Flow Connectivity of a Lake-Stream System in a Semi-arid Precambrian Shield Environment

Many small lakes occupy Precambrian Shield and lowland areas in the boreal region. To investigate the processes causing seasonal severance of flow connection in the lake-stream system, a chain of lakes in northern Canada was studied in 2004. Water balance shows that rapid and substantial runoff from the local basin slopes during the snowmelt period led to a rise of lake levels above their outlet elevations to generate outflow. Continued summer evaporation caused drawdown of lake storage below the outflow thresholds, represented by the lake outlet elevations. Outflow ceased and the lakes became disconnected. Summer rainfall in a semi-arid environment was insufficient to overcome storage deficit to re-establish flow connectivity among all lakes. For the drainage system as a whole, streamflow interruption or continuity depends on linkage of its lake-stream sub-units. The principle of fill and spill of lakes should be considered in modeling Shield hydrology under semi-arid conditions, to take account of (1) antecedent storage in individual lakes, (2) their storage change calculated through water balance, and (3) the thresholds to be exceeded for outflows to occur.