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.^[1] 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.^[2]

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.^[1] 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.^[3] 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.^[3] 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.^[3]