CHANGES IN STREAMFLOW REGIME FOLLOWING URBANIZATION IN GREATER TORONTO AREA

The effects of urbanization on streamflow regime in the Greater Toronto Area is being assessed using a paired basin approach comparing changes in regime since the late 1950s on East Humber River, Little Rouge Creek and Highland Creek. Highland Creek urbanized rapidly beginning in the late 1960s and early1970s while the other two nearby watersheds, with similar area, have undergone almost no urban development. Highland Creek is large (88 km-2 at the stream gauge) compared to watersheds analyzed in many other urbanization studies, and yet has over 80% urban land cover. There has been extensive channel straightening and lining in the headwaters and little storm water management. While the rural watersheds show no obvious or significant change in stream flow regime over almost 40 years, Highland Creek has experienced progressive and dramatic changes beginning in the early 1970s. Seasonality of discharge has almost disappeared; numerous flashy large events occur throughout the year and have replaced the historical dominance of the spring melt. Ratio of daily maximum instantaneous:daily mean discharge have increased along with daily runoff:precipitation ratios. Extreme events are now up to 5 times larger than those of pre-urban Highland Creek and rural watersheds. This may be among the most dramatic urbanization effects on streamflow regime documented for a basin this size.