MORPHOLOGICAL RESPONSE OF A SEMI-ALLUVIAL CHANNEL TO URBANIZATION: HIGHLAND CREEK, TORONTO, ONTARIO

Over the past 30-40 years many drainage basins in the Toronto area have undergone urbanization. Compared to non-urban catchments, the highly urbanized basins show upward trends in maximum annual mean daily flows, and even greater increases in maximum instantaneous flows. Of these urban basins, Highland Creek shows the largest increases in maximum instantaneous discharge beginning in the mid to late 1960s. Flows of a given recurrence interval are now up to 5 times larger than in the pre-urban state. Urban land cover accounts for 85% of the total drainage area of 102 km2 of Highland Creek. Its headwaters originate in a low-relief glacial till upland, the creek then becomes more incised and confined further downstream towards Lake Ontario. Alluvial cover, typically coarse gravel, is thin or absent in many reaches, exposing till and other glacial deposits in the bed and banks. The headwaters, are completely artificial and are typically straight channels with trapezoidal cross-sections and either grass banks or lining of concrete or gabion baskets. In the downstream reaches the channel remains largely natural but the frequent large flows and extreme stream power associated with them have caused chronic and acute erosion problems beginning at the time of urbanization, resulting in engineering responses to reduce infrastructure loss that may, in the long term, exacerbate the problems caused by this extremely energetic channel. Until now the geomorphic response to urbanization has not been systematically documented for this extreme case. Historical air photo analysis provides data on trends in channel width, channel pattern and bank migration over the past 40 years and an understanding of the timing and magnitude of channel response in this semi-alluvial river system.