Hydrology of subarctic Canadian shield: soil-filled valleys

The Canadian Shield landscape includes bedrock uplands and valleys infilled with soil. A site near Yellowknife in subarctic Canada was studied to elucidate the hydrological behaviour of soil-filled valleys. A suite of hydrological processes was found to be important in the studied valley, including snowmelt and rainfall, ground frost development, evaporation, infiltration, lateral inflow from adjacent uplands, and surface and subsurface flows. Valley storage requirements during snowmelt are met by local meltwater infiltration, but lateral inflows in the summer are needed to satisfy the storage before runoff can be generated from the valley. The valleys perform three functions of collecting vertical and lateral water inputs, retaining and losing the water held in storage, and transferring water down the valley and generating local outflow. Unlike channel flows in humid areas, runoff from the upper valley has to meet the storage demands of the lower valley segments, often causing seepage loss along flow paths to render the flows intermittent. A fill-and-spill runoff system is proposed in which the valley physiography results in a series of segments with varying storage conditions. As water is supplied to the valley, each segment has to be filled until its storage threshold for runoff is exceeded. Then, subsurface or surface flows will be generated, but these flows may be arrested to furnish water to satisfy the storage requirements of the segments downstream. Such a flow system applies also to other valleys in the Shield environment.