Northwest Territories Geological Survey

Freshwater lakes are prominent features throughout northern boreal regions which provide important habitat for wildlife and resources for local communities. Increasingly, there have been concerns regarding how these northern lakes respond to fire disturbance and drought, which have been associated with changes in climate conditions during recent decades. Wildfires have been particularly intense in the Yellowknife, NT area over the past several years, and the influence on aquatic systems is not fully understood.

This research project integrates a number of analyses to identify the relative importance of climatic and catchment controls on the hydrology of 20 study lakes in the Yellowknife region. The study lakes reflect a range of catchment characteristics, with lake surface areas ranging from 1.6 to 3,280 ha (median= 10.9 ha). Nine of the study lakes are situated within catchments that have experienced either full or partial burn since 2012. The 2017 and 2018 years of study reflect disparate, ice-free season (i.e., May to September) precipitation conditions. The 2017 ice-free season was indicative of relatively “dryer” conditions (i.e., total precip.= 148 mm), slightly below the seasonal average. The 2018 ice-free season reflected pronounced “wetter” conditions (i.e., total precip.= 259 mm), and the most seasonal rain experienced in Yellowknife since 1943 when climate records were first maintained.

Lake water isotope data, specifically d2H and d18O, were obtained twice (i.e., spring and late summer) during each of the 2017 and 2018 ice-free seasons, respectively. Study lake catchments were modeled in ESRI ArcMap 10.5 software using available 5-m and 10-m Digital Elevation Model (DEM) data, utilizing Spatial Analyst and ArcHydro extensions. Preliminary isotope data and lake catchment analyses indicate that the lakes exhibit strong hydrological variability. Basins with higher lake area to catchment area ratios appear to be more susceptible to drying compared to lakes with larger catchments. In-situ lake level monitoring was employed for 19 of the 20 study lakes using installed HOBO water level loggers, with relative depth changes derived via air/water pressure calibration. Net-average lake level changes were determined to be approximately -0.22 m (max= +0.02 m, min= -0.51 m) during the 2017 (i.e., dryer) ice-free season and +0.13 m (max= +0.42 m, min= -0.17 m) during the 2018 (i.e., wetter) ice-free season, respectively. Overall, lake level drawdown during periods of drought during summer months was more pronounced during 2017 compared to 2018. The influence of catchment properties including size, land cover and burn area on water isotope and lake level data continue to be investigated.

Lake sediment cores were obtained from three lakes spanning identified hydrological gradients. Historical records of hydrological conditions are being reconstructed using cellulose-inferred oxygen isotope (d18O) analyses of the sediment cores. These data will be used to evaluate whether contemporary lake hydrological conditions are within the range of natural variability. The findings of this research program will enhance our knowledge of how warming northern climate and associated landscape changes are influencing lake hydrology. The work is part of a collaborative effort supported by the Government of NWT Cumulative Impact Monitoring Program (CIMP).