Multi-year landscape-scale assessment of lakewater balances in the Slave River Delta, NWT, using water isotope tracers

We apply a coupled-isotope tracer model to quantify end-of-thaw-season (fall) lakewater balances in the Slave River Delta (SRD), Canada, during 2003-2005, which effectively differentiates the relative importance of hydrological processes across this complex northern freshwater landscape. The model incorporates Great Slave Lake evaporated vapour to the ambient atmospheric vapour pool and is thus tailored to the hydroclimatic setting of the delta, which experiences onshore winds. Results, expressed as evaporation-to-inflow ratios (E/I) for 41 delta lakes, reflect the role of spring break-up flooding and local hydrological setting. Fall E/I ratios for lakes where water balances are dominated by exchange with the Slave River or Great Slave Lake are low (0.06-0.53) and do not vary substantially during the three-year monitoring period. E/I ratios for flood-dominated lakes in the active delta are moderate (0.26-0.98) and have low inter-annual variability, even in the absence of spring flooding. This suggests that annual flooding during the spring break-up period is not necessary to maintain positive (E/I < 1) water balances in flood-dominated lakes, but multiple years without flooding would clearly lead to greater cumulative evaporation. Fall E/I ratios are generally higher and more variable in evaporation-dominated lakes in the relict delta (0.42 to > 1), although greater snowmelt runoff tends to occur in sub-sectors with mature spruce forest and offsets open-water vapour loss. Our results indicate that spring inputs (river flooding and snowmelt runoff) are key components of the hydrological evolution of SRD lakes during the openwater season, and distinguish regions of the delta where expected declines in river discharge and climate warming will likely cause lake-level drawdown. Such findings have particular relevance for informed ecosystem management in the Peace-Athabasca-Slave watershed, where unprecedented industrial development is imposing substantial additional pressure on freshwater resources. (C) 2009 Elsevier B.V. All rights reserved.