Original ArticlesApplication of lithotopo units for automatic classification of rivers: Concept, development and validation

River classification is one of the recommendations of the European Water Framework Directive 2000/60/EC, which establishes that classifications should be carried out according to different variables hierarchically organized from a smaller to a larger scale. We suggest incorporating into the Directive's hierarchical system a geoecological unit (lithotopo unit) that discriminates rivers with similar geomorphological features and ecological funtionality. The lithotopo units are not an alternative to the Directive typology, they are a complement intended to improve it.Our method is divided into two stages, the first focused on the development of LTUs and the second on their validation. We applied the concept of lithotopo units to a 30,000 km2 region in the NW of the Iberian Peninsula (Spain) using a Geographic Information System and field work. Seven kinds of lithotopo units were identified for the study area, each with its own geomorphological processes and dynamics, and, as a consequence, particular associated habitats. Cartographic validation was done through the analysis of 122 sample sites distributed in eight basins. Of the five validation variables originally employed, specific stream power and median grain size are the two that yielded the best results. Each kind of lithotopo unit displays a range of values of specific stream power and median grain size that is internally homogeneous but different from that of the other units. The methodology thus produced, which can be applied to other regions, is transparent, objective and quantitative.