Groundwater-surface water interactions in headwater forested wetlands of the Canadian shield

Groundwater and surface water interaction in two conifer swamps located in headwater catchments with contrasting till depth, typical of the southern Canadian Shield, were studied from June 1990 to August 1992, Both swamps had little influence on the regulation or attenuation of seasonal runoff response in the catchment. The two valley bottom swamps were connected to local aquifers but the upland-wetland connection was continuous in the catchment with deeper till and ephemeral in the catchment with thin till-rock ridges. Groundwater movement through the wetlands was restricted mainly to the surface peat layer in both wetlands, because a large portion of inputs from shallow soil layers and stream inflows enter near the peat surface. However, differences in upland-wetland connections resulted in contrasting hydrologic regimes in the two swamps. During seasons with larger inputs, both swamps were hydrologically connected to uplands and had a similar hydrology characterized by a high water table, rapid storm response, and predominance of saturated overland flow, In summer, upland inputs were absent in the catchment with thin till-rock ridges, resulting in cessation of baseflow and a lower water table that varied in response to variations in rainfall. Continuous upland inputs throughout the summer in the catchment with deeper tills (1-3 m) sustained baseflow and kept the water table near the peat surface. This study demonstrates the control of morphology and shallow subsurface geology on the hydrology of valley bottom swamps influenced by local aquifers.