Different national assessment concepts impede the harmonization of river quality classifications using macrophytes in Europe. This study describes a procedure to identify similarities between the national methods for ecological quality assessment of Austria, Belgium (Flanders and Wallonia), France, Germany, Great Britain and Poland. Based on an international data set covering three European stream types we identified sites commonly assessed as high status by most methods. A mean index derived from averaging the national assessment results per stream site was then correlated with the abundance of each macrophyte taxon. We defined common macrophyte indicator scores using these correlation coefficients. This enabled the description of type-specific macrophyte communities under near-natural and degraded conditions, and the development of a common metric (mICM) that was correlated with all national methods. The weaker relations of the Flemish and German methods were improved by adjusting national indicator scores of selected macrophyte taxa that deviated from the common indicator scores. The analysis of common high status sites provided mICM reference values. This study offers a general approach to harmonize the national assessment methods for biological elements of any water category.