PHYSICAL-PROPERTIES GOVERNING GROUNDWATER-FLOW IN A GLACIAL TILL CATCHMENT

Hydraulic conductivity, surface topography and sediment thickness influence the spatial and temporal pattern of groundwater flow and stream discharge in an unconfined aquifer in a 3.7 ha headwater catchment in central Ontario, Canada. Groundwater levels in the soils adjacent to the stream also significantly influence the magnitude and spatial distribution of stream discharge. Topographic convergence in plan and decreasing sediment thickness along flowpaths result in surface saturation and groundwater discharge. Hillslope gradients adjacent to discharge areas determine how fluctuating groundwater levels influence the extent of spatial variations in surface saturation and saturation within the soils. As a result of spatial differences in hillslope gradients, the spatial patterns of both groundwater and stream discharge change with fluctuating groundwater levels. Unsaturated sediments in upslope locations store water infiltrating during wet periods such that groundwater flow from upslope sediments maintains high groundwater levels near the stream and supplies baseflow during dry periods. The direction of horizontal groundwater flow in Harp 4-21 is not perpendicular to topographic contours so that subcatchment boundaries based on topographic divides differ by as much as 57% from their true subcatchment boundaries based on groundwater divides. Therefore, groundwater flow models based on surface topography may incorrectly predict the spatial pattern of stream discharge.