The Mullaperiyar dam is located on the confluence of the Mullayar and Periyar rivers in Kerala’s Idukki district. The dam intends to divert the waters of the west-flowing river Periyar eastward to the arid rain shadow regions of the Tamil Nadu. 

According to a 999-year lease agreement made during the British rule the operational rights were handed over to Tamil Nadu.  So, the dam is operated and maintained by the Tamil Nadu for meeting the drinking water and irrigation requirements of five of its southern districts. 

Source: The Hindu