In Assam State, 46 numbers of river stretches have been assessed for bio-mapping of 14 rivers and tributaries. Out of 46 rivers stretches, 7 river stretches belonging to 5 major rivers namely, River Buridihing at Miaow, River Disang at Dillighat, River Subansiri at Gerukamukh, Dhulumukh, in North Lakhimpur, River Brahmaputra at Tinsukia and River Malidor at New Malidor were indicating the clean water quality (Class `A’). Nine river stretches of total 8 numbers of Rivers such as River Jhanji at Rajabari, River Dhansiri at Nambar, River Brahmaputra at Dibrugarh, River Subansiri at, River Borak at Fuler Tal, River Boginadi at Lakhimpur District, River Dikrong at Bandardua and River Disang at Desangmukh were slightly polluted (Class `B’). Moderate pollution in water quality of Class `C’ was observed in 26 number of river stretches of 9 rivers and tributaries like River Buridihing, River Jhanji, River Dhansiri, River Subansiri, Borak River, River Brahmaputra, River Jia-Bharali and Ellenga Beel Pond System at various locations and different seasons (Table 9). Water quality of Ellenga Beel System Pond at Belguri and Morigaon in May and December was found polluted. Therefore, 4 river stretches of 3 number of water bodies including River Ranganadi and River Disang at Namrup Industrial Township were lying in Class `D’ of highly polluted water quality. None of the water bodies showed severe pollution Class `E’ of water quality.

River Brahmaputra from its origin to its downstream reaches exhibits different classes of water quality at nine locations (A to I) as shown in Fig. 1. River Brahmaputra water quality is clean only in its upstream stretches at Saikowaghat in Tinsukia. At downstream of this location River Brahmaputra gets slightly polluted at Maizan in Dibrugarh. Rest of the seven locations from downstream Dibrugarh, the water quality of River Brahmaputra remains moderately polluted in Class `C’ of water quality. These locations are at Sibsagar, Jorhat, Golaghat, Silighat, Guwahati, Jogighopa and Dhubri. At Dhubri, the water quality of River Brahmaputra gets diluted by the confluence of several tributaries and thus regains its water quality from moderate to slightly polluted water quality class `B’ specially during month of December, 2003.

The taxonomic composition of Benthic macro-invertebrates collected from clean water quality stretches (Class `A’) of Rivers (Table 7) in Assam supported 20 number of families whereas slightly polluted (Class `B’) river stretches were indicated by 19 numbers of families (Table 8). Maximum of 24 numbers of families showed moderate pollution (Class `C’) of water quality (Table 9). Highly polluted (Class `D’) water quality inhabited only 10 numbers of families (Table 10). 80% of Arthropods dominated the clean water quality stretches. Their population gradually reduced with the increase in pollution of water quality of Class `B, C & D. Molluscs and Annelids were maximum in highly polluted water of Class `D’. No benthic animals belonging to Class `E’ were observed (Table 12).

A comparison of Bio-map (Fig. 1 & 2) of year 2003 & 2004 indicates no change in water quality at most of the locations on River Brahmaputra & its tributaries except for few locations. For example, the biological water quality of River Disang at Lalpagari Ghat at Namrup Industrial Township has changed, from Moderate Pollution (Class `C’) in 2003 to Heavy Pollution (Class `D’) in year 2004. Similarly, the biological water quality of River Brahmaputra at Nagagholli, Maizan in Dibrugarh was slightly polluted in year 2003 and degraded to Heavy Pollution (Class `D’) in year 2004.

Water quality of River Brahmaputra at Nagagholli Maizan in Dibrugarh was slightly polluted (Class `B’) in 2003 and shifted to Moderate Pollution (Class `C’) in 2004. On the other hand, water quality of Ellenga Beel System Pond at Jagiroad, Belguri indicates improvement in year 2004 indicating Moderate Pollution and upgrading from Heavy Pollution during year 2003. Dhubri Station is the last sampling location on River Brahmaputra, which depicts the overall water quality of River Brahmaputra at downstream of Assam State. The biological water quality of River Brahmaputra improved to slight pollution (Class `D’) in 2004 compared to Moderate Pollution (Class `C’) in year 2003. Following table indicates the total number of families of benthic macro-invertebrates collected during year 2003 and 2004:

An actual water quality assessment relies on collection of mature colonization of benthic macro-invertebrates. Quite often the change in observations of water quality could be due to insufficient biological sampling.