Hydrological conditions at the mouth of the Kolyma River in summer 2019

A poorly-studied Kolyma River mouth section (near-delta section and the delta) was surveyed in summer 2019, first since the 1950-s. The complex hydrological survey included measurements of flow rates, water discharges, water depths, water surface slopes, water turbidity, temperatures, and conductivity. We also obtained remote sensing data using quadcopter, took samples of water and sediments for chemical and other analyses, and collected field data for the satellite data calibration. Five temporary hydrological gauges with loggers were organized to monitor hydrological conditions and the passage of river floods. Some measurements were made for the first time. Several series of runoff measurements at 24 cross-sections made it possible to reveal the along-channel increase of the Kolyma River flow, the contribution of main tributaries and the Stadukhinskaya anastomosing branch, velocities structure of the flow and the actual runoff distribution within the delta. The latter is already different from hydrological calculations of the late 20^th century, because the proportion of the Kamennaya Kolyma branch has increased by nearly 10%. Two zones of different water masses mixing were identified within the mouth area. The first zone is within the near-delta section where waters of the Kolyma River and its right tributaries are mixed. The second one is in the delta and on the seashore, where fresh and salt waters are mixed. The water of the Omolon and Anui rivers are less turbid (5–15 vs 120 mg/l) and less mineralized (55–70 vs 140–160 mg/l), and have basic salt composition as compared with the Kolyma River water. Waters of these tributaries are cooler. Despite gradual mixing, they flow along the right bank of the neardelta section of the Kolyma River channel reaching the head of the delta and moving further on. Thus the temperature data of the Cherskiy gauge are not representative. The influence of small inflows on the temperature regime is more complex. Mineralization of river waters increases towards the sea within the delta, but the real sea waters were found only in the lower part of the Chukochia branch. The spatial variability of the suspended sediment runoff is influenced by the dilution of Kolyma River water with waters of Anui and Omolon rivers, decreasing water surface slopes and current velocities downstream, runoff distribution in the delta, and a great amount of sediment material from eroded riverbanks, e.g. near the well-known Duvanny Yar. A new sediment trap, ADCP, turbidimeters and different probes were applied to collect unique data about sediment composition and their distribution within the stream.

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