The Impact of Urbanization on Temporal Changes in Sediment Transport in a Gravel Bed Channel in Southern Ontario, Canada

A field investigation has been undertaken to characterize the event-based bed load transport dynamics of a highly urbanized gravel bed stream. A combination of direct bed load and tracer particle measurements were taken over a 3 year period during which time approximately 30 sediment mobilizing events occurred. Sediment transport measurements were used to calibrate a fractional bed load transport model and combined with hydrometric data which represent four different land use conditions (ranging from rural to highly urbanized) to analyze the differences in discharge magnitude and frequency and its impact on sediment transport. Fractional transport analysis of the bed load measurements indicates that frequent intermediate discharge events can mobilize sand and fine gravel to an approximate equally mobile condition, however, the transport rates at these discharges exhibit greater variability than at discharges above the bankfull discharge. Path lengths of the coarse fraction, measured using tracer clasts, are insensitive to peak discharge, and instead transport at distances less than those reported in other gravel bed channels, which is attributed to the shorter duration discharge events common to urban streams. The magnitude-frequency, analysis reveals that the frequency, time, and volume of competent sediment mobilizing events are increasing with urbanization. Variability in effective discharges suggests that a range of discharges, spanning between frequent, low magnitude events to less frequent, high magnitude events are geomorphically significant. However, trends in the different land use scenarios suggest that urbanization is shifting the geomorphic significance toward more frequent, lower magnitude events. Plain Language Summary Urbanization is known to have impacts on river health, typically resulting in a degraded ecosystem and erosion problems. This article investigates the impact of urbanization on sediment transport, which is known to be a primary factor controlling how rivers evolve over time. Both field measurements and analytical modeling were employed encompassing different approaches to achieve the study objectives. The increases in floods common to urbanization change the way sediment moves in a gravel bed river in Toronto, Ontario, Canada. It is also shown that more (frequent, lower magnitude floods (which are known to increase with urbanization) are becoming more important for controlling how gravel bed rivers are evolving over time.