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A channel evolution model to guide sustainable urban stream restoration

Channel evolution models (CEMs) are used to structure the interpretation of observed channel morphology to support long-term restoration of these systems. However, channels reflect the variety of their watersheds' climatological, ecological and physiographic contexts, and so no single CEM can be truly ‘global’. Unrecognised differences between the assumptions and the reality of evolutionary trajectories of particular streams can subsequently lead to restoration actions that neither fully achieve their intended objectives nor successfully self-maintain even limited improvements. Despite the daunting variety of biophysical settings, however, urbanisation imposes distinctive, homogenising influences on virtually all watercourses, suggesting that even a relatively small set of evolutionary pathways can embrace much of the diversity of critical watershed drivers on urban channels. CEMs describing single-thread channel response to incision are most common in the published literature, but not every urban disturbance yields this classic sequence, initiated by excess transport capacity followed by incision, bank erosion, widening and ultimately a lowered re-equilibrated channel. A comprehensive urban CEM must also include responses under less common (but locally ubiquitous) conditions, such as excess sediment relative to transport capacity (the ‘inverse’ of the classic CEM), imposed constraints on vertical and/or lateral adjustment, and multi-thread channels or those influenced by instream or riparian vegetation. An urban CEM also requires a hierarchical framework that acknowledges fundamental differences in the process drivers within any given watershed, because a single observation of channel form can rarely pinpoint the context or evolutionary trajectory of every stream. We present a geomorphic framework for diagnosing and predicting the evolution of urban streams, potentially guiding the selection of restoration targets that are achievable within an urban context and sustainable without ongoing maintenance.