GROUNDWATER-SURFACE WATER INTERACTIONS IN HEADWATER FORESTED WETLANDSOF THE CANADIAN SHIELD

Groundwater and surface water interaction in two conifer swamps locate d 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 s easonal runoff response in the catchment. The two valley bottom swamps were connected to local aquifers but the upland-wetland connection wa s continuous in the catchment with deeper till and ephemeral in the ca tchment with thin till-rock ridges. Groundwater movement through the w etlands was restricted mainly to the surface peat layer in both wetlan ds, because a large portion of inputs from shallow soil layers and str eam inflows enter near the peat surface. However, differences in uplan d-wetland connections resulted in contrasting hydrologic regimes in th e two swamps. During seasons with larger inputs, both swamps were hydr ologically connected to uplands and had a similar hydrology characteri zed by a high water table, rapid storm response, and predominance of s aturated overland flow, In summer, upland inputs were absent in the ca tchment with thin till-rock ridges, resulting in cessation of baseflow and a lower water table that varied in response to variations in rain fall. 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 morpholo gy and shallow subsurface geology on the hydrology of valley bottom sw amps influenced by local aquifers.