THE FRESH-WATER-SALTWATER INTERFACE AND ITS RELATIONSHIP TO THE TURBIDITY MAXIMUM IN THE TAMAR ESTUARY, UNITED-KINGDOM

Data are presented from several experiments in the freshwater-saltwate r interface (FSI) region of the Tamar Estuary. Longitudinal surveys of salinity and suspended particulate matter (SPM) at high water showed that the location of the FSI could be predicted in terms of a power-la w regression with freshwater runoff. Longitudinal transects also were surveyed over periods of several hours. The FSI was observed to advect into the region on the flood with strong vertical mixing. After high water, stratification became intense as fresher water ebbed in the sur face layers. The near-bed water in the stratified region began to ebb between 2 h and 3 h before low water. A model of the vertical structur e of longitudinal currents showed that the enhanced stratification on the ebb, coupled with the longitudinal density gradient, partly produc ed this long period of slack, near-bed currents following high water. A strong turbidity maximum (TM) occurred during spring tides and was l ocated slightly up-estuary of the FSI at high water. Longitudinal tran sects during a period of low freshwater runoff and large neap tide sho wed that at the start of the flood the TM was associated with the FSI region. As the FSI advected up-estuary on the flood there was consider able resuspension of sediment at the FSI. Some of this SPM moved with the FSI and reached the limit of saline intrusion, where it formed a s lowly-eroding TM as particles settled during the long, high-water slac k period. As the near-bed currents increased on the ebb and the FSI mo ved down-estuary, strong vertical mixing and resuspension of recently deposited sediment occurred in the unstratified water behind the FSI a nd the associated TM advected down-estuary. Additional effects were pr esent with stronger tides and increased runoff.