ALLLUVIAL SEDIMENT SINKS AND SOURCES TO THE LOWER GREAT LAKES: THE GEOMORPHIC ROLE OF RIVERS IN PENINSULAR SOUTHERN ONTARIO

Fluvial processes are responsible for a substantial proportion of sediment yield to the lower Great Lakes of eastern North America. Fluvial sediments from the relatively small watersheds of peninsular southern Ontario (< 5000 km²) are derived from glacial legacy sediments that have been re-worked by Holocene river erosion and floodplain development. This palimpsest creates challenges for identifying both contemporary and Holocene sediment sources and for developing detailed sediment budgets. Twenty four years (1970-1993) of daily suspended sediment observations from 30+ watersheds provide some evidence of the temporal and spatial variability of thesesediment sources. The spring nival melt dominates the flood record and yields the majority of annual sediment load. Multiple clock-wise hysteresis loops indicate a continuous supply of sediment during the early season snowmelt period. As the runoff season progresses, there is a tendency towards counter-clockwise loops suggesting delayed inputs from river bank erosion. Glacial sediments exposed in tall moraine embankments are particularly important sources in entrenched river reaches. The overall effects of Holocene river entrenchment and historical land use changes are strong contributing factors in sediment budget exercises.

Alluvial floodplains in these watersheds act as both important sinks and sources for sediments. Forty two ¹⁴C dates extracted from the contact between basal lag gravels and the overlying accreted floodplain sediments provide some insight to the timing and role of floodplain sediment sources. Net floodplain accumulation rates prior to 1500 cal BP were nearly constant at around 0.18 mm/yr. Increases began after that and were an order of magnitude higher by 1700 AD. Land clearance during and following European settlement resulted in rates as high as 9 mm/yr. The accumulation chronology, combined with interpretations of the style of floodplain accretionary processes at selected sites, suggests that the average residence time of alluvial sediments is < 3000 years.