Complex sentence: The Ponmudi Dam, in the Idukki district, State of Kerala, India, is a hydroelectric project constructed across the Panniar River, a tributary of the Periyar River which is the longest river in Kerala.
Split into simpler sentences: The Ponmudi Dam, in the Idukki district, State of Kerala, India. It's a hydroelectric project constructed across the Panniar River, a tributary of the Periyar River which is the longest river in Kerala.