In this paper, we conceptualise rivers as an assemblage of physical, social and metaphysical properties, knowledges and relations that connects people, places and waters with the spiritual dimension. We are particularly interested in taniwha—supernatural creatures that inhabit rivers, lakes, or caves—whom, we argue, may be seen as symbols of the mythical and metaphorical embodiment of the relations between Māori and their rivers. We address the social nature of rivers and waters and how taniwha affect, and are affected by, interactions with humans and other nonhumans. Based on our encounters with Waiwaia, a taniwha and kaitiaki (guardian) of one of the authors’ iwi (tribe), we argue that revitalising rivers, rather than merely restoring or rehabilitating, attends to the wider relational as well as material dimensions of rivers. Our approach to reimagining ontologies of rivers is timely following settlements under the Treaty of Waitangi Act 1975, which are reconfiguring the institutional arrangements for governing rivers in New Zealand.