SALT WEDGE CONTROLLED SEDIMENT DYNAMICS OF THE BRAZOS RIVER, TX: STORAGE IN THE LOWER RIVER, TRANSPORT TO THE SHELF

The Brazos River is being used to investigate the role salt wedge dynamics plays in the initial placement of flood deposits during various flood stages. Stations have been established every 0.5 km from the mouth of the river and extending 15 km upstream and also every 0.5 km from the mouth of the river 7 km across the shelf. A time series has been conducted sporadically for each of these stations where profiles of water column salinity, temperature and turbidity for each sampling event. Additionally, surface sediment grab samples and/or 30 cm long gravity cores were collected at each station along with surface and bottom water samples. For select sampling events, ultra high resolution swath bathymetry was conducted to detect changes in bathymetry. During low flow conditions (~50-150 m³s^-1), a well stratified salt wedge extends 1-8 km up river from the river mouth and the suspended sediment is trapped upstream of the salt wedge as ephemeral layers decimeters thick. During a moderate river flow survey of 340 m³s^-1, the salt wedge was pushed to the seaward side of the river mouth bar and the suspended sediment was trapped near the seabed, across the mouth bar landward of the salt wedge. During the one high discharge event (2040 m³s^-1) sampled (July 12, 2007) 10 days after peak discharge (2190 m³s^-1), the shelf waters seaward of the seabed intersection of salt wedge (null point) were well stratified, with a highly turbid fresh water hypopycnal plume and a highly turbid bottom boundary layer both extending ~5 km offshore. The highly turbid bottom layer appears to be a wave supported boundary layer of recently deposited flood sediments, deposited when the salt wedge was displaced further offshore during peak discharge. Results to-date suggests that the salt wedge provides a highly effective trap of suspended sediment. The initial placement offshore flood deposition of the Brazos River’s suspended load is controlled by the offshore displacement of the salt wedge