Late Holocene aggradation in the lower Humber River valley, Toronto, Ontario

Abstract

Alluvial fills are common in the lower reaches of rivers along the western shore of Lake Ontario. The Humber River floodplain at Toronto is underlain by a 2.5 km long wedge of alluvium that thins upstream from Lake Ontario. Floodplain sediments were studied for their lithology, 14C age, and fossil pollen. On the levees, grey clay is overlain by oxidized silt and sand. Sediment cores from two flood ponds grade upward from gravel, sand, and silt, to silty marl, mineral peat, and clay, to heterogeneous silt and sand. Base-level (Lake Ontario) rise directly controlled aggradation between 6500 and 1800 years ago, after which time base level no longer directly controlled aggradation because levees had emerged alongside the channel and reduced the supply of sediment to the floodplain. For the past 150 years, upstream forest clearance and urbanization increased sediment input to the floodplain, broadened the levees, and filled the flood ponds.Average flood-pond aggradation rates were estimated from seven 14C dates; these rates declined from 65 cm/100 years between 6500 and 3800 years ago, to 47 cm/100 years between 3800 and 3400 years ago, to 26 cm/100 years between 3400 and 1800 years ago. These rates reflect contemporaneous lake-level rise. Between 1800 and 150 years ago, the average aggradation rate declined below the estimated rate of lake-level rise to 14 cm/100 years. Since then, the average aggradation rate has increased tenfold to 140 cm/100 years, surpassing the historic rate of lake-level rise of 23 cm/100 years. Fossil pollen from the flood ponds reflects local flood plain and regional upland vegetation during the past 4000 years.