Landscape controls on the hydrology of stream riparian zones

Increased knowledge of hydrology is essential to an understanding of the water quality function of stream riparian zones. We examined the effect of upland surficial aquifer size, topography and riparian sediment lithology on the subsurface hydrology of eight riparian sites on glacial till and outwash landscapes in southern Ontario, Canada. Riparian sites had a permanent subsurface hydrologic connection to the adjacent upland in landscapes where upland permeable sediment depths were 2-15 m. In contrast, a hydrologic connection was absent in summer and autumn at riparian sites with <2 m of permeable sediments overlying an aquitard. Riparian zones linked to thicker and more extensive upland aquifers had large relatively constant groundwater inputs that maintained a stable riparian water table. Sites that were seasonally disconnected from uplands had large annual water table drawdowns that dried out the riparian area unless the water table was sustained by the stream. Peats with low hydraulic conductivity and a thinning of highly permeable sediment layers occurred in some riparian zones producing upward groundwater flow that created seeps and surface rivulets. Subsurface flow data indicated a consistent downslope flow path in areas of the riparian zone where the slope gradient was >5%. Where the riparian zone was level to gently sloping (<5%), subsurface flow directions were influenced by stream water level. Riparian subsurface flows at these sites often shifted from a hillslope to stream pattern to a down valley and even a stream to hillslope direction during summers. A conceptual framework of riparian hydrologic types based on hydrogeologic setting is developed to generalize the hydrology of riparian zones at the landscape scale.