Temporal and spatial dynamics of urban stream geomorphology: The role of effective imperviousness and development age

Urbanisation has been widely studied for its impact on stream channel geomorphology, primarily using space-for-time substitution approaches. However, continuous monitoring of urban geomorphic responses, to understand the temporal dynamics and influence of development age, remains underexplored within this framework. In this study, we combined repeated field observations collected 13 years apart with land cover analysis to assess geomorphic change across a gradient of urbanisation. We addressed three research questions: (i) to what extent do space-for-time substitution interpretations align with directly observed changes in stream channel morphology, and are post-study changes in stream channels consistent with earlier observations? (ii) how has the change in the extent of urbanisation over the study period directly influenced observed changes in stream channel morphology? (iii) at what level of urban development is stream incision most severe (i.e., does channel morphology stabilise during the latter stages of urban development)? We found that predictions made through space-for-time substitution in 2011 (i.e. regression models) were largely consistent with relationships observed in 2024 following further urbanisation of the catchments. However, change in geomorphic attributes at each site over the 13-year period were not significantly related to change in effective impervious cover (EI), possibly due to lag in geomorphic responses to ongoing urbanisation, along with the presence of hardpoints such as rock protection in the urbanised streams. Notably, several individual sites did show signs of active channel adjustment; all of these sites had moderate levels of urbanisation (<30 % EI), which had developed relatively recently (in the last 25 years). Severe channel incision was also observed within this range, suggesting a critical window for morphological instability. Whether channels eventually stabilise (without heavy intervention to harden channels) remains uncertain. Overall, the findings reinforce the importance of validating space-for-time substitution approaches through repeated observations to better develop more reliable urban channel evolution and stream management strategies. • Long-term site revisits confirm that space-for-time predictions broadly reflect observed temporal channel changes. • Stream channels do not respond immediately to increasing urbanisation due to ongoing disturbances and delayed responses • Recently developed, moderately urbanised catchments show the greatest channel instability. • Early intervention during this development window may reduce long-term stream degradation.