Conceptual Model for Development of Karst–Erosion Processes in Large Dam Reservoir Coastal Geosystem: Bratsk Reservoir, Baikal-Angara Hydroengineering System, Russia

The sulphate–carbonate karst in the southern part of the Bratsk Reservoir has been active throughout the reservoir’s operation. Long-term monitoring of the coastal zone, interpretation of multi-temporal images, and field studies at the Khadakhan key site resulted in the creation of a conceptual model of coastal geosystem functioning in areas of sulphate–carbonate rock development under conditions of long-term and seasonal fluctuations in the reservoir water level. The structure of interactions within the coastal geosystem is organized at three hierarchical levels: (1) the intra-rock level, (2) the level of interacting factors, and (3) the level of interacting exogenous geological processes, whose activation is driven by an external factor—changes in the reservoir’s water level. We identified five stages of gully formation and the cyclic nature of the karst–erosion process in the coastal geosystem under conditions of seasonal and long-term reservoir water-level fluctuations. Our findings indicate that, when regulating reservoir water levels, dramatic drawdowns should be avoided. This conceptual model aims to improve the understanding of the impact of large reservoir operation on the dynamics of a complex of interacting coastal processes, as well as on the peculiarities of karst development in a boreal climate.