Transforming river morphology over 60 years of urban development

Urban land development causes changes to river channel form and function and to community relationships and visions of rivers.  The story of urbanization of Highland Creek, Toronto, Canada involves the transformation of the 100km² watershed from rural to almost 100% urban land-use over 6-7 decades. The unusually rich documentation of the watershed changes allows for understanding of the long-term trajectory of transformation of the channels based on geomorphic principles. It also shows the evolving institutional responses to managing and redesigning the river channels and valleys, and mitigating risks, which can be seen as part of the urbanization of rivers along with the physical response of the river channels. Urbanization has transformed the main channels to an extremely energetic state more like mountain streams despite the low relief terrain of the Toronto region with total stream power increasing by up to 10 times following urbanization. Measurements from historical air photos for multiple epochs over 60 years show that channels have widened progressively by a factor of 4 or 5, with accompanying changes in planform, triggered, in particular, by two or three large flood events, and consistent with hydraulic geometry predictions. The time trajectory and spatial pattern of changes to morphology of reaches that were free to adjust to the increased stream power are well-predicted by land-cover based predictions of stream power using the SPIN watershed analysis tool (Stream Power Index for Networks) which provides a means of predicting possible future changes as well as explaining historical change.  Increases in specific stream power show potentially large increases in bed material particle mobility consistent with observed morphology changes. Freedom to evolve to a new state is partly the result of institutional decisions and vision to set aside valley land in the 1950s to reduce flood damage. In other places the process of urbanization has involved progressive engineering of the channel, reflecting approaches in 1960s and 70s to protect infrastructure and institute an engineering vision of river control which has eliminated fluvial processes of channel development and adjustment. Recent moves to channel restoration using geomorphic design approaches reflect create novel river morphology. Urbanization of channels does not end with land cover change and its physical hydro-geomorphic effects, nor is there a clear end-point and time-span of response that is often claimed in studies of urban fluvial change. Understanding urbanization as an ongoing process of ‘re-storying’ river channels within a socio-geomorphic system is essential to understanding urban river histories and futures, explaining urban river morphology, and recognising the entwined physical and socio-political power and processes that transform urban rivers.