Observations at the mouth of the Fraser River (British Columbia, Canada) indicate an abrupt frontal transition between unstratified river outflow and a highly stratified river plume with differences in salinity greater than 25 psu across a few meters in the vertical direction and several hundred meters in the horizontal direction. The front roughly follows a natural break in the bathymetry, crossing the channel at an angle of approximately 45°, and is essentially stationary for a period of approximately 3.5 h centered on the low tide following the larger of two daily ebbs. The location of the front is coincident with observations of significantly supercritical internal Froude numbers at the front, based on velocities in the along-flow direction. This observation contradicts the one-dimensional theory, which indicates that the Froude number should be 1. However, because the front is oriented obliquely to the outflow, a coordinate system can be selected that is normal to the front and for which a critical Froude number of 1 is obtained. This indicates that a Froude angle, similar in application to a Mach angle for transonic flows, can be used to determine critical conditions when the front is oblique to the principal flow direction.

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%.

Off‐river coves and embayments provide accommodation space for sediment accumulation, particularly for sandy estuaries where high energy in the main channel prevents significant long‐term storage of fine‐grained material. Seasonal sediment inputs to Hamburg Cove in the Connecticut River estuary (USA) were monitored to understand the timing and mechanisms for sediment storage there. Unlike in freshwater tidal coves, sediment was primarily trapped here during periods of low discharge, when the salinity intrusion extended upriver to the cove entrance. During periods of low discharge and high sediment accumulation, deposited sediment displayed geochemical signatures consistent with a marine source. Numerical simulations reveal that low discharge conditions provide several important characteristics that maximize sediment trapping. First, these conditions allow the estuarine turbidity maximum (ETM) to be located in the vicinity of the cove entrance, which increases sediment concentrations during flood tide. Second, the saltier water in the main channel can enter the cove as a density current, enhancing near‐bed velocities and resuspending sediment, providing an efficient delivery mechanism. Finally, higher salinity water accumulates in the deep basin of the cove, creating a stratified region that becomes decoupled from ebb currents, promoting retention of sediment in the cove. This process of estuarine‐enhanced sediment accumulation in off‐river coves will likely extend upriver during future sea level rise.