Impact of ice regime of small and medium-sized rivers in the permafrost zone on the formation of their riverbeds (the case study of Central Yakutia)

In small and medium-sized rivers of the permafrost zone, ice formation lasts for most of the year, and the ice cover often grows to the bottom along the entire length or in some of its sections. However, its impact on the morphology and dynamics of the riverbed, runoff, sediment, dissolved substances, and surrounding deposits is still unexplored. Observations on the formation and destruction of ice cover, freezing and thawing of riverbed sediments, and water turbidity on three small and medium-sized rivers were made in Central Yakutia. The data obtained from hydrographic stations of the Yakutsk Hydromet Office in 2008–2022 were used for our analysis. The character of the river’s freezing, whether the ice cover grows to the bottom along the entire length or the river or only in certain areas, depends on its morphology. Even in the absence of water sources in winter, lenses of unfrozen water still remain in the deepest (more than 1.5 m) sections of rivers, such as pools of beaded channels or meandering rivers. Local tallks up to 4 m thick are preserved under sections of the river with floating ice, while under sections of the rivers with bedfast ice, the sediments completely freeze in winter. On rivers with bedfast ice a significant part of the snowmelt runoff passes over the ice cover. The presence of ice in the riverbed promotes rising of water levels and increased water flow rates, but at the same time, it protects the sediments on the bottom and banks of the river from thawing and subsequent erosion. The peak of water discharges during the spring flood on the smallest rivers passes over the ice; but as the size of the river increases, the peak of water discharge shifts to later dates, so it occurs on the ice-free riverbed. Thus, the effect of spring floods on the erosion of the bed and banks of rivers with bedfast ice is reduced due to the energy expenditure of the water stream in the first phase of the flood on the destruction of ice filling the channel and the thawing of the bed and banks material. This phenomenon is more pronounced on the smallest rivers, which have lower thermal energy, than on larger ones.

freezing of the rivers, river ice, aufels, under-channel talk, beaded channel, turbidity, permafrost, effective discharge