Hydrogeomorphic controls on runoff in a temperate swamp

Abstract. Beverly Swamp, a high-order forested temperate wetland near Hamilton, Ontario was studied during wet (2000) and dry (2001) summer seasons to determine and compare runoff pathways and storage mechanisms in two hydrogeomorphically different sub-basins. A channelized (Fletcher Swamp) and an unchannelized (Spencer Swamp) sub-basin were examined. During wet periods, the Fletcher Swamp displayed a consistent interaction between the wetland and stream, resulting in a gaining stream, maintaining a lateral hydrological connection and yielding high runoff ratios. When dry periods dominated, water tables dropped low enough in the Fletcher Swamp such that water moved from the stream channel into the underlying substrate, creating a lateral hydrological disconnection between the stream and wetland. This disconnection reduced runoff ratios during storm events. The Spencer Swamp, with no well-defined stream channel, relied on overland flow and a longitudinal hydrological connection to deliver water to the sub-basin outflow. This connection was maintained in the wet season (2000) but ceased in the dry season (2001) despite consistent inputs from an upstream reservoir. Available depression storage prevented overland flow during 2001, and although a shallow groundwater flux maintained discharge for a period of time, drought conditions led to zero discharge at the sub-basin outflow in mid-summer. Runoff ratios were reduced dramatically from season to season as a result of this disconnection. Implications of these hydrogeomorphic runoff controls on wetland biogeochemistry are discussed.