River plume

A river plume is a freshened water mass that is formed in the sea as a result of mixing of river discharge and saline seawater. River plumes are formed in coastal sea areas at many regions in the World. River plumes generally occupy wide-but-shallow sea surface layers bounded by sharp density gradients. The area of a river plume is 3-5 orders of magnitude greater than its depth; therefore, even small rivers with discharge rates ~1–10 m/s form river plumes with horizontal spatial extents ~10–100 m. Areas of river plumes formed by the largest rivers are ~100–1000 km2. Despite the relatively small volume of total freshwater runoff to the World Ocean, river plumes occupy up to 21% of shelf areas of the ocean, i.e., several million square kilometers. In some occasions river plumes are spoken of as regions of fresh water influence (ROFIs), although it is preferred to use this term for regions in which multiple sources add to the fresh water input of the zone or for shallow, frictional shelves. ROFIs and river plumes differ in the variation at temporal and spatial scales. The river plume can be identified as a buoyant water mass that emerges due to river discharge into the coastal ocean and varies over diurnal to synoptic timescales. At the edges of this water mass mixing takes place, creating a region adjacent to the river plume which is diluted and fresher compared to the open ocean, but does not have a clear boundary. This extended region is called the region of freshwater influence, ROFI. Due to the indirect influence of freshwater discharge, ROFIs incorporate the dynamics and spatial extent of the river plumes but are typically assessed on seasonal, annual, and decadal timescales.

Processes River plumes play an important role in global and regional land-ocean interactions. River discharges provide large fluxes of buoyancy, heat, terrigenous sediments, nutrients, and anthropogenic pollutants to the ocean. River plumes strongly influence many physical, biological, and geochemical processes in the coastal and shelf sea areas including stratification of seawater, coastal currents, carbon and biogeochemical cycles, primary production, and seabed morphology. A river plume is a dynamical system influenced by processes with a wide range of temporal and spatial scales, which depend on the size and shape of the estuary as well as on the type and variation of the forcing from the estuary and the ocean. Feedback mechanisms between sediment deposited by the plume at the submarine delta and the geometry of the delta make for a complex system. Due to this complexity there is not (yet) a general, simple theory that offers quantitative predictability for the motion of particles and the structure of river plumes; however, some theories incorporating simplified assumptions have helped in understanding the important aspects of buoyancy-influenced coastal flows. As is commonly used in fluid dynamics, the description of these complex flows is aided by scaling analysis to determine the relevant processes. The primary parameters which define the structure and scale of an individual river plume are freshwater discharge, tidal energy, coastline bathymetry/geometry, ambient ocean currents, wind, and the rotation of the Earth.

Structure The balance between the important processes varies over the position in the plume. The following regions can be distinguished: the source region, the liftoff point, the front, and the near field region. Beyond the plume itself but within its area of influence are the mid-field region and the far field region.

Source region In the source or estuarine region, the buoyancy and momentum of the freshwater inflow from the estuary are the dominant properties that determine the initiation of the river plume. The competition between river-induced stratification and tidal mixing sets the river plume's characteristic properties. This competition can be captured in the (dimensionless) estuarine Richardson number, which is defined as

R i E = g r ′ Q r W E u t i d a l 3 , {\displaystyle Ri_{E}=g_{r}'{\frac {Q_{r}}{W_{E}u_{tidal}^{3}}},}

where

the reduced gravity g r ′ = g ( Δ ρ / ρ 0 ) {\displaystyle g'_{r}=g(\Delta \rho /\rho _{0})} is the gravitational acceleration due to the density difference between fresh river water and saline ocean water,

Q r {\displaystyle Q_{r}} is the river discharge,

W E {\displaystyle W_{E}} is the estuary width, and

u t i d a l {\displaystyle u_{tidal}} is the tidal velocity. where

g r ′ = g ( Δ ρ / ρ 0 ) {\displaystyle g'_{r}=g(\Delta \rho /\rho _{0})} is the gravitational acceleration due to the density difference between fresh river water and saline ocean water,

Q r {\displaystyle Q_{r}} is the river discharge,

W E {\displaystyle W_{E}} is the estuary width, and

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