The transport of fresh water from river to coastal zone through a temperate estuary

This paper analyses the transport of fresh water from rivers to coastal zone through the Tamar Estuary, a temperate, partially mixed estuary in Southwest England. Anchor stations were worked for 28 separate tidal cycles. Surface-to-bed salinity difference (stratification) and velocity difference (current layering) were generally significantly greater during the ebb portion of the tide than during the flood in the upper estuarine reaches. In the deeper, lower reaches of the estuary, salinity stratification reached maximum values at, or soon after, low water. Current layering was generally more symmetric between ebb and flood portions of the cycle, although maximum layering still occurred during the ebb. Maximum salinity stratification tended to be smaller at stations located farther down-estuary, to decrease with greater tides and to increase with greater runoff (the most important factor). Maximum current layering increased with both increasing tidal range and runoff, but did not vary significantly with location. In the upper reaches of the estuary the instantaneous, vertical shear flux of fresh water was directed down-estuary during the ebb and reached its maximum transport on the early to mid ebb tide. The vertical shear flux tended to be very small during the flood. In the lower reaches, the instantaneous vertical shear flux of fresh water was again down-estuary during the ebb, but reached its maximum transport at (or near) low water, rather than on the early to mid ebb. At both spring and neap tides, the tidally averaged, advective flux of fresh water in the upper estuary accounted for most of the seaward transport of fresh water there. Generally, tidally averaged, ebb-directed, vertical shear flux increased down-estuary. The vertical shear flux at neap tides exceeded that at spring tides in the lower reaches. Strong tidal pumping of fresh water occurred both in the central reaches and, generally, also in the lower reaches at spring tides.