Here there is a very strong halocline between the upper water and deep water, particularly at the head where vertical salinity gradients of 10 to 20 %o per meter may occur in summer during the peroid of greatest river runoff. There is a vertical mixing but it results predominantly in an upward movement of salt water from below.(4) Salt Wedge (TYPE D)Here the saline water intrudes from the sea as a wedge below the river water. This situation is typical of rivers of large volume transport. The salt wedge estuary has features in common with the stratified estuaries. There is a horizontal gradient of salinity at the bottom as in a slightly stratified estuaryand a pronounced vertical salinity gradient as in a highly stratified estuary. The distinction is in the lack of saline water at the surface until it reasches the sea.Estuarince CirculationA feature of the stratified estuaries is that depth of the halocline (the thickness of the upper, low salinity layer) remains substantially constant from head to mouth of an estuary for a given runoff. If the estuary width does not change much, the constancy of depth of the upper layer means that the corss-sectional area of the upper layer outflow remains the same while its volume transport increases because of the entrainment of salt water from below. In consequence the speed of the ouflowing surface layer increases markedly along the estuary from head to mouth. The increase in volume and speed can be very considerable, the outflow at the mouth being as much as 10 to 20 times the volume flow of the river....
