Channels in the Making – An Appraisal of Channel Evolution Models

Over the past 30 years, fluvial geomorphologists have sought to understand the historical trajectory of channel adjustments after disturbance by developing channel evolution models (CEMs). These models use a combination of quantitative data and qualitative indicators to describe the spatial progression of channel evolution. This article surveys the historical development of CEMs, how they function, and what types of CEMs are best suited for different geomorphic settings. It describes the logic underlying classic CEMs, and their subsequent application to predict channel response following dam removal. After this, it recounts the emergence of multi‐pathway CEMs, which have been used to capture the evolutionary trajectories in complex fluvial systems (e.g., anastomosing rivers). Lastly, it argues that a state‐and‐transition framework offers an appropriate modeling template to document channel evolution in a wide variety of fluvial systems, including those originally modeled using classic CEMs. A state‐and‐transition approach sees rivers as composed of interrelated process and form (morphological) states. They identify under what conditions a river channel would undergo a transition from one formal state to another (e.g., from a single‐threaded to braided planform). They can also be used to understand the form‐process dynamics underlying subtle morphological transitions that produce changes in a river's biogeomorphological structure without triggering wholesale river metamorphosis.