River Plumes and Estuaries
The Columbia River brings buoyant fresh water to the Pacific Ocean, along with dissolved silicate and iron, crucial to phytoplankton growth. The plume increases near-surface stratification in coastal waters so may trap momentum, heat and biomass, and may inhibit turbulent exchanges with nutrient-rich subsurface waters.
Turbulence and acoustics measurements made during the River Influences in Shelf Ecosystems (RISE) experiment elucidate small-scale processes within the river-plume-ocean system, including:
- turbulence in the estuary and nearfield plume
- plume-driven bottom stress
- frontal dynamics and nonlinear internal wave generation
- mixing in the farfield plume
Columbia River Estuary
Tidally-resolving timeseries were aquired within the estuary during low (Aug 2005) and high (May 2006) river flows. While the salinity and stratification were markedly different, turbulence intensity within the estuary is dominated by the tidal dynamics and similar during both field seasons.
Contrasting the river estuary, the plume nearfield depends strongly on the near surface stratification. Under low river flows, turbulence extends from surface to bottom several kilometers from the river mouth. Under high flow conditions, the plume lift-off moves into the river mouth and the nearfield is more strongly stratified. Turbulence under high river flow conditions is trapped to the surface, having important consequences to sediment transport.
During the ebb of each tidal cycle, the plume front propagates as a gravity current with an extremely sharp (0.5 m/s changes in 10 m) front. The front is highly nonhydrostatic and grows in time. As its Froude number approaches 1 (i.e., it's propagation speed matches the internal wave speed), it may release large amplitude internal waves. Such waves are ubiquitous in coastal waters, where they displacing fluid >30-m downward and may be unstable and generate turbulence.
For more information, see
Nash, J.D. and J.N. Moum, 2005: River plumes as a source of large-amplitude internal waves in the coastal ocean, Nature 437, 400-403 (15 September 2005) | doi: 10.1038/nature03936, (500 kB pdf).
In the plume farfield, turbulence is similar to that observed elsewhere on the coast, with K~10-5 - 10-4 m2/s.