ИСТИНА

Water tracks (or dells) are widespread hillslope flowpaths in permafrost areas. They are shaped as hollows with gentle slopes and are 1-30 m width and several hundred meters length. Devoid of pronounced talveg, they stretch from the top of uplands down to the valley bottoms, forming a dense network on gentle and medium slopes. Depending on the slope topography, they develop parallel to each other, branch downwards or form dendritic system. There is still no consensus on genesis of water tracks. They are considered a rudimentary channel network that is not fully developed due to the erosional resistance of permafrost [McNamara, 1997], a result of thermokarst processes on ice wedges, or a consequence of uneven movement of slope material [Katasonova, 1959]. Water tracks are known as preferential suprapermafrost flowpaths, providing an area for fast runoff generation [McNamara et al., 1997]. We investigated the mechanisms and pathways by which water travels through the small basin of Krestyakh River located in continuous permafrost near the Lena Delta. The slopes of the basin are composed of ice rich silt (up to 300% of dry weight). The Paleozoic rocks (aleurolites) are exposed in the upper part of the ridges and in the incised channel of the creek, where gravelly material appears in the sediments. The active layer of an average thickness of 20–25 cm is over saturated. Slopes are covered with tussock sedge tundra and dissected by numerous water tracks. We measured level and water temperature at three gauging stations located in the nested basins: in the water track (0.015 km2), in the stream with an overgrown channel (1.41 km2) and the stream with an incised pebbly channel (2.07 km2). During the measurements (Aug 1-11 2018) the air temperature increased in the absence of precipitation. Suprapermafrost groundwater level measured in water track consistently increase during this period, which can be explained by seasonal thawing of ice rich sediments. Water level in the overgrown channel also rose, but at a lower rate. Water level in the incised pebbly channel had no significant dynamics. Discharge measured by floats in the incised pebbly channel was 1-1.5 l/s. The water temperature at all gauging stations had a pronounced daily variation with amplitude of up to 3°С and changed from 1.85°С at the water track to 6.4°С at the middle and 4.9°С at the lower gauging stations. The decrease of the water temperature at the pebbly channel we credit to local percolation of water through cold gravel sediments upstream the gauging station. Despite the polar day, there was a pronounced diurnal variation of water level in the water track with minimum values in the evening and maximum in the early morning. Since the fluctuations in the levels were in antiphase with fluctuations in water and air temperature, this can be explained by evapotranspiration in the daytime and condensation as dew at night. The absence of significant variations of water levels in the overgrown channel can be explained by the high accumulating capacity of the wetland and by larger drainage area. Filtration rates in active layer measured using the NaCl-tracer in the water track and in the intertrack are close (3–4 m/day). However, the active layer in the water track is deeper and the volume of filtered water there is somewhat larger. At the same time, in case of the appearance of surface runoff in the water track, discharge can increase significantly. The maximum filtration rates (14 m/day) occur on the top of uplands covered with patterned ground, where loamy aggregate is washed out of the gravelly material.