In this study, the downstream effects of pollutants spreading due to hydromorphological gradients and associated changes in sediment transport conditions along the braided-meandering and deltaic distributary reach of a large river downstream section are discussed. We demonstrate the significance of hydrodynamic control for sediment-associated metal partitioning along the river. Typically, the downward decline of the sediment and metals spreading towards Lake Baikal is observed due to buffer effects in the delta. During peak flow, the longitudinal gradients in heavy metal concentration along the distributary delta reach are neglected due to higher concentrations delivered from the upper parts of the river. In particular, significant variations of heavy metal concentrations associated with the river depth are related to sediment concentration and flow velocity profiles. Various particulate metal behavior in silt-sand delta channels and the sand–gravel Selenga main stem emphasize the importance of near-bottom exchange for particles spreading with the river flow. Using empirically derived Rouse numbers, we found quantitative relationships between the ratio of particulate metals sorting throughout depth in a single river channel and the hydrodynamic conditions of sediment transport.

The article considers the features of biogenic elements and organic carbon (Corg) content spatial variability in the Lake Baikal tributaries. The role of hydrological conditions and landscape-geochemical features in the chemical elements flow formation is shown. It was found that organic carbon in the river waters is represented mainly by its dissolved form. In the lower reaches of the Selenga, erosion processes and economic activity in the catchment area increase the proportion of suspended forms of organic carbon and mineral phosphorus. In the river delta under the influence of the sedimentation and the intake of organic matter formed during aquatic vegetation decomposition, the content of dissolved Corg increases with a contemporary decrease in its suspended form. As a result of intensification of production and destruction processes in well-heated areas of the delta, a significant decrease in the concentrations of mineral phosphorus and an increase in organic phosphorus occur.

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