The word estuary refers to a semienclosed body of water such as river mouth or a coastal bay, where the salinity is intermediate between salt and freshwater and where tidal action is an important physical regulator and energy subsidy. These are not just transition zones but have ecological characteristics of their own (they are true ecotones). The major estuarine systems in India pertain to river Ganga in West Bengal, Mahanadi in Orissa, Godavari and Krishna in Andhra Pradesh Cauvery in Tamil Nadu and the Narmada and Tapti in Gujarat.