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.

A regional basinwide assessment of the formation of the water and suspended matter runoff was carried out for theSelengaRiver–Baikal Lakesystem. It is based on the results of integrated hydrological and geochemical research of more than 100 locations withinRussiaandMongoliaperformed by the authors in 2011–2013 using the unified methodology during various hydrological periods. It is shown that the quantitative parameters of the suspended sediment transport are largely determined by the seasonal hydrology. Contribution of floods in the suspended sediment transport ranges from 52% of annual runoff for the large rivers up to 99% for the small ones. Chemical composition of 400 samples of the water and suspended sediment was analyzed by ICPMS / ICP-AES method. The obtained results demonstrate that Selenga River basin is enriched relative to the average content in the World Rivers by dissolved Sr, Li, U, Br, B, Mo, As, migrating predominantly in the ionic form, as well as by Fe, Al, Zn and Pb, migrating in the form of complexes with organic matter. Suspended sediments of the Selenga River and its tributaries are enriched with As, Cd, Mo, Pb, Zn, Mn, Co, which is due to both the geochemical specialization of the region (As) and the technogenic influence (Cd, Mo, Pb). Analysis of particulate/dissolved modes of chemical elements in river water showed the prevalence of particulate modes of most heavy metals and metalloids (HMM) in the upper part of the basin and dissolved ones in the middle and lower parts. The share of suspended forms of migration increases dramatically during the floods, especially in the upper reaches of the rivers; in the lower part of the basin the impact of floods on the ratio of forms decreases, which is especially noticeable in theSelengadelta. During the floods geochemical fluxes of dissolved HMM increased towards theSelengadelta with increasing water runoff. Extremely high flows of suspended forms caused by heavy rainfall were determined in the upper part of the basin that rapidly decreased downstream due to sedimentation. In the lower part of the basin, the flows increased again due to the confluence of major tributaries. The largest anthropogenic changes in fluxes are typical for small rivers (Modonkul, Hangalyngol, Tuul, Uda et al.), impacted by the towns and mining facilities. Extremely high pollution by Cd and other metals was revealed for theModonkulRiverthat is significantly affecting the geochemical flows of HMM in theDzhidaRiverup to its confluence with theSelengaRiver.

Sediment concentration (SC)-water discharge (Q) relations in rivers are typically governed by multiple and relatively complex processes. Due to hysteresis effects, sediment discharges can differ for similar or equivalent water discharges, which causes scatter in empirical datasets and may decrease the predictive power of SC rating curves. Such hysteresis effects must therefore be understood and accounted for to make dependable predictions for river system management. The overall objectives of this study are to develop modelling approaches suitable for reproducing and predicting hysteresis effects at larger scales and to investigate the possible contribution of in-channel processes (erosion and deposition) to sediment concentration hysteresis loops. To investigate relevant field-scale conditions, we develop a one-dimensional dynamic sediment transport model of the downstream Tuul River (northern Mongolia), investigating in-channel processes along a 141 km stretch during a hydrological year. The results show that the present modelling approach can reproduce both anti-clockwise and clockwise hysteresis effects. Importantly, in-channel processes alone can cause considerable anti-clockwise hysteresis effects without being reinforced by catchment processes such as hillslope erosion. Such specific contributions from in-channel processes introduced data scatter into the sediment rating curves, decreasing their R-2-values from unity to approximately 0.5 to 0.6. More generally, possible changes in the number or magnitude of high-flow events, caused by climatic or other anthropogenic factors, could influence total sediment deposition, which was primarily found to occur during relatively short high-flow events. Such potential changes also have important implications for the possible spreading of polluted sediments. (C) 2015 The Authors. Published by Elsevier B.V.

An assessment is made of the contribution from different sources supplying sediments to the rivers flowing in areas of open-cast mining of placers in places of their contemporary exploitation (using the Russian Federation and Mongolia as an example). We examine the sediment yield transformation processes in conditions of open-cast gold and platinum mining in the valleys of creeks and small and medium-sized rivers where extraction of mineral resources is an exceptionally important kind of activity (the rivers of the Vyvenka basin, Kamchatka krai) or characteristic for a given territory (Tuul river downstream of the city of Ulaanbaatar, Selenge river basin, Mongolia). We summarized the assessments of the sediment yield transformation downstream of the areas of mining placer deposits along the rivers of Russia, Mongolia, USA, India, Australia, and Surinam. It is shown that stream-channel erosion in anthropogenically modified channels is responsible for up to 90% of technogenic changes in sediment yield. The most significant (by several orders of magnitude) increase in sediment yield has been recorded for small rivers. For large rivers that are characterized by low background values of sediment yield, its change is by a factor of 1.1‒1.2 in the case of single mining operations in the drainage area and reaches a factor of five for large-scale mining operations.

Many Asian rivers have been intensively used to boost economic growth, resulting in sudden and drastic changes in sediment transport patterns. However, a few rivers are still undisturbed. The present paper considers the unregulated Selenga River and its basin, located in Russia and Mongolia. The river contributes to 50 % of the total inflow to Lake Baikal. Pending scientific challenges include the quantification of sediment loads and erosion–deposition patterns along the Selenga River system, the understanding of suspended particulate matter composition and the importance of peak flow events for total sediment discharge and heavy metal transport. Field data and hydraulic modeling converge on showing that peak flow events during spring and summer contribute to the main part (70–80 %) of the annual sediment and pollution loads in upstream parts of the basin. The Selenga River carries mostly silt and sand. The average particle size differs by a factor of four between summer floods and base flow periods. The low amount of particulate organic matter (ranging between 1 and 16 % in the studied rivers) is consistent with the significant role of sediments originating from mining areas and in-channel sources. The bed load transport in the downstream part of the river basin is high (up to 50 % of the total transport), and channel storage plays an important role in the total sediment transport to Lake Baikal. Reported statistically significant multi-decadal declines in sediment fluxes in the downstream Selenga River can be attributed to the abandonment of cultivated lands and (most likely) to changing hydroclimatic factors.