Vembanad is the longest lake in India, as well as the largest lake in the state of Kerala. The lake has an area of 230 square kilometers and a maximum length of 96.5 km. Spanning several districts in the state of Kerala, it is known as Vembanadu Lake in Kottayam, Vaikom, Changanassery, Punnamada Lake in Alappuzha, Punnappra, Kuttanadu and Kochi Lake in Kochi. Several groups of small islands including Vypin, Mulavukad, Maradu, Udayamperoor, Vallarpadam, and Willingdon Island are located in the Kochi Lake portion. Kochi Port is built around the Willingdon Island and the Vallarpadam island.
Kuttanad, also known as The Rice Bowl of Kerala, has the lowest altitude in India, and is also one of the few places in world where cultivation takes place below sea level. Kuttanad lies on the southern portion of Vembanad.
The Nehru Trophy Boat Race is conducted in a portion of the lake. High levels of pollution have been noticed at certain hotspots of the Vembanad backwaters. The Government of India has identified the Vembanad wetland under the National Wetlands Conservation Programme.
A unique characteristic of the lake is the 1,252 metres (4,108 ft)-long Thanneermukkom salt water barrier constructed as a part of the Kuttanad Development Scheme to prevent tidal action and intrusion of salt water into the Kuttanad low-lands. It is the largest mud
regulator in India and essentially divides the lake into two parts – one with perennial brackish water and the other with fresh water from rivers draining into the lake. This barrier has helped farmers in Kuttanad by freeing the area of salinity and allowing them an
additional crop in the dry season. The Thanneermukkom barrier is located at one of the narrower parts of the Vembanad Lake. Only two-thirds of the original number of gates are opened in July to release flood flow. These gates remain closed until mid-November. The main drawback of the structure has been the loss of opportunity for fish and prawns to migrate upstream, and also an increase in weed growth in the upstream, severely restricting the natural flushing of pollutants. The Thanneermukkom bund has also created ecological problems, primarily, the rampant propagation of the Water Hyacinth in fresh water.
