GLACIAL CONDITIONING OF STREAM GEOMORPHOLOGY IN LOW-RELIEF VERSUS MOUNTAIN LANDSCAPES

The imprint of late Pleistocene glaciation on river systems is an essential theme in Canadian geomorphology. Existing ideas about glacial legacy effects tend to focus on mountainous environments, which are different from the low-relief physiography of the Laurentian Great Lakes region. This study investigates river landforms in southern Ontario to develop an adapted fluvial landscape model for low-relief settings, reflecting glacial legacy effects and post-glacial fluvial processes. The analysis is based on an original dataset, including basic channel observations from over 500 field sites, alluvial floodplain properties from 109 sites, and published physiographic mapping from digital sources.

Glacial signatures are evident in river profiles extracted from a digital elevation model for 22 river catchments in southern Ontario. Stream power and slope–area analysis stratify river slopes by glacial landform types and demonstrate significant differences between rivers incised into glacial moraines versus plains. A stream length–gradient index provides a relative measure of how river profiles are oversteepened or understeepened by glacial landforms relative to a theoretical graded profile. River landforms can be organized based on four first-order alluvial floodplain types tested using a k-means clustering analysis. Predictive variables are explored using PCA and discriminant analysis, producing two principal components: (1) stream power-resistance and (2) floodplain sedimentology (or floodplain sand equivalent, FSE).

As compared to literature on mountain landscapes, this study demonstrates glacial conditioning of river and stream landforms for a low-relief setting. At the source, these river catchments lack steep headwaters and are not coupled with hillslope sediment production. At the outlets, the river channels (with drainage areas typically <<10⁴ km²) have experienced dramatic fluctuations in lake baselevel associated with post-glacial isostatic and hydrological adjustments over the Holocene. Depositional glacial features in the landscape govern patterns of sediment supply and thus the spatial arrangement of fluvial landforms. In particular, variations in alluvial floodplain deposits are strongly influenced by the relative availability of glacially inherited cobble and sand.