Structure and Dynamics of Plumes Generated by Small Rivers DOI: http://dx.doi.org/10.5772/intechopen.87843

a river plume, but also influences physical, biological, and geochemical characteristics of ambient sea. Strength and extent of this influence mainly depend on volume of freshwater discharge and varies from negligible impact of small plumes formed by rivers with low discharge rates on coastal sea [20, 24, 38] to formation of stable freshened water masses in the upper sea layer on wide coastal and shelf areas [11, 39–41]. The latter water masses, commonly referred as regions of freshwater influence (ROFI), are characterized by more homogenous structure, significantly as greater spatial scales and lower temporal variability, as compared to river plumes.

We regard river plumes as water masses formed as a result of transformation of freshwater discharge in coastal sea on diurnal to synoptic time scales, while ROFI reproduce transformation of freshwater discharge on seasonal to annual time scales. River plume embedded into ROFI reproduces a continuous process of transformation of freshwater discharge and, therefore, cannot be distinctly distinguished. On the other hand, river plumes and ROFI have strongly different thermohaline characteristics and dynamics. Therefore, interaction between river plumes and surrounding ROFI significantly influences spreading dynamics and mixing of river plumes on synoptic time scale [5, 25, 31–33].

In this chapter, we focus on small river plumes; therefore, we need to determine characteristic properties of small plumes to distinguish them from large plumes. We define small plumes as plumes that do not form ROFI; i.e., residence time of freshened water in a small river plume is equal to hours and days. Dissipation of freshened water as a result of mixing of a small plume with subjacent saline sea limitedly influence ambient sea and does not result in accumulation of freshwater in adjacent sea area. As a result, small plumes are characterized by sharp salinity and, therefore, density gradient at their boundaries with ambient sea. This density gradient hinders vertical energy transfer between a small plume and subjacent sea.

This feature strongly affects spreading dynamics of a small plume due to following reasons. First, the majority of wind energy transferred to sea remains in a small plume, because vertical momentum flux diminishes at density gradient between a plume and subjacent sea. Thus, wind stress is concentrated in a shallow freshened surface layer that causes higher motion velocity and more quick response of dynamics of a small plume to variability of wind forcing, as compared to ambient sea [42, 43]. Second, circulation of adjacent sea limitedly affects spreading dynamics of a small plume, because density gradient hinders upward momentum flux from subjacent sea to a small plume [44]. It results in wind-driven dynamics of small plumes, which is characterized by very energetic temporal variability of their positions, shapes, and areas [45–47]. Spreading pattern of a small plume can dramatically change during several hours that is regularly registered by in situ and satellite data. High temporal variability of small plumes and their small vertical sizes often result in large inhomogeneity of their horizontal structure.
