The operational management of water quality requires a methodology that can provide precise information on cycles and trends in water quality in an objective and reproducible manner. Such information can be provided by the adoption of a water quality indexing system. The continuous scale afforded by a water quality index allows changes in river water quality to be highlighted. At the same time the sub-division of this scale into a series of water quality and water use categories provides an easy means of relating information to government and public. The development of four independent water quality indices (WQIs) is outlined. These have been applied to data for a number of UK river reaches. The results of these applications indicate the utility of these indices in the classification of water quality and the monitoring of ecosystem change.