Quasi‐equilibrium conditions of urban gravel‐bed stream channels in southern Ontario,Canada

Urban gravel‐bed stream channels in southern Ontario, Canada identified to be in a state of quasi‐equilibrium have been studied over the past 15 years and compared against rural gravel bed stream channels of the same hydrophysiographic region. Bankfull width and depth versus bankfull discharge were not found to increase as a function of increasing urbanization as has been found in many other studies. The observed annual frequency of bankfull discharge was typically less than a 1‐year return period with many sites ranging between 2 and 18 bankfull events per year with higher intensity and shorter duration urban flood responses, which further identified significant limitations in using annual peak discharge methods for predicting morphological forming flows in urban watersheds. The cumulative volume of bankfull and larger flood events from the urban stream channels were very similar to the same annual event volumes in the rural comparison study reaches. Bed material supply was found to decrease with increasing urbanization and the reduction in bed material supply appears to be off‐set by the smaller bankfull channel width, depth and access to floodplains during large flood events. Field evidence may also suggest a even greater reduction in channel width trajectory, relative to the rural setting, with expansive floodplains to maintain quasi‐equilibrium conditions as bed material supply continues to decrease with increased anthropogenic activity. Copyright © 2010 John Wiley & Sons, Ltd.