In this study a new chironomid deformity-based index, the ETSI,was developed. The development of the index followed a five-stepempirical approach. The index was found to be an appropriateand useful tool for riverine ecosystem biomonitoring, distinguish-ing between the ecological conditions at the sampling sites in thepresent study, and it can easily be used in other riverine ecosys-tems both locally and internationally. A key limitation of the indexis that it has been developed based on a single river and althoughit can be used in other rivers in its present form, further research isnecessary for its refinement and standardisation. A potential sourceof error is that apart from the pollution tolerance of a taxon, whichmay influence its expression of deformities, other factors such asniche inhabited and history of previous exposure may also influ-ence expression of deformities. These were not taken into accountin the present study and deserve investigations in a future study.Overall, the index proved sensitive to pollution and the species-level responses of chironomids shed light on the importance ofidentifying species in freshwater biomonitoring and in conserva-tion planning and management.