The Kerala Backwaters are brackish lagoons and lakes which are formed by the action of the waves and the shore currents. Small small low barrier islands are formed at the mouth of that many rivers flowing down from the western ghats range of mountains.
The network of canals include five different large lakes, namely, Ashtamudi kayal, Vembanattu kayal, Sasthamkotta Lake, Akkulam lake, and Pookot Lake and the rainfed rivers of Kerala.
There are many rivers which form the backwaters of Kerala, in the state of Kerala. All of these rivers except three, flows to the west in to the sea. All of them emerge from the western ghats and flows to the Arabian Sea. These rivers of Kerala are rain fed and non perennial, are found only during the monsoons and at other times they are just streams.
The three rivers which flow out of Kerala towards east are Kabini in the the state of Karnataka and Bhavani and Pambar in the the state of Tamilnadu.
The important rivers and their length out of these 49 rivers in Kerala are
Valapattanam - 110 kilometers
Chaliar - 130 kilometers
Kadalundipuzha - 130 kilometers
Bharathapuzha - 209 kilometers
Chalakudy river - 130 kilometers
Periyar - 244 kilometers
Pamba - 176 kilometers
Achankoil - 128 kilometers
Kalladayar - 121 kilometers
Kallada river - 121 kilometers
Muvattupuzha river - 121 kilometers
Meenachil river - 78 kilometers
Ithikkara river - 56 kilometers
Neyyar river- 56 kilometers