A 9000-YEAR RECORD OF VERTICAL AND LATERAL ACCRETION ON THE FLOODPLAIN OF THE LOWER THAMES RIVER, SOUTHWESTERN ONTARIO, CANADA

Results of ground-penetrating radar and grain size analysis of sediments from three, 5-7 m-long cores generally support a classic, fining-up point bar model of floodplain development on a reach of the lower Thames River, a single-thread, meandering channel that flows southwest into Lake St Clair (bankfull discharge = 411 m3s-1). Four radiocarbon dates on organic material recovered from cores indicate that floodplain development in this reach occurred over at least the past 9000 years.

These results are not typical for southern Ontario channels. Previous studies have shown that other rivers in southern Ontario tend to have low-energy, cohesive-bank channels with floodplains strongly dominated by vertical accretion.

The distribution of culturally diagnostic archaeological artifacts on the floodplain surface dating back to the Middle Archaic period, but mostly confined to the Early Woodland period or later, suggests that human settlement of the floodplain was restricted to the later Holocene. The observation that a Middle Archaic site component is located in what should be a younger part of the floodplain challenges the notion that a simple lateral channel migration occurred with orderly (channelward) emplacement of sediment by the river. It causes us to consider the possibility that the channel was anabranching in this reach of the river.