Features and Factors of Hydrological and Morphological Changes in the Agrakhan Bay at the Mouth of the Terek River in the 20th–early 21st Centuries

The Agrakhan Bay of the Caspian Sea is a unique hydrographic and environmentally valuable object at the mouth of the Terek River. At the beginning of the 20th century, it was a typical sea bay; however, by the 21st century, it lost the features of a marine area and a single water body and was divided into two parts with their specific hydrographic network, water regime, landscapes, biota, and anthropogenic load. Rehabilitation of the wetlands of the former sea bay requires urgent and scientifically grounded measures, as well as good knowledge of the causes and features of its rapid degradation and data on its current hydrological-morphological and hydroecological state. In 2018–2020, the authors carried out a series of field reconnaissance, geodetic, hydrometric, botanical, and hydrochemical works in this part of the mouth, created a network of stations, took numerous water samples and bottom soil samples, and analyzed them in laboratories. A vast array of data from Roshydromet (Russian Hydrometeorological Service) stations, as well as multi-temporal cartographic materials and satellite images, have been accumulated and studied; a multilayer GIS and various maps, empirical dependencies, diagrams, and graphs have been constructed; the results of studies by outside specialists have been considered. This study has made it possible to comprehensively describe the current state of the former Agrakhan Bay, build a depth map, a landscape map, a map of bottom sediments, a map of macrophytes, and a map of hydrochemical parameters, describe the water regime and water balance of the main water bodies of the former bay, determine the relationships between the main hydrological and morphological characteristics, etc. Patterns, causes, long-term stages, and parameters of the unfavorable morphodynamics of the Agrakhan Bay in the 20th century and at the very beginning of the 21st century have been studied. This has made it possible to justify measures for partial restoration of the hydrological and ecological potential of the water bodies of the former Agrakhan Bay.