Historical Flashiness Index Dynamics in Urbanizing Streams in Southwestern Ontario

Urban-growth increases impervious-surface cover and ‘flashy’ streamflow responses. Research has suggested that flashy streamflow occurs when total impervious area (TIA) approaches and crosses 10% of a watershed. This study examines the spatiotemporal variability in river flashiness of rural/urbanizing watersheds in southwestern Ontario. This research addresses relationships between river flashiness and watershed TIA across 37 watersheds as they approach and/or cross 10% TIA between 1990 and 2017. The Richards-Baker Flashiness Index (RBFI) were calculated using hydrometric-data for each watershed. Watershed TIA was estimated for the years 1990/2000/2010/2017 using land-use-data. Strength and direction of the RBFI-TIA relationship is compared to other watershed characteristics (area, soil type). Results show that spatial variability in RBFI is best explained by soil type rather than TIA. Mann-Kendall trend-analysis revealed 10/37 watersheds exhibited significantly increasing RBFI trends. Watersheds with larger increases in RBFI over the study period had lower urban-growth suggesting factors beyond TIA influencing watershed flashiness.