4. Dynamics of small river plumes

In this section, we address dynamical features of small river plumes using the case study of the Mzymta plume and other small river plumes formed along the Russian coast of the Black Sea (RCBS) between the city of Novorossiysk and the city of Sochi (Figure 2). The drainage basin of RCBS is a narrow area (10–40 km wide) limited by the Greater Caucasus range at the east and the sea coast at the west. Steep gorges of this range form the drainage basins of several dozens of rivers that discharge to the sea at RCBS. Watershed basin areas of these rivers are relatively small, and the total freshwater runoff from the study region to the sea is estimated as 7 km<sup>3</sup> in a year [52].

Multiple buoyant plumes are formed along the coast of the study area. The largest plume is generally formed by the Mzymta River, which is the largest river of RCBS with mean monthly discharge equal to 20–120 m<sup>3</sup> /s. The area of the Mzymta does not exceed 10 km<sup>2</sup> under average climatic discharge conditions. However, it can increase up to 50 km<sup>2</sup> during spring and summer freshet periods. Areas of the other river plumes of RCBS are even smaller except for rain-induced flash flooding

#### Figure 2.

Location of the largest rivers and cities of the Russian coast of the Black Sea.

periods. The rivers of the study region are significantly more turbid, as compared to sea water, due to elevated concentrations of terrigenous suspended sediments. As a result, surface salinity and turbidity show good correlation within the river plumes. Thus, optical satellite imagery can be effectively used to detect river plumes of RCBS [45, 46]. Both in situ and satellite measurements revealed high spatial and temporal variability of the river plumes of RCBS. Their areas, positions, and shapes can significantly change during several hours in response to variations of river discharge and local wind forcing [42, 43, 45].

### 4.1 Wind forcing and small river plumes

We used near-simultaneous ocean-color satellite imagery from NASA's Landsat 8 and ESA's Sentinel-2 missions to reconstruct surface currents along RCBS and study spreading dynamics of small river plumes formed in this area [53]. Several times a year, Landsat 8 and Sentinel-2 satellites both pass over the study area within a time interval of less than 10 minutes. The obtained near-simultaneous ocean-color composites can be used to reconstruct surface sea currents. In particular, they can be applied for detection of motion of frontal zones of river plumes, which are visible in optical satellite images. We used an optical flow algorithm applied to nearsimultaneous Landsat 8 and Sentinel-2 images to reconstruct surface currents within the Mzymta plume. The obtained results reveal significant differences in wind-driven dynamics of the Mzymta plume and large plumes [12, 16, 54–58].

The main features of the dynamics of the Mzymta plume reconstructed from the satellite imagery are the following. First, the near-field part of the Mzymta plume is smaller than it is estimated by relevant parameterizations based on river discharge parameters designed for large plumes. Second, under low-wind-forcing conditions, the mid-field plume, i.e., a recirculating bulge adjacent to the river mouth, is not formed. The near-field freshwater jet directly transitions to the far-field part of the plume near the Mzymta mouth. Finally, during upwelling, onshore, and offshore wind-forcing periods, the wind-induced Ekman transport within the Mzymta plume occurs at a wide range of angles to the wind direction. It changes from values of 60–80° near the Mzymta mouth to 30–40° at the far-field part of the plume.
