Which is the largest river in Kerala?

Periyar is the largest river in Kerala with a length of 244 km. it is one of the few perennial rivers in the region and provides drinking water for several major towns. The Idukki Dam on the Periyar generates a significant proportion of Kerala’s electrical power. Its largest tributaries are the Muthirapuzha River, the Mullayar River, the Cheruthoni River, the Perinjankutti River and the Edamala River. 

Further Reading:

• The Major Indian River Systems
• West Flowing Rivers of Peninsular India