Developing an optimal river typology for biological elements within the Water Framework Directive.

The Water Framework Directive requires a river classification based on environmental variables (a typology) to be created as a structure for reporting ecological status. A single permutation procedure, utilising the same variables repeatedly but with different categorical divisions, enabled both the choice of variables and the boundary divisions for these variables to be optimised simultaneously in the development of the typology. This, in addition to a data set which appropriately combined different biological elements, enabled a typology to be developed which was far more effective than a System A, CCA-derived or expert opinion-based typology in segregating communities. This optimal typology could be used to improve the performance of ecological quality assessment methods.