ИСТИНА

The detailed results of the study could be found in (Chalov et al., 2012; 2013; 2015, Pietroń et al., 2015). Below we summarize the main conclusions on the hydroclimatic change in water, metal and carbon fluxes 1. Long-lasting low water period (since around 1995) is observed for the Selenga River. The runoff records for downstream Selenga (Mostovoy) shows statistically significant downward trend from 903 m3/s (for the period since 1941 to 1982) to 888 m3/s (for the period since 1983 to 2011), or even more drastic for the recent decades – from 940 m3/s (for the period since 1975 to 1995) to 689 m3/s (for the period since 1995 to 2011). The mean annual discharge at some gauging stations within the Mongolian part of the basin) demonstrated even more significant decrease during the last decades, from an average of 53,3 m3/s during the period 1975-1995 to only 18,1 for the years 1996-2011 at the upper Orkhon river (Orkhon-Orkhon) and from 35,3 m3/s to 14,9 m3/s for the Tuul river at UlaanBaatar respectively. The changes were caused mostly by intensive rainfall floods which determined high water period in the early 1990 and late 1970-1980. Present and expected hydrological changes in the Selenga River Basin are caused by three processes: land use changes, the impacts of global climate change on precipitation and evaporation and permafrost, and increasing water withdrawals. This lead to decline in sediment delivery to Lake Baikal: the annual total sediment load (suspended and bed load) delivered to the Selenga River Delta at Lake Baikal is about 2.5 Mt/year. Reported multi-decadal declines in sediment loads (up to 20%) in the downstream part of Selenga River can be attributed to the abandonment of cultivated lands and changing hydroclimatic factors, such as in particular climate-driven decrease of water flows and intensified water use for irrigation purposes. 2. Results indicate that high sediment and metal fluxes were reported both for altered and natural rivers. Mining impacts on sediment loads were seen mostly during relatively short hydrological events, during which an intensified slope wash near floodplain mining activities could flush large amounts of turbid water into the river. Whereas sediment flows were connected with the hydroclimatic conditions in the catchment, elemental composition of the mass flows mostly relate to the soil/petrologic conditions. With the exception of small impacted rivers where water quality impacts associated with mining were found, the formation of elemental compositions and sediment-associated chemical constituents generally reflects catchment characteristics. 3. Silt (< 0.05 mm) and sand (> 0.05 mm) are primary constituents of the observed sediment load in the Selenga River Basin. Fine sediments that range in grain size from 0.01 to 0.05 mm represent more than 50 % of the total suspended sediment load. The volume of particles that have a grain size of <0.05 mm constitutes over 60 % of the total particle volume. Since significant correlations were found for concentrations of Fe, Co, As, Cu and Zn with grain size fractions we conclude that particle size is one of the main factors that govern HMM contamination levels. This provide an important conclusion that long-lasted low water period associated mostly with changes in precipitation patters have a significant impact on geochemical fluxes due to changes in sediment delivery and sources related to the shifts in catchment erosion processes. 4. Significant inter-annual variability in mean values of particulate organic carbon POC has been found. POC content in total SPM reached 23,6 % during flood in 2013 and was negligibly small during winter low water season. The annual Selenga River discharge of particulate organic carbon (POC) can be as high as 0,0005 Tg (1Tg = 1012 g) which is 3-4 orders of magnitude less then POC flux in the Enisey River outlet (Holmes et al, 2015). As far as catchment area of Enisey and Selenga are differed in just 1 order of magnitude (2540000 km2 against 477000 km2), this emphasize significant differences in fluvial carbon input between headwaters and downstream of large Arctic catchment (Enisey river). 5. Important role of the fluxes delivery to Lake Baikal is played by delta. The delta network has a large influence on the mass of metals reaching Lake Baikal at the delta outlet. Up to 50 % of suspended and up to 70 % of the total sediment loads are stored in certain parts (sectors) of the delta. Although the differences in average conditions between the channels are not large, there are generally larger differences within each channel reach.