From Periyar Thekkady lake and reservoir, some water is diverted eastwards to Tamil Nadu via a tunnel. The diverted water, after power generation, is let into the Suruliar river (a tributary of the Vaigai River) thereby resulting in an inter-basin transfer of water. Downstream of the Mullaperiyar dam, the Periyar continues to flow northwestward for , exiting the Periyar Tiger Reserve and passing through Vandiperiyar, Elappara and Ayyappancoil to the Idukki reservoir formed by the Idukki, Cheruthoni and Kulamavu dams. The Idukki dam is constructed across the Periyar river on the famous Idukki gorge formed by the Kuravan and Kurathi hills.