Structure and composition of a strongly stratified, tidally pulsed river plume

Abstract

The initial composition of a river plume depends on the cumulative turbulent entrainment within the estuary and how this dilutes the supplied freshwater. Here we examine the relative roles of turbulence and freshwater input using observations from the Columbia River estuary and plume during two periods with contrasting river flow. Within the estuary, intense turbulence observed on flood and ebb stages is controlled by the bottom stress and scales with tidally dominated near‐bottom velocity as utidal3. Shear associated with the estuarine circulation is found to have a much weaker influence on turbulence dissipation rates. On the basis of these observations, we suggest that properties of the Columbia River tidal plume should be controlled by the ratio of horizontal advection to turbulent mixing within the estuary. This ratio depends on the magnitude of freshwater river input (characterized by its volumetric flow rate Qf) as compared to turbulent fluxes due to tidal mixing. This is summarized in terms of the estuary Richardson number RiE, a nondimensional ratio between Qf and utidal3. From 17 tidally resolving offshore surveys during spring/neap tides and low/high river flows, we find that the plume's median salinity, thickness, and turbulent mixing are each predicted through RiE. It is hoped that these simple formulations will provide guidance in assessing critical properties of river plumes and their influence on coastal circulation.