ABSTRACT: Urbanization effects on stream-flow and channel morphology in Toronto: the case of Highland Creek.

CGRG Bibliography of Canadian Geomorphology
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Abstract

Many small drainage basins in the Toronto region have undergone rapid urbanization over the past 30-40 years. Stream-flow regimes have shown some dramatic changes in some, but not all cases. Compared to the non-urban catchments, the highly urbanized basins show upward trends in maximum annual daily mean flows but much more obvious increases in instantaneous maximum flows. Highland Creek in Scarborough (east Toronto) shows the largest changes increases` in annual maximum instantaneous discharge. Like many of the streams in the region, Highland Creek originates in a low-relief upland and then becomes more incised and confined further downstream as it approaches the Lake Ontario shoreline. The steep valley sides are cut into glacial deposits and alluvial deposition is very limited within the valleys – many channels have only a thin (or no) alluvial cover (typically coarse gravel) overlying Quaternary deposits. Stream power is very low in the headwaters and peaks in the downstream reaches where gradients are steeper. The headwaters are entirely artificial grass or concrete-lined drainage channels for light industrial areas. The natural reaches further downstream have suffered from chronic erosion problems following local channelization and increases in maximum flood flows, beginning with rapid urbanization in the 1960s and 1970s. Substantial infrastructure is at risk. Channel widening and bend cutoff have occurred in many places in response to large increases in peak stream power and channel incision may have accelerated but channel engineering has mitigated this response. Currently, channel gradients in some reaches are up to 4 times steeper than predicted from regime equations for gravel-bed rivers and widths are lower than predicted. With sufficient sediment supply braiding is possible in some reaches. Local erosion mitigation has been partially successful but major problems persist, requiring research on the morpho-dynamics of these semi-alluvial, incising streams and analysis of the historical changes in channel morphology.

Bibliography of Canadian Geomorphology