Prime Minister Manmohan Singh had declared the ______ as India's first national river with a separate river basin authority to monitor its cleanliness in the states through which it flows.

Prime Minister Manmohan Singh declared the Ganga as India's first national river in 2008, establishing the National Ganga River Basin Authority to monitor its cleanliness and conservation. This recognized the Ganga's cultural and ecological importance as India's holiest river. Other major rivers like Yamuna, Brahmaputra, and Narmada were not given this national river status.