ABSTRACT
Urban rivers and streams, collectively 'urban watercourses', play a unique role in the ecology of urban environments. This chapter focuses on free-flowing urban rivers and streams and, in particular, the hydrological regime that determines much of their attributes and behavior. Recent research and practice have addressed both of these broad settings, which for convenience are distinguished here as 'urban streams' and 'urban rivers'. Nearly all of the impairments described in urban streams, except those that physically alter the channel itself, result from one underlying cause: loss of the water-retaining function of the soil and vegetation in the urban landscape. Despite the obvious consequences for natural channel form and biotic communities that result from putting small streams into large pipes, the most widespread and pervasive impacts to urban streams arise from the indirect effects of urban stormwater runoff. In urban streams these are normally expressed as instantaneous measurements of concentration, with threshold values intended to highlight particularly problematic levels.
