The question posed by the title of celebrated British nature writer Robert Macfarlane’s keenly felt study of rivers is answered in the first few pages by his nine-year-old son: “Duh.” Yet, self-evident values are not always treated self evidentally. In 1971, when law professor Christopher Stone proposed rivers and other natural elements might have legal rights, he was ridiculed. Five decades later, Ecuador became the first country to recognize these rights formally, and that’s where Macfarlane begins his search for the animating force behind running waters. As he travels from South America to India to eastern Canada and back home to England, accompanied by guides who are themselves characters, Macfarlane finds people who are radically reimagining what a river can be—and, more provocatively, exploring the exciting consequences from such revolutionary thought.