Satellite images have long revealed the surface expression of large amplitude internal waves that propagate along density interfaces beneath the sea surface^{1, 2, 3}. Internal waves are typically the most energetic high-frequency events in the coastal ocean^{4, 5, 6}, displacing water parcels by up to 100 m and generating strong currents and turbulence⁷ that mix nutrients into near-surface waters for biological utilization. While internal waves are known to be generated by tidal currents over ocean-bottom topography^{8, 9, 10, 11, 12, 13}, they have also been observed frequently in the absence of any apparent tide–topography interactions^{1, 7, 14}. Here we present repeated measurements of velocity, density and acoustic backscatter across the Columbia River plume front. These show how internal waves can be generated from a river plume that flows as a gravity current into the coastal ocean. We find that the convergence of horizontal velocities at the plume front causes frontal growth and subsequent displacement downward of near-surface waters. Individual freely propagating waves are released from the river plume front when the front's propagation speed decreases below the wave speed in the water ahead of it. This mechanism generates internal waves of similar amplitude and steepness as internal waves from tide–topography interactions observed elsewhere¹¹, and is therefore important to the understanding of coastal ocean mixing.

1. College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin Bldg, Oregon State University, Corvallis, Oregon 97331, USA

Correspondence to: Jonathan D. Nash¹ Correspondence and requests for materials should be addressed to J.D.N. (Email: nash@coas.oregonstate.edu).

Received 10 March 2005 | Accepted 13 June 2005

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