Assessing variability in snowmelt bypass among thermokarst lakes using water isotope tracers, Northwest Territories, Canada

Abstract. Snow represents the largest potential source of water for thermokarst lakes, but the runoff generated by snowmelt (freshet) can flow beneath lake ice and out of lakes without mixing with and replacing pre-snowmelt lake water. Although this phenomenon, called “snowmelt bypass”, is common in ice-covered lakes, it is unknown what lake and watershed properties cause variation in snowmelt bypass among lakes. Understanding the variability of snowmelt bypass is important because the amount of freshet that is mixed into a lake affects the limnological and biogeochemical properties of the lake. To explore lake and watershed attributes that influence snowmelt bypass, we sampled 17 open-drainage thermokarst lakes for isotope analysis before and after snowmelt. Isotope data were used to estimate the amount of lake water replaced by freshet and to observe how the water source of lakes changed in response to the freshet. A median of 25.2 % of lake water was replaced by freshet, with values ranging widely from 5.2 to 52.8 %. For every metre lake depth increased, the portion of lake water replaced by freshet decreased by an average of 13 %, regardless of the size of the lake’s watershed. Vertical mixing is more restricted in deeper lakes, which reduces the relative thickness of the layer where freshet can mix with lake water, leading to more snowmelt bypass at deeper lakes. We expect a similar relationship between increasing lake depth and greater snowmelt bypass could be present at all ice-covered open-drainage lakes, since the limited vertical mixing conditions that lead to this relationship are present at all ice-covered lakes. The water source of freshet that was mixed into lakes was not exclusively snowmelt, but a combination of snowmelt mixed with rain-sourced water that was released as the soil thawed after snowmelt. As climate warming increases rainfall and shrubification causes earlier snowmelt timing relative to lake ice melt, snowmelt bypass may become more prevalent with the water remaining in thermokarst lakes post-freshet becoming increasingly rainfall sourced. However, if climate change causes lake levels to fall below the outlet level (i.e., lakes become closed drainage) more freshet may be retained by thermokarst lakes as snowmelt bypass will not be able to occur until lakes reach their outlet level.

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