Isotope tracing of water balance and climatic variability along the Mackenzie River

Full text: The Mackenzie River, draining an area of 1.7 million km2, incorporates a diverse range of geographic source regions, including 8 of the 15 distinct ecoclimatic regions identified in Canada. The basin is mountainous in the west and relatively flat-lying in the east with strong north-south climatic gradients, and generally cold, dry climate conditions compared to other large river basins in the world. As a major contributor of freshwater discharge to the Arctic Ocean, the river is also distinct due to the occurrence of several large lakes (Lesser Slave, Athabasca, Great Slave, Great Bear) which naturally act as flow, sedimentation, and biogeochemical regulators along its main drainage network. Evolution of the isotopic composition of river discharge from the headwaters of the Mackenzie River to the mouth also reflects many of these complexities, particularly the mixing of tributary inflows and the buffering effect of large lakes. A map of the oxygen-18 composition of summer runoff in tributaries reveals distinct spatial patterns across the basin. Notably, the most depleted isotope signatures (<-20 per mille in δ18O) are observed for tributaries of the Western Cordillera, especially the Mackenzie Mountains (min. of -22.9 per mille in δ18O), which are characterized by higher altitude precipitation, greater snowfall, and higher runoff/precipitation ratios than other parts of the basin. In shield-dominated areas to the east of Great Slave Lake and Lake Athabasca, and to a lesser extent in the central boreal-taiga plains, tributary runoff is typically enriched by lake and wetland evaporation in low-relief areas where rivers traverse extensive string-of-lakes and bog-fen drainage networks. Oxygen-18 values in major tributaries typically range between -16 to -14 per mille in shield areas with peak enrichment observed in wetland dominated drainage of the south-central Boreal Plain (Wabasca R. ∼-13.9 per mille). A synoptic survey along the main stem of the Mackenzie-Athabasca River reveals the periodic fluctuation of oxygen-18 from headwaters to mouth due to interaction of tributaries draining both western alpine regions (with depleted isotope signatures) and eastern lowlands (with enriched isotope signatures), overprinted by lake storage effects. In general, lakes serve a regulatory role in the runoff regime by reducing seasonality of discharge and amplitude of isotope variations. The 2-3 per mille overall enrichment of oxygen-18 from headwaters to mouth, despite the north-flowing drainage network and northeastward decrease in oxygen-18 in precipitation across the region emphasizes the cumulative importance of open-water evaporation losses in the basin water budget (∼15%). Seasonality in the isotope composition of discharge, as measured near the river mouth upstream from the Mackenzie Delta is pronounced. Peak flow produced by snowmelt typically occurs in April (∼ Day 150) at the basin outlet, and coincided with roughly a 2 per mille depletion in δ18O during the early 1980s. Significantly higher depletions during freshet are often observed in smaller tributaries, with similar recessions to higher δ18O values in summer and late fall. Reduced isotope variability is generally observed during extended winter periods with thick ice cover, and reflect the predominance of groundwater sources (∼19.0 per mille in δ18O) although the signature of water derived from lake storage is also evident during some years (max of -17.5 per mille in δ18O). This paper presents an overview of isotope datasets collected within the Mackenzie Basin since 1969 as well as more recent hydrograph separation, water balance and evaporation-transpiration partitioning studies conducted under the auspices of the Global Energy and Water Cycle Experiment, Mackenzie Study (GEWEX-MAGS) and the IAEA Coordinated Research Project 'Isotope tracing of hydrological processes in large river basins'. One long-term objective of the research is to incorporate tracers in the suite of models currently being implemented and tested with the MAGS study. Coupled simulation of isotopic and hydrologic fluxes and storages, particularly on the land surface and in streamflow will be used to evaluate and refine these models so that they more realistically depict processes, and so that they evolve to become more relevant for evaluating and predicting both water quantity and quality impacts, especially in vast ungauged regions of the basin and other parts of northern Canada. (author)