Observations at the tidal plume front of a high-volume river outflow

We present shipboard observations of very strong convergence, vertical velocities and mixing, and nearbed impacts associated with the leading-edge front of the tidally-pulsed Columbia River plume. With upwelling-favorable winds and riverflow of 4900 m³s^-1, the plume propagates as a buoyant gravity current with a rotary, borelike vertical frontal circulation and downwelling as strong as 0.35 m s^-1. In waters as deep as 65 m, near-bed currents intensify to as much as 1.0 m s^-1 after frontal passage, and are often associated with elevated acoustic backscatter. Mixing is locally strong, with an eddy diffusivity of O(0.2 m²s^-1) 50 m behind the front, and T-S diagrams imply plume mixing with 10 m deep ocean water. These observations indicate that the leading-edge front of a surface-advected plume can cause exchanges of (a) nutrients between cold subsurface shelf waters and the river plume, and (b) nutrients and sediments across the sediment-water interface.