1. Introduction

River discharges inflow to sea and form buoyant river plumes at coastal areas in many world regions. The total surface area and volume of river plumes are relatively small as compared to the saline ambient sea. However, river plumes govern land-ocean fluxes of fluvial water, sediments, nutrients, and pollutants and, thus, significantly influence many physical, biological, and geochemical processes on the continental shelf [1–6]. Structure, dynamics, and variability of river plumes are key factors for understanding mechanisms of advection, convection, transformation, accumulation, and dissipation of fluvial discharge as well as suspended and dissolved river-borne constituents in the coastal sea [7–9].

Two groups of factors govern the processes of formation, spreading, and mixing of river plumes. Immanent characteristics of local landscapes, namely shoreline and sea bottom features, morphology of river mouths, and latitude, which define the local magnitude of the Coriolis force, define the first group of factors [10–13]. The second group consists of variable external forcing conditions, which include river discharge, local wind, coastal circulation, tides, waves, and stratification of the ambient ocean [14–21]. The structure and dynamics of a river plume also strongly depend on its spatial scale. Sizes of river plumes vary from meters to hundreds of kilometers due to large ranges of freshwater discharge rate among world river

systems. Also, spatial scales of many river plumes have large variability within a year caused by seasonal changes in river discharge rates. It results in diverse patterns of formation, spreading, and mixing of a river plume on intra-annual time scale [22–25].

General aspects of the structure and dynamics of river plumes as well as their regional features were addressed in many previous studies. Nevertheless, these works were mostly focused on large river plumes, while small rivers plumes received relatively little attention. This is presumably caused by small influence of individual small plumes on coastal sea as compared to large plumes. Also, most of the world's small rivers are not covered by regular hydrological and discharge measurements, which result in a lack of information about their runoff volume and variability [26, 27].

The total share of small rivers in the influxes of fluvial water and suspended sediments to the world ocean is estimated at about 25 and 40%, respectively [28, 29]. Furthermore, this contribution is much more significant on a regional scale for many coastal regions. Under certain terrain and climatic conditions, the cumulative discharge from small rivers can greatly increase in response to heavy rains and become comparable to or even exceed the runoff of large rivers [30–33]. Flash floods at small rivers caused by active precipitation events can strongly influence the land-ocean buoyancy fluxes, heat, terrigenous sediments, nutrients, and anthropogenic pollutants. Many studies showed that they can affect coastal dynamics of certain world regions [13, 28, 34–37].

In this chapter, we focus on specific features of structure and dynamics of small river plumes, which are not typical for large plumes. In Section 2, we address spatial structure and temporal variability of small river plumes and analyze general aspects of difference between small and large plumes. In Section 3, we describe the Lagrangian numerical model that was specifically designed for simulation of spreading and mixing of small river plumes and the associated transport of riverborne suspended matter. Section 4 provides description and analysis of several important dynamical features of small plumes including the response of small plumes to wind forcing and river discharge variability (Section 4.1), the interaction between neighboring small plumes (Section 4.2), and the generation of highfrequency internal waves in small plumes by river discharge (Section 4.3). The conclusions are given in Section 5.
