low Gradient River Deltas and Implications for Both Sand and Mud Transport Onto the Shelf: Brazos River

Sediment transport from river mouths via hyperpycnal flow is a well documented process for high gradient rivers worldwide. However, many important hydrocarbon bearing deltaic systems were derived from lower gradient systems where hyperpycnal flow is not normally attributed. The Brazos River provides a modern example of such a low gradient river. We had the unique opportunity to sample the mouth and proximal shelf of the Brazos River during the flooding of July 2007. Using a CTD equipped with a turbidity sensor, water column profiles were taken on an along shelf transect from the river mouth 10km northeast in the direction of the plume transport and an across shelf transect from the river mouth 8km offshore to the seaward edge of the plume. In addition, bottom water samples and shallow gravity cores were collected to determine the thickness of the flood deposit, suspended sediment concentration, porewater salinity and grain size distribution. We found both a high turbidity hypopycnal plume as well as a high turbidity bottom layer, with low turbidity in the middle of the water column. Brazos River mud is characteristically red, while marine sediment is olive-grey. Preliminary result reveal a distinctively red, porous storm layer, composed of up to 30% sand that extended 5 km from the river mouth. The presence of sand within the storm layer and the high turbidity bottom layer suggest hyperpycnal flow existed during the flood. If this is the case, hyperpycnal flow may be a regular occurrence on the Brazos River and may be more common within low gradient river systems than previously believed, providing an additional mechanism for transporting both sand and mud across the inner shelf.