The longest river and the one with the largest discharge potential in Kerala, Periyar is one of the few perennial rivers in the region. The Marthandavarma branch of the Periyar flows southwards, before draining into the Vembanad backwaters at Varapuzha.
The Pamba originates in the Western Ghats at an altitude of 1650 metres and flows through Ranni, Kozhenchery, Tiruvalla, Chengannur, Kuttanad, Karthikapally, Ambalappuzha taluks and finally empties into the Vembanad Lake. Kuttanad, an important rice cultivating area in Kerala gets water for irrigation from Pamba.
The Kallada River is one of two major rivers that flow through the Kollam district. It travels for 121 km, flowing through Punalur, Pathanapuram, Kunnathur and Kallada before ending at Ashtamudi Lake. The Chalikkari River also joins Kallada a little downstream and the two flow westward, before falling into the Ashtamudi Lake.