ИСТИНА

One of the typical results of the interactions of hydrological and cryogenic processes is the formation of specific small river channel planforms represented by prominent alternations of widened and deepened lake-like pools and connecting narrow runs best termed in the literature as «beaded channels». Beaded channels are widespread throughout the northern hemisphere in permafrost zone and occupy up to 50% of the length of all small watercourses (1-3 orders) of the tundra landscapes. Our study conducted in Yamal Peninsula (arctic part of Western Siberia) and Central Yakutia (continental part of Eastern Siberia) aims at the revealing of modern processes of beaded channels formation and also reconstruction of its Holocene history. We used pressure and temperature data loggers, automatic photo camera, bathymetry and tacheometry, ground and ice drilling, geophysical measurements, paleochannel analyze to determine morphological characteristics of beaded channels, their hydrological, thermal and ice regime, geological structure, history and causal relationships between meteorological parameters, ice thickness, water, ground and ice temperatures. We revealed that specifics of the planform and longitudinal profile shapes of beaded channels determine characteristics of their thermal, chemical and water regimes. Deepened lake-like pools («beads») are sufficiently (up to 20°C) warmed during summers and don’t freeze over entirely during winters, even under harsh climatic conditions of the Central Yakutia. As a result there is usually a lens of pressurized water (up to 2-3 m depth) under surface ice underlain by lens (up to 1.5-2 m thickness) of the perennially thawed sediment (taliks). Shallows (runs) freeze up to the bottom and there is no thawed sediment under the channel. Freezing of the pool leads to pressure increasing under the ice cover and ice cracking several times during winter. Biogenic gases release due to ice cracking and, thus, the water enriches by oxygen. It has an important impact on the hydrobionts (minnows, frogs, insects, leeches) overwintering in these pools. Such regime distinguishes beaded river channels from other rivers of permafrost zone. It has important implications for flow discharge regulations and permafrost ecosystems, providing overwintering habitats, biodiversity dispersal, leading to generation and release of biogenic gases into the atmosphere. Planform of beaded channels also is a good sign of permafrost condition and drainage network development in the past and present. Majority of scientists considers that a significant role in the formation of lake-like pools belongs to thermokarst and thermal erosion processes. However alternative opinions also exist, arguing that beaded channels were formed by freezing over of bedload accumulation forms in channels and «beads» are inherent from the former pools preserved since warmer and more humid period. Preliminary analysis carried out by the authors allows suggesting that beaded channel pattern formation is likely a result of complex processes interaction triggered by decrease in flow discharges caused by natural climate changes in the past. This assumption is based on the fact that small 2-3 order stream channels in Arctic often have a specific transitional meandering-to-beaded form resembling the shape of unconfined meandering rivers, but consisting of «beads» and runs. However, such channels exhibit no evidences of present-day erosion of concave banks and sediment accumulation at the convex banks as typically being observed in normally meandering rivers. The meandering shape of some beaded channels and presence of sickle-shaped oxbow lakes on the floodplain in the absence of modern development of bends indicate that it was formed under different conditions. It can be suggested that their formation occurred under unconfined meandering conditions governed by the greater channel-forming flow discharges. Transition to the beaded channel planform took place only later, presumably as a result of climate changes – reduction of water runoff and increase of the seasonal freeze-thaw depth.