Thesis Giving a river legal personhood is the right idea, not a legal gimmick
Where it stands
I teach at a Māori-language school in Rotorua. Since the Whanganui River was recognised in law as a person, Te Awa Tupua, in 2017, my students have found the idea completely obvious, and I've been surprised by how many adults outside our community find it strange or even silly.
The Whanganui iwi have a saying, "Ko au te awa, ko te awa ko au": I am the river and the river is me. For them, the river is an ancestor and a living whole, from the mountains to the sea. The law now names two guardians to speak for it, one appointed by the iwi and one by the Crown. Te Urewera, a former national park, was given a similar status a few years earlier.
My thesis: this is not a gimmick or a metaphor. Legal personhood is how the law recognises that something has interests of its own that can be harmed and that someone must be able to defend in court. Corporations got it for far worse reasons. A river that people have cared for and depended on for centuries deserves at least as much.
I know the objections: rivers can't sue, can't be sued, can't consent; the guardians will really decide; it's sentimental. Make them, and I'll try to answer from tikanga as well as law.