The longest river and one with the largest discharge potential in Kerala, Periyar is one of the few perennial rivers in the region. The Marthandavarma branch of Periyar flows southwards, before draining in to the Vembanad backwaters at Varapuzha.
The Pamba originates in the Western Ghat at an altitude of 1650 metres and flows through Ranni, Kozhanchery, Tiruvalla, Chengannur, Kuttanad, Karthikapally, Ambalapuzha taluks and finally empties in to the Vemband Lake, Kuttand, 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.