New data on the transformation of water and sediment runoff into the Lena river delta based on the results of expedition measurements in August 2022

Abstract

Field hydrological measurements were performed in the Lena River delta from the 10th to the 16th of August 2022. 25 values of water discharge and 58 suspended sediment concentrations (SSC) were measured, multiple chemical composition water samples were taken, along with samples of bottom and bank river sediments, and thermal abrasion coasts were highlighted. Particle size and chemical composition analyses of the sediment samples were carried out. The aim of the study was to estimate the water flow distribution in the delta and to determine the directional SSC, sediment particle size and chemical composition changes along the delta and inside the depth of the river flow, and define the roles of the local hydrological and morphological factors of river flow transformation. Moreover, field measurements are crucial for SSC estimation and monitoring based on satellite image data. The laboratory analyses included particle size definition in suspended and bottom sediment samples, organic matter and SSC definition. The August 2022 expedition results were compared to the previous surveys. It was established that the flow distribution around the Stolb island corresponds with that of 2016: the Bykovsky, Trophimovsky, Tumatsky and Oleneksky branches receive 24.9–25.5, 58–59.2, 6 and 6.6 % of the water discharge from the Kusur gauging station on the Lena River, respectively. However, the role of the Main channel was slightly overestimated. The new data includes water discharges in the branches around the Samoilovsky island and in the channel systems of the Oleneksky and Tumatsky branches. SSC was relatively low and amounted to around 12–24 mg/l, rising from the water surface to the bottom 1.2–2 times. SSC decreased along the course of the Oleneksky and Tumatsky branches (1.5 and 1.1 times, respectively) due to the ratio between SSC and the potential stream transport capacity. SSC rose along the course of the Bykovsky branch, even though it wasn’t shown by the Landsat-8 satellite image (16th of August 2022). A significant sediment source in the delta are eroded and thermo eroded shores of the edoma island systems. The average sediment size proved to be from 0.011 to 0.019 mm. SSC does not vary much around the main channel and the nearest branches. Important relations between SSC, optical turbidity and ADCP backscatter intensity were estimated. The coarse diameter of 30 bottom sediment samples was 0.46 mm. The largest sediments were discovered in the Bykovsky branch, while the smallest sediments were found in the smaller transverse branches. Along the Tumatsky and Oleneksky branches the bottom sediments alternate from fine to medium sands.