The Selenga River is the main artery feeding Lake Baikal. It has a catchment of ~450000 km² in the boundary region between Northern Mongolia and Southern Siberia. Climate, land use and dynamic socioeconomic changes go along with rising water abstractions and contaminant loads originating from mining sites and urban wastewater. In the future, these pressures might have negative impacts on the ecosystems of Lake Baikal and the Selenga River Delta, which is an important wetland region in itself and forms the last geobiochemical barrier before the Selenga drains into Lake Baikal. The following study aims to assess current trends in hydrology and water quality in the Selenga-Baikal basin, identify their drivers and to set up models (WaterGAP3 framework and ECOMAG) for the prediction of future changes. Of particular relevance for hydrological and water quality changes in the recent past were climate and land use trends as well as contaminant influx from mining areas and urban settlements. In the near future, additional hydrological modifications due to the construction of dams and abstractions/water diversions from the Selenga’s Mongolian tributaries could lead to additional alterations.

The runoff of the Selenga R., the largest tributary of Lake Baikal, in recent two decades corresponds to a low-water period. Such decrease can be due to the global climate processes, which have an effect on the amounts of precipitation onto and evaporation from Selenga drainage basin, which is located in arid climate zone. The adaptation of Ecomag software complex to simulating river runoff in the Selenga Basin based on global databases (relief, soils, vegetation, and weather information) is described. The model was calibrated and verified, and the statistical estimates of calculation efficiency were constructed. The obtained model of runoff formation in the Selenga Basin was used to assess the possible changes in the climate and water regime in the XXI century with the use of data of global climate models under different scenarios of greenhouse gas emissions. Throughout the XXI century, the Selenga R. runoff may decrease by 10–40%, depending on the forecasted climate conditions.

The major formation factors of a disastrous flood in the Amur basin in July–September 2013 are discussed. The role of the Zeya and Bureya reservoirs in reducing the flood hazard is discussed. Preliminary estimates of the recurrence of peak flood discharge are given. It is shown that, considering the deficiency of data on water discharges along the river and the short duration of the available observation series, the estimates of flood hazard on the Amur R. obtained by statistical treatment of those data shows considerable uncertainty. A physical-mathematical model was developed to describe runoff formation in the Middle-Amur basin based on the ECOMAG information-simulation complex and a one-dimensional hydrodynamic model describing the motion of flood wave over a 600-km segment of the Amur R. was developed based on MIKE 11. The potentialities of the joint use of the proposed models to reproduce the behavior of water discharges and levels in different parts of the Amur R. during the propagation of the flood in 2013 and to evaluate the effect of withdrawals from the Zeya Reservoir to the water level regime of the Amur downstream of the Zeya R. mouth.