The flow regime is recognized as a key factor determining biological and physical processes and characteristics in rivers. Because of this, there is interest in classification and regionalization of rivers in order to delineate patterns in flow regime character at landscape scales. The River Environment Classification (REC) is an a priori mapped classification of rivers. The REC is based on a hierarchical model of 'controlling factors', which are assumed to be the dominant causes of variation in physical and biological characteristics of rivers at a variety of spatial scales. The first and second levels of the REC are based on climate and topography and are expected to discriminate rivers according to differences in their flow regimes. Classes are assigned to individual 'sections' of the river network based on categorical description of the climate and topography of each section's unique watershed. This paper describes a test of the REC's ability to explain variation in hydrological character of rivers. Flows that were measured continuously at 335 sites distributed throughout New Zealand were summarized by 13 flow variables and were classified using the REC. Principal components analysis was used to show that the REC classes have distinctive flow regime characteristics. We quantified the classification strength (i.e. the extent to which the mean between-class inter-site dissimilarity exceeds the mean within-class inter-site hydrological dissimilarity) of the REC based on the 13 flow variables. The classification strength of the REC was greater than for two existing regionalizations and a classification that is based on climate, but which does not account for the river network. We attribute the increased classification strength of the REC to its explicit consideration of the causes of spatial variation in flow regimes among rivers and its representation of the network spatial structure of rivers. Copyright (c) 2005 John Wiley & Sons, Ltd.