To develop a biological method in river water quality classification for Nepal is the main aim of the present research. With this goal in mind, 178 sites were sampled including 106 rivers, 5 ponds, 3 lakes, and 3 canals.

This research covers nearly all ecoregions with an altitudinal variation of 80 meters in the south to 3802 meters to the north above sea level and provides important faunistic informations. The biological assessment method adapted for Nepal combines the Central European System and the BMWP/ASPT method used in UK.

The Central European principles governing the saprobic water quality classes is found suitable also for Nepal. Four main water quality classes defined were Ònone to very slightly pollutedÓ (oligosaprobic), Òslightly pollutedÓ (oligosaprobic to beta-mesosaprobic), Òmoderately pollutedÓ (beta-mesosaprobic), Òcritically pollutedÓ (beta-mesosaprobic to polysaprobic) and Òextremely pollutedÓ (polysaprobic). The classes are designated in Roman numericals from I to IV and differentiated in colours such as blue, green, yellow, and red according to the increasing degree of saprobity.

After an extensive application of ten different biotic index and score methods, a new system was adopted for Nepal called as Nepalese Biotic Score and abbreviated as NEPBIOS. NEPBIOS contains the following information: (1) number of family-level indicator organisms added; (2) score values of the family-level indicator organisms modified; (3) higher taxonomic precision is obtained.

In aggregate 82 families of macrozoobenthos are scored. Some genera of mayflies, water beetles, stoneflies, and chironomids are additionally scored. Some important improvement in NEPBIOS is done with some recommendations for satisfactory application of the system.

With the development of this system, the water resources authorities in Nepal have now in their hand a fast and cheap informative instrument useful for water quality monitoring.