Can changes in geomorphic responses to urbanisation be predicted from stormwater outfalls?

Stormwater drainage is a primary pathway through which urbanisation degrades physical channel form and ecologically relevant in-stream attributes, such as the presence of large wood. However, there is a gap in the literature regarding methods or study designs that effectively isolate the specific effect of stormwater from those of catchment context, geology and other geomorphic controls. This study examines how stream geomorphology, characterised through variables such as bankfull cross-sectional area, pool-to-riffle spacing and large wood, relates to stormwater drainage inputs from urban areas. To achieve this, we employed historical data reviews, GIS techniques and field observations to assess morphological changes along a stream channel (Toomuc Creek, Melbourne, Australia), focusing on the differences between upstream and downstream of stormwater inputs. We hypothesised that: (i) stream bankfull cross-sectional area increases with catchment urbanisation and (ii) significant differences in geomorphic response variables exist between upstream and downstream of stormwater outfalls. However, contrary to our expectations, stream bankfull cross-sectional area did not follow a systematic downstream increase with catchment urbanisation, largely due to historical land-use practices (e.g. vegetation clearing) and channel stabilisation interventions (e.g. grade control structures, rock lining). Nonetheless, some outfall locations did show clear evidence of disturbance, confirming that widening, deepening and a combination of both occur locally and in a spatially discontinuous manner. These findings highlight two key directions for future research. Firstly, to properly isolate urban influences on stream geomorphic adjustments, controlled study designs should prioritise sites with minimal historical disturbance and no hardpoint interventions. Secondly, and perhaps more importantly, the influence of past channel corridor management and channel evolution on contemporary geomorphic responses needs to be specifically studied in urban settings. Understanding the complex interplay between urbanisation, channel geomorphology, historical land-use and in-stream features is vital for developing more accurate predictive models to mitigate urban stormwater impacts.