ABSTRACT: The large lake effect on Mackenzie River Flow: an isotopic perspective.

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Abstract

The Mackenzie Basin (1.7 million sq. km) incorporates a diverse range of geographic source regions, including eight of the fifteen 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 distinct due to the occurrence of several large lakes (Lake Athabasca, Great Slave Lake, Great Bear Lake) which act as flow, sedimentation, and biogeochemical regulators along its main drainage network. A detailed water sampling network was established in 2002 to support isotope hydrology studies in the basin by Environment Canada, as well as modelling efforts conducted in association with the Mackenzie GEWEX Study and the IAEA Coordinated Research Project on Rivers. Results from this survey reveal the complex evolution of the isotopic composition of river discharge from the headwaters of the Mackenzie River to the mouth, particularly the mixing of tributary inflows and the buffering effect of the large lakes. Spatially, the most depleted isotope signatures are observed for tributaries of the Western Cordillera, especially the Mackenzie Mountains, which are characterized by higher-altitude precipitation, greater snowfall, and higher runoff/precipitation ratios than other parts of the basin. In contrast, shield-dominated eastern areas and the central boreal-taiga plains tend to have enriched isotopic signatures reflecting lower altitude precipitation as well as significant contributions from evaporated lake and wetland sources. Here, rivers commonly traverse extensive string-of-lakes and bog-fen drainage networks. In addition, seasonality of the riverine isotopic signals is often pronounced, reflecting varying proportions of flow derived from snowmelt, groundwater, and surface waters during the ice-on, freshet, and ice-off periods. This presentation will highlight our current understanding of isotope balance constraints on tributary mixing, water balance of the large lakes, and the seasonal timing of large lake contributions to Arctic Ocean discharge, factors not readily traced by physical gauging alone.

Bibliography of Canadian Geomorphology