The shores of the Bratsk reservoir are widely affected by bank gullies. We monitored the short- and medium-term gully dynamics at three key sites. The interaction of karst-erosional, aeolian-erosional, and gully and shore erosion processes were determined and presented as conceptual models. Gully and shore erosion were the dominating processes in the Rassvet site in the period of heavy rainfall and the maximum shore edge retreat at high water level stand (2009–2010). During 2011–2013 aeolian process prevailed over erosion due to small rainfall and low water-level stand, and aeolian material accumulated in the gullies. A high water-level period at the Ulei site resulted in shore edge retreat, hanging of gully mouths, formation of wave-cut holes in the shore scarp, and rejuvenation of retrogressive erosion in gully thalwegs. The combined action of karst and gully erosion processes led to a maximum increment in gully size (4486.34 m³), recorded at the Khadakhan site in 2004–2008 (at periods of reservoir water-level lowering following water saturation of shore massif at high water levels). This case study shows that the dynamics of the processes, their interactions, and environmental changes are characterized by a certain cyclicity corresponding to water-level changes.

The study area is located in the Ulaanbaatar basin and is characterized by complicated environmental conditions determined by the peculiarities of an extremely continental climate with high rainfall intensity, a structural–tectonic pattern, the geological structure of the Ulaanbaatar intermountain depression, high seismic activity reaching 8 points, and the occurrence of catastrophic flash floods. In the conditions of rapid urban expansion, the high population density and housing development of Ulaanbaatar, and the implementation of large investment projects, it is an urgent task to assess the exogeodynamic hazard of the Ulaanbaatar catchment basin and develop the concept of geoecological safety of the territory. These methods and approaches consist of combining a complex of methods of engineering geology and geomorphology: monitoring the dynamics of exogenous processes; environmental assessment; the analysis of development factors (climatic, geomorphologic, tectonic, and lithological) and interaction of processes; laboratory methods of analyzing the composition, structure, and properties of soils; a detailed morphometric analysis of catchment areas; and the application of the basin approach. Based on the analysis of topographic maps on a scale of 1 : 50 000 and of digital relief models in GIS programs, the parameters of the drainage network of the catchment areas and of energy and basin geometry were calculated and reflected on map layers, and specialized maps were compiled. A detailed assessment of erosion-prone soils (composition, structure, and specific properties) revealed a moderate resistance of sediments to geodynamic impacts. As a result, maps of flood hazard of catchments under different rainfalls—short intensive heavy rainfalls and long-term rains—were compiled. The elaborated concept of geoecological safety of territories includes successive stages of works aimed at (a) identifying the exogeodynamic hazard, (b) the organization and implementation of monitoring works, (c) assessing geoecological safety with respect to hazardous exogenous geological processes, and (d) the development of recommendations for further use and specification of information.