Investigating the Hydrology of a Thermokarst Landscape (Old Crow Flats, Yukon, Canada) Using Water Isotope Tracers

Lake-rich thermokarst landscapes occupy vast regions and provide important natural resources for northern communities. Recent research has shown that these lake-rich environments are sensitive to changes in climate and are in a state of transition. Old Crow Flats (OCF, Yukon, Canada) is one such thermokarst landscape where people of the Vuntut Gwitchin First Nation (VGFN) have thrived for many generations. However, local residents have observed changes in OCF during recent decades including unpredictable weather patterns, expansion of shrub coverage, and drastic lake and river water level fluctuations. Concerns that these changes may affect their traditional lifestyles fueled initiation of a multidisciplinary Government of Canada International Polar Year project in OCF. A central component of the project was an investigation of landscape variability in contemporary lake and river hydrological responses to climate conditions during 2007-09. Analysis of lake water isotope tracers elucidated patterns of landscape hydrological variability. These results were integrated with spatial analysis of catchment land cover characteristics to provide insight of the drivers of lake hydrological variability and the basis to anticipate lake hydrological responses to future climate. To complete the assessment of the OCF water balance, patterns in river water isotope compositions were used to capture upstream hydrological fluctuations. Emerging from this comprehensive study are hydrological monitoring approaches that may be readily implemented in OCF or elsewhere to detect landscape responses to climate change.;During the 2007 ice-free season, spatial variability in lake hydrological conditions was identified using water isotope tracers. Water samples were collected from 56 lakes in early June and late July, and 26 lakes were sampled in late September / early October. Lakes were classified as either snowmelt-dominated, rainfall-dominated, groundwater-influenced, evaporation-dominated, and drained, according to isotopic trajectories in δ18O–δ 2H space, input water compositions (δ1), evaporation-to-inflow (E/I) ratios, and field observations. Hydrological classes represent dominant hydrological processes (input and evaporation) that influenced lake water balances. These characterizations demonstrate the broad range in lake hydrological conditions in the OCF landscape. Reference to oblique aerial photographs and field observations suggest that snowmelt-dominated lakes are located more in southern and north central areas of OCF where larger shrub and woodlands vegetation seems more prominent. In contrast, lakes are typically rainfall-dominated in other areas of OCF where tundra vegetation is more prevalent.;Lake water isotope monitoring continued during the following two years and provided the basis to identify drivers of lake hydrological variability. Utilizing a novel approach, relations were quantified between lake hydrological conditions, catchment characteristics, and climate. Water isotope tracers tracked the hydrological responses of 57 lakes to varying seasonal and inter-annual meteorological conditions during 2007-09. Isotope results were integrated with analysis of lake catchment characteristics to evaluate their relations. Quantitative analysis confirmed that lake hydrology is strongly linked with catchment land cover and physiography. The catchments of snowmelt-dominated lakes are typically located in peripheral areas of OCF and contain ∼60% woodland /forest and tall shrub vegetation. These catchments effectively intercept snow and provide elevated spring inflow to lakes, which was sufficient to offset the effects of evaporation during the relatively dry 2008 ice-free season. Notably, multiple hydrological trajectories are expected. On one hand, more lakes could become evaporation-dominated as the duration of the ice-free season increases, yet this tendency may be offset by increases in precipitation, thermokarst lake expansion, and greater snowmelt generation in response to expansion of shrub vegetation coverage. However, increasing summer precipitation may accelerate permafrost thaw and lake expansion, and lead to a higher frequency of lake drainage events. (Abstract shortened by UMI.).