WATER EXCHANGES IN A SHORELINE TYPHA MARSH ON LAKE-ONTARIO

A Typha-dominated marsh bordering Lake Ontario had a strong hydraulic connection with the lake during spring 1991, when the water level was more than 10-20 cm above the marsh surface. During this period, water-level changes were dominated by the general decline of Lake Ontario, by short-term fluctuations caused by seiche activity (1.7 h periodicity) and by setup (diurnal). These were transmitted into the marsh at 65-135 m h-1. The water surface profile was generally flat over the marsh during this period but water depth was uneven owing to peat surface adjustment. High buoyancy of the surface in the 30 m margin adjacent to the lake minimized water depth there. By 21 June, water depth at the margin was essentially zero, but elsewhere was 0.07-0.1 m; all water exchanges between the marsh and lake were thereafter constrained to relatively slow subsurface flow (approximately 10(-3)-10(-4) m h-1). Short-term fluctuations caused by lake-level variability were then restricted to the margin. The Lake Ontario water level then dropped faster than that on the marsh, as did the water table on the adjacent mineral terrain, resulting in an upwardly convex water surface profile on the marsh, where water flow was away from the marsh. Between 14 June and 6 August, 116 mm of rain and 264 mm of evapotranspiration resulted in 30 mm of lateral water loss from the marsh centre; only 1 mm was lost at the marsh margin, reflecting the frequent reversals of hydraulic gradient there.