The Cauvery river is one of the major rivers of India, and perhaps, physically most remarkable river of the Peninsula (Jayaram, 1981). The river originates at Talakaveri, Kodagu district of Karnataka in the Western Ghats and flows generally south east through Karnataka and Tamil Nadu and across the southern Deccan plateau through the south-eastern lowlands, and finally emptying into the Bay of Bengal. It is considered India's fourth largest river, draining about $9,600 sq. km (Jayaram et al, 1982). Since historical times the river water is extensively utilized for agriculture, fisheries, irrigation and navigation purposes. In recent years they have been further subjected to many multi-purpose hydro-electric and other projects. As a result, a number of old and new barrages, weirs, anicuts or dams, have been constructed across the river. It is considered as the lifeline of the ancient kingdoms and modern cities of south India.

The Cauvery estuary, located along the east coast of India, (Fig. 1) is a part of the Cauvery river basin (Ramanathan et al, 1988). The Cauvery delta begins from the Grand Anicut near Tiruchirapalli. It was constructed more than 1800 years ago by King Karikalcholan. The gross area imigated by the rivers and canals commencing from this point is nearly 5.60 lakh ha spread over the erstwhile Thanjavur district (currently split into Thanjavur, Tiruvarur and Nagapattinam districts) and a portion of Pudukottai district (Sivanappan et al., 2007). The Cauvery River bifurcates into two large branches, the Kollidam (Coleroon) and Cauvery at Grand Anscut (a) dam near Trichirapalli). Cauvery discharges an average of 602 m's at Grand Anaicut into the Cauvery estuary (Ramanathan et al, 1993). Below the Anaicut, the river flows almost in a flat plain, having a gentle slope from west to east. The Kollidam River the main distributary of the Cauvery River is the northern branch from Grand Anaicut and discharges into Bay of Bengal at Palaiyar near Mahendrapalli, Nagapattinam district, Tamil Nadu. The Cauvery River branches off into numerous smaller branches before discharging into the Bay of Bengal Some of the important branches are Arasalar, Vennar, Vattar, Tirumulairaja, Adaparu, Uppanaru, Korai etc. In the present study the Cauvery Estuarine System has been applied to the lower tidal reaches of all branches of Cauvery river along with Kollidam river in the Nagapattinam and Thirurur districts of Tamil Nadu and Karaikal of Pudducherry During the recent years large scale developmental activities, industrialization, construction of check dams for urigation and drinking purposes, increasing pollution, etc have change the scenario of the river system and its aquatic biodiversity, particularly the construction of check dams at every possible branch of the river has drastically reduced the outflow of water into the sea through the estuaries Because of this reason, the estuarine part of the river system has been reduced, which in turn affects the diversity of the fauna of the estuary.

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