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. The main goal of this study is to examine how stream geomorphology, characterised through variables such as bankfull cross-sectional area, relates to stormwater drainage inputs from urban areas. To achieve this, we employed a combination of historical data reviews, GIS techniques, and field observations to assess morphological changes along a stream channel (Toomuc creek, Melbourne, Australia), with a particular focus on the differences between upstream and downstream of stormwater inputs. Two key hypotheses were tested: (i) stream bankfull cross-sectional area increases with catchment urbanisation and (ii) significant differences in geomorphic attributes or response variables exist between upstream and downstream of stormwater outfalls. However, contrary to our expectations, stream channel capacity or cross-sectional area did not follow a systematic downstream increase with catchment urbanisation due to arrays of factors. 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 point to two important 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 conditioning influence of past channel corridors management and channel evolution on contemporary geomorphic responses needs to be specifically studied in urban settings. Understanding the complex interplay between urbanisation, channel morphology, historical land-use context, and in-stream features is vital for developing more accurate predictive models that could inform effective and broad management strategies.