Synoptic and time-series stable isotope surveys of the Mackenzie River from Great Slave Lake to the Arctic Ocean, 2003 to 2006

We report oxygen and hydrogen isotopic composition of river discharges in the main stem and tributaries in Mackenzie River system, Canada, based upon a synoptic survey and a time-series monitoring program undertaken between 2003 and 2006. The water isotope signals in the Mackenzie River system exhibit distinct geographic variations. Isotopic results from the comprehensive synoptic survey offset from meteoric water lines in δ18O–δ2H space, and suggest a snowmelt-driven discharge regime with mixing between heavy-isotope depleted discharge from Liard River and Mackenzie Mountains, and evaporatively-enriched water from Great Slave Lake catchment. Based upon these isotopic data in river discharge, we propose an evaporative-mixing line for the Mackenzie River system (δ2H = 6.0 × δ18O − 40.8), which highlights the integrated effects of the two most important hydrologic processes (i.e., evaporation and mixing). Time-series investigation of water isotope signatures and river discharge at three key hydrometric stations demonstrates the isotopic response to the strong seasonality of river discharge in northern Canada. In general, isotopic compositions in river discharge tend to be negatively correlated with discharge amount, which is a typical snowmelt-driven isotope-discharge pattern. However, low flow conditions are found to be noisy with a wide range of isotopic variability. More importantly, we also observe that extreme high flow events, especially in the Liard River, appear to be unexpectedly enriched in heavy isotopes, which indicate contributions from evaporated water that are probably derived from wetland surface storage. This is a situation, which produces a reversal in the isotope-discharge pattern and marks important episodes that may impact chemical and riverine/estuarine biotic processes at times of extremely high runoff.