How to quantify the hydromorphological alteration of a river, ie its deviation from a “natural river”, in balance with its catchment and capable of selfregulation? Hypothesis: The difference between the measured morphological parameters of the river and predicted values for a given catchment size is an estimation of the hydromorphological alteration of the river. Two steps: 1. Definition of reference models from a large amount of « non-altered » rivers (« natural » rivers). 2. Quantification of the deviation from it for altered rivers. -Quantification of hydromorphological alteration? -Relationship between hydromorphological alteration and ecological quality