Chiang Mai Journal of Science

       Present study was carried out to evaluate quality of water in eight selected sites in the head waters and down streams of Nilwala River Basin, in Matara district. Four sites (St1, St2, St3, st4) are located in the upper catchment area while the rest of the sites (St5, St6, St7 and St8) are in middle and lower catchment, convering 68 km along the length of the river. Macroinvertebrates were sampled using surber sampler and by kick sampling, from September 1999 to February 2000. Macroinvertebrates collected and sorted out were identified up to family level. Taxa or family richness, modified family biotic index, Average Score Per Taxa (ASPT) values based on Biological Monitoring Working Party (BMWP) adopted for India, number of Ephemeroptera, Plecoptera and Trichoptera (EPT) families, ratio of EPT and Chironomids abundance, percent contribution of dominant family were calculated using collected data on macroinvertebrates. Taxa or family richness and number of EPT families were highest in head stream sites. Highest BMWP and ASPT values showed sites in head streams, while these values were lower in sites St7 and St8. Physicochemical parameters such as conductivity and biological Oxygen demand were highest in sites located in down streams, while these values were lower in sites in head stream. Values calculated from biological monitoring methods have revealed the occurrence of high quality water in head stream sites and low quality water in down stream sites.