Spatiotemporal Plume Variability of Terek and Sulak Rivers from Satellite Data and Concurrent In Situ Measurements

The article is devoted to the study of spatiotemporal variability of the plumes of the Terek and Sulak rivers flowing into the Caspian Sea and to the determination of the three-dimensional structure of the plumes based on a joint analysis of satellite data and concurrent in situ measurements. In the study, high spatial resolution visible data from Sentinel‑2 MSI and Landsat-8, -9 OLI/OLI-2 satellite sensors were used. In situ measurements of the main plume parameters were made in June 2023, from a small vessel, concurrently with the satellite survey. As a result of satellite monitoring of the Terek and Sulak estuarine zones for the period 2014–2022, it was determined that the 73-km-long coastline of Dagestan north of the city of Makhachkala is potentially exposed to pollution entering the sea with waters of these rivers. An analysis of the satellite images showed that there is a pronounced spatial asymmetry in the direction of plume spreading. For both rivers, the dominant spreading direction over a large time interval is southeast. A seasonal variability of Terek and Sulak plumes signatures in satellite images was revealed. Based on satellite data and concurrent in situ measurements in June 2023, the three-dimensional structure of the Terek and Sulak plumes was obtained and the spatial distributions of the main parameters, such as temperature, salinity, turbidity and chlorophyll-a concentration, were determined. It was found that the depth of river water penetration does not exceed 1.5–2 m and, regardless of the amount of suspended matter carried with river runoff and river discharge, the bulk of the suspended matter is deposited in the same localized area, within a distance of about 500 m from the shore. On the basis of XRD analysis of water samples taken at the stations nearest to the river mouths and at plume boundaries, the mineral composition of suspended matter and its variation in different plume areas were determined. Repeated coincidence of the boundaries of the area affected by river water, established from satellite data and measured in situ, makes it possible to confidently use remote sensing methods for monitoring estuarine zones without involving in situ data.