Channel widening taking place during large flood events can be substantial in mountain rivers, with consequent great potential damages to infrastructures and buildings. The purpose of this work is twofold: i) to provide a quantitative assessment of geomorphic effects of an extreme flood event (recurrence interval > 100 years); ii) to test on this study case a new hydromorphological methodological framework (IDRAIM) developed to guide river corridor planning and management. As to the first objective, field surveys were integrated with remote sensing, GIS and statistical analyses for a flood event occurred in 2011 in Northwestern Italy. Channel widening ratios (width after / width before the flood) were calculated and then correlated with different controlling factors, and envelope relationships were then obtained. The tool of the IDRAIM framework used for the second objective was the Event Dynamics Classification (EDC) applied to selected study reaches, whose widening ratios turned out to correspond well with the EDC classes. Based on the results obtained, a practical procedure for predicting the expected widening is finally proposed.