ИСТИНА

Sediments fluxes and particulate matter composition abruptly change along the lower reaches of large rivers where a decrease of sediment and metals spreading is observed due to channel gradient decline towards the receiving water body related to buffer effects in the delta. Here we investigate estuaries of the largest tributaries of the Lake Baikal. Based on hydrogeomorphological classification we separate transient and depositional estuaries (buffers). Based on in-situ monitoring, field sampling, and satellite datasets the suspended sediment budget for the largest depositional estuaries of the Selenga and Upper Angara rivers was estimated. Both rivers experience sediment deficit which is observed during recent decades are characterized by the reduction of the sediment deposition rates. Under low-flow conditions we observed a longitudinal increase of the sediment transport rates along the Selenga River delta. Sediment transport rates in the Upper Angara river are constantly reduced under various hydrological conditions. Our results highlight that intense winds induce fine sediment (≤ 100 μm) export to the lake, whereas increased river discharges induce pronounced sediment deposition across delta wetlands. Extreme waves and streamflow events were less intense since 1995 reducing fine sediment export to the Lake Baikal. Additionally, based on Rouse number (Ro) analyses for the 28 transects along lower Selenga River where vertical geochemical sampling was done both with the simultaneous discharge measurements, we demonstrated the significance of the hydraulic control for sediment-associated metal partitioning along the rivercourse. Due to interaction between the transient flow and channel bed (near-bottom sediments (bed load) and bottom deposits), pronounced increase of coarse mineral fractions (50-2000 mm) in the near-bottom layer in the depositional delta pattern was revealed. Along the hydraulic gradient from the main river course to the estuary, the increase of the metals in the near-bottom layers is observed at the low values of Ro.