GEOMORPHIC RESPONSES TO CHANGES IN INSTREAM FLOWS: THE FLOW‐CHANNEL FITNESS MODEL

The flow‐channel fitness model is a conceptual and practical model for predicting the qualitative response of alluvial channels to modifications of flow regimes. ‘Fitness’ refers to the size of channels compared with the flows they convey, with the terminology derived from traditional geomorphic concepts of overfit and underfit streams. The qualitative predictions refer to whether channels experience aggradation, degradation or relative stability, and whether aggradation or degradation is dominated by width or depth. The model is based on transitions among seven possible fitness states, triggered by key thresholds of sediment supply versus transport capacity and shear stress versus shear strength, and requires that potential changes in sediment supply and water surface or energy‐grade slope also be accounted for. The fitness approach can be used where only relative values and changes are known, as is illustrated in three example applications from Texas. The flow‐channel fitness model synthesizes key elements from several existing approaches to predicting geomorphic responses to changes in flow and is intended to augment rather than replace quantitative approaches, providing a predictive tool where the data requirements and assumptions for quantitative models cannot be fully met. Copyright © 2012 John Wiley & Sons, Ltd.