Horizontal Fine Structure of River Water Flow in the Volga Delta According to Satellite Data of High Spatial Resolution

From 1995 to the present, the level of the Caspian Sea decreased by almost 3 m, which significantly affects the morphometry and the ecological state of shallow water areas. First of all, this concerns the shallow Northern Caspian and, especially, the Volga Delta, which over the years has moved 10–20 km forward and grown by about 3000 km 2 in area. Changes in the morphometric characteristics of the Volga Delta inevitably affect the hydrodynamic, hydrophysical, hydrochemical, and hydrobiological characteristics of river water in the foredelta. From May 18 to 20, 2021, complex hydrophysical, hydrochemical, and hydrobiological studies of river water were carried out in the southwestern part of the Volga Delta. A total of 32 stations were completed, at which water surface temperature, electrical conductivity, turbidity, and chlorophyll a and oxygen concentrations were measured and samples were taken for further analysis in the laboratory. They showed a complex picture of the distribution of the studied parameters, both along the watercourses and across them. Only high spatial resolution optical and infrared Landsat-8 OLI/TIRS satellite images, obtained on May 20, 2021, almost simultaneously with the ongoing studies in the Volga Delta, helped the spatial distribution of these parameters to be understood. As a result of the analysis of the images, it was possible for the first time to identify a horizontal fine structure of watercourses, which is an alternation of clear immiscible narrow jets with significantly different characteristics of temperature, suspended matter, turbidity, and chlorophyll a .