Chilika is a shallow, brackish-water, lake on the eastern coast of Orissa. This lake which is the largest in the subcontinent, is roughly pear shaped and varies in its extent in the dry and wet seasons between about 560 and 1100square kilometres and is about 32 km wide at its broadest. It has been formed due to the silting action of the Mahanadi river, which drains into the northern end of the lake, and the northerly currents in the Bay of Bengal, which have formed a sandbar along the eastern shore leading to the formation of a shallow lagoon.
The lake is divided into an outer channel with a narrow neck leading into the sea and the main body of the lake with a muddy bottom rich in organic matter. Nalaban, one of the biggest islands, is a 10km marsh which is submerged during the four or five monsoon months, but is a major feeding and roosting habitat during winter for over a hundred species of migratory birds, which arrive in October from their temperate breeding grounds.
These birds include a few species of flamingos, over a dozen species of ducks, and several migratory birds. Due to the varying degrees of salinity in different parts of the lake, the fauna is interestingly diverse, with a variety of animals adapting to a marine or riverine existence to survive in different parts of the lake. Animal life recorded in the lake ranges from planktonic microorganisms to a vast variety of fish, which together sustain the migratory birds population in winter. A few estuarine turtles and snakes are found here along with species of dolphins, otters and several rodents, bats and sloth bears on the hills. Around 158 species of fishes and prawns have so far been recorded. In 1917, a rare reptile, the limbless skink (a type of lizard) was discovered for the first time in the loose soil of the Barakudia island.