In 2008, The River Why was adapted into a "low-budget film" of the same name starring William Hurt and Amber Heard. Since April 30, 2008, the film rights to The River Why have become the subject of a lawsuit by Duncan alleging copyright infringement, among other issues.

Duncan has written a collection of short stories, River Teeth (1996, ISBN 0553378279), and a memoir of sorts, My Story As Told By Water (2001, ISBN 1578050839). His latest work is God Laughs and Plays: Churchless Sermons in Response to the Preachments of the Fundamentalist Right, published in 2006 (ISBN 0977717003).

An essay, "Bird Watching as a Blood Sport", appeared in Harper's Magazine in 1998; Duncan wrote the foreword to Thoreau on Water: Reflecting Heaven (2001, ISBN 0395953863).

An essay, "A Mickey Mantle koan: The obstinate grip of an autographed baseball" appeared in Harper's Magazine in 1992.

Duncan was born in Portland, Oregon and lives in Lolo in Missoula County, Montana. He has written op-ed pieces in support of preservation of Montana's Blackfoot River.