Three Quarters Alluvial: Constraining the Semi-Alluvial River Condition in the Glacial Landscape of Southern Ontario

English Abstract:

Rivers and streams of southern Ontario are carved into a diverse landscape of glacial landforms and deposits of varying thickness. Semi-alluvial channels are incised into non-alluvial boundary materials of glacial clay or exposed bedrock, with varying amounts of alluvial cover. In this paper, we question the degree to which semi-alluvial conditions are a “natural” state for fluvial systems in the low-relief glacial landscape of southern Ontario, and the degree to which intermittent contact with glacial materials modifies alluvial channel processes. To examine these questions, we consider the frequency and extent of semi-alluvial channels in the region, and the potential effects of post-settlement deforestation and urban land use history on their distribution.

An exploration of hundreds of field sites across peninsular southern Ontario provides evidence that reaches with extensive non-alluvial boundaries occupy only a small proportion of drainage networks in the region. For this study, field observations of semi-alluvial and alluvial reaches are organized within a recently published classification framework for alluvial floodplain types. Many river reaches have thick alluvial cover, with very little evidence of exposed glacial materials (or bedrock). Where alluvial channels intermittently come into contact with glacial materials, these glacial beds and bluffs are important sources of sediment, but they only impose local controls on channel behaviour. The morphodynamics of channels in intermittent contact with glacial boundaries are likely to be very similar to that of alluvial channels. Channels with thin alluvial cover and extensive glacial materials exposed on the bed and banks are often located on oversteepened tributary reaches and/or in watersheds impacted by urban development. The implications for channel morphodynamics and floodplain accretion processes in these glacial boundary reaches are still poorly understood.