Change in event-scale hydrologic response in two urbanizing watersheds of the Great Lakes St Lawrence Basin 1969–2010

[Display omitted] •High temporal resolution hydrologic records indicate chronic flow regime disruption.•Total seasonal discharge in two urbanizing watersheds increased 45% in 4 decades.•Peak rain event flows increased almost 0.1m3s−1yr−1 over four decades.•Available data indicate urban land use explains same statistical variation as year.•Quantified loss of hydrologic stationarity is attributable to urbanization. The cumulative impacts of urban land use change on natural stream flow regimes and lotic ecosystems are poorly understood, and generally under-studied within the hydrologic sciences literature. Moreover, flow assessments using daily or monthly flows cannot adequately characterize long-term trends in event-scale flow dynamics in urbanizing watersheds. Accordingly, we analyzed high temporal resolution (15-min flows) growing season discharge records for two urbanizing watersheds in Canada’s Great Lakes Basin, the Don and Humber, over a 42-year period. Results show that total discharge between May 26th and November 15th in the mainstem rivers has increased by about 45%, independent of total rainfall depth, over four decades. Peak rain event flow rates have increased by almost 0.1m3s−1yr−1 in both watersheds and event flow variability has increased two-fold in the Don and almost five-fold in the Humber. In the Don, the ratio of rising limb event flows to median flow (for the period May 26 to November 15) increased from 1.5 in the 1970’s to 2.3 in the 2000’s. A similar comparison of ratios in the Humber showed similar results, with higher variation in flow response Rising limb event flow acceleration increased 2-fold over 4 decades in the Don and slightly more in the Humber. This study provides a new understanding of the changes in event-scale flow regime dynamics associated with over four decades of intensive urbanization, including increased magnitude of rising limb flows and flow acceleration, and systematic increases in the variability of peak discharges. Overall, our analysis demonstrates marked alteration in total and event flow regimes resulting in chronic perturbation of stream flows. The results demonstrate an important application of long-term, high temporal resolution hydrological records. Furthermore, we quantify the degree to which hydrologic stationarity within the Don and Humber watersheds has been compromised over four decades, during a period prior to detectable climate-induced changes in rainfall patterns.