The impact of the extreme flood in August 2005 (up to 943 m/s discharge, with an approximated 5000 year return period flow) on the channel morphology of the Austrian gravel-bed river Lech (over 49 running kilometers) was investigated in terms of quality and quantity. The aim of this work was to develop a better understanding of the interaction between flood flows and morphodynamic processes. Based on the River Scaling Concept, 14 flood-based morphodynamic processes (10 lateral erosions, 3 sediment relocations und 1 natural dislocation) were determined. The hydrodynamic-numerical model HEC-RAS©, historical maps from the third surveying in the period 1869 to 1887 and aerial photographs from 2000 and the flood in 2005 were used to analyse the morphodynamic processes. The comparison of morphodynamically active and inactive river reaches, based on channel geometry data from 1992, as well as the analysis of the channel geometry data from 1992 and after the flood in 2005, allowed to estimate the ability of various parameters to act as indicators for morphodynamic effectiveness. In that context, it turned out that the parameters Specific Stream Power and Froude-number are good indicators. Moreover, the stability of morphodynamically inactive river reaches as well as the instability of reaches with morphodynamic activity could be demonstrated by performing an analysis of historical maps, applying the regime-equation according to BRAY and a novel approach to estimate the maximum river width (SCHMAUTZ).