Temporal and spatial variability of vertical salt flux in a highly stratified estuary

The temporal and spatial variability of vertical salt flux in the Fraser River Estuary, British Columbia, was investigated observationally, using several different direct and indirect indicators of buoyancy flux. Data were collected from the estuary using shipboard instrumentation, primarily an acoustic Doppler current profiler and a towed conductivity-temperature-depth unit. Direct estimates of buoyancy flux were made from along-channel control volume analyses and from measurements of overturn scales. The spatial and temporal evolution of the salt wedge structure through a tidal cycle was evaluated using the results of these buoyancy flux calculations, as well as gradient Richardson number, Froude number, and stratification profiles. Observations indicate that buoyancy flux is driven primarily by interfacial stress, and not bottom stress, as is common in many estuaries. Vertical salt flux, as opposed to seaward advection of high-salinity fluid, was found to be the dominant mechanism responsible for removal of salt from the estuarine channel during each daily tidal cycle. Buoyancy flux was highly variable in time and space, however, with vertical salt flux during ebb tides on the order of 2 to 3 times greater than that estimated during floods. This is due to an increase in the vertical shear of horizontal velocity and a sharp increase in stratification, which was observed during early ebb. Enhanced mixing was observed spatially within a region dominated by a channel constriction in which the channel narrows by approximately 25%.