AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Sediment Dynamics of the Brazos River, Texas: Storage in the Lower River, Transport to the Shelf
(1) Marine Science, Texas A&M University at Galveston, Galveston, TX.
(2) Oceanography, Texas A&M University, Galveston, TX.
The Brazos River, located along the northwestern Gulf of Mexico, is the 11th longest in the US. Starting in May 2010, a monthly series of research cruises along the lower 11 km of the Brazos River and across the proximal shelf has revealed that during low flow conditions (~50-150 m3s-1), a well stratified salt wedge extends 1-8 km up river from the river mouth. Water column profiles of suspended sediment concentration, salinity and temperature collected during each of these cruises reveal that the suspended load of the river is trapped up stream of the salt wedge, with very little suspended sediment escaping to the shelf, revealing a classic estuarine turbidity maximum. On October 13, 2007, a cruise was conducted collecting profiles within the lower river and extending across the mouth bar and out onto the shelf. The river flow was 340 m3s-1, the salt wedge was pushed to the seaward side of the river mouth bar and the bulk of the suspended sediment was trapped near the seabed, across the mouth bar. In July of 2007 the Brazos River experienced a major flood, with river discharge in excess of 1500 m3s-1. During this flood, 10 days after peak discharge, a sampling cruise was conducted only on the shelf, and it was revealed that a well established buoyant surface (hypopycnal) plume of suspended sediment was present, as well as a modest mid water column (homopycnal) plume and there was also a turbid layer along the seabed, extending 5 km offshore of the river mouth. We believe that the presence of this turbid bottom layer suggests that during peak discharge, the salt wedge extended as far as 5 km offshore and that the turbid layer observed in the survey was a wave boundary layer resulting from the suspension of the turbid layer created by the rough sea conditions existing during the survey. The preliminary results of this study suggest that most of the suspended sediment load of the river is trapped upstream of the salt wedge, that the only time this load is delivered to the shelf is when the wedge is pushed offshore of the mouth bar, out into the Gulf of Mexico and initial deposition of this load primarily occurs landward of the salt wedge.