Periyar River which has a length of 244 kms is the longest river in Kerala. It is a perennial river and is used for supplying drinking water. It is popularly called as the Lifeline of Kerala. Periyar River originates from Sivagiri hills of Western Ghats and flows through the Periyar National Park and reaches the Periyar Lake. The water from here then flows into the Vembanad Lake and then finally to the Arabian Sea. A portion of the water from the lake is diverted through a tunnel into the Vaigai River in Tamil Nadu state. This water is used for supplying drinking water in almost 5 drought affected districts in Tamil Nadu. The Periyar Lake dam is maintained by the Tamil Nadu government based on an agreement between the two states.

The major river in Ernakulam is Periyar and plays an important role in the development of this district. Some of the main tributaries of Periyar are Muthirapuzha, Mullayar, Cheruthoni, Perinjankutti and the Edamala Rivers. The Idukki dam is built on this river and electricity is generated from here. The largest Hydro electricity project called as Idukki Hydro Electric Project is commissioned here. It is a very large dam and the biggest of its kind in Asia. There are some other dams also on this river which includes Neriyamangalam, Mullaperiyar, Pallivasal etc. Recently the river is being polluted due to the discharge of industrial wastes.

There are many important places situated on the banks of this river such as the birthplace of Sankaracharya at Kaladi, pilgrim centers at Malayattoor and Aluva, Thattekad Bird Sanctuary, Thekkady etc.