The role of turbulence stress divergence in decelerating a river plume

[1] Turbulence controls the composition of river plumes through mixing and alters the plume’s trajectory by diffusing its momentum. While believed to play a crucial role in decelerating river-source waters, the turbulence stress in a near-field river plume has not previously been observationally quantified. In this study, finely resolved density, velocity, and turbulence observations are combined with a control-volume technique to describe the momentum balance in the Columbia River’s near-field plume during 10 tidal cycles that encompass both large and small river flow. Turbulence stress varies by 2–3 orders of magnitude, both within a given ebb and between ebbs with different tidal or river forcing; its magnitude scales with the strength of the instantaneous ebb outflow, i.e., high stresses occur during peak flow of strong ebbs. During these periods, the momentum equation is represented by a balance between stress divergence and plume deceleration. As the flow relaxes, the stress divergence weakens and other terms (pressure gradient and Coriolis) may become appreciable and influence plume deceleration. While the momentum balance could not be closed during these weaker flow periods, during strong tidal pulses the time scale for decay based on observed stress is significantly less than a tidal half-period, indicating that stress divergence plays a fundamental role in the initial deceleration of the plume.