This near six-hour film, which takes the form of a three-act opera, was inspired by Norman Mailer's 700-page historical novel Ancient Evenings (1983). Mailer's novel, which met with critical derision on its release, deals with ancient Egyptian history and mythology, death and reincarnation, and sexuality. Featuring a cast including Maggie Gyllenhaal, Elaine Stritch, Salman Rushdie, Debbie Harry, YouTube "beatboxer" Ryan Robinson and Paul Giamatti, River of Fundament draws upon and reconstructs Ancient Evenings, simultaneously incorporating the modern history of the rise and fall of the American car industry. Automotive processes are presented with what art critic Adrian Searle calls "the character of rites and rituals," such as scenes in which a car is dismembered in a Los Angeles showroom, and subsequently smelted in Detroit.

The film also includes a number of explicit sexual scenes that Searle describes as "hard to take". In one scene, a man, whose penis is covered in human excrement and gold leaf, engages in anal sex. In another, a woman copulates with a car engine, and in yet another, two women engage in sexual play with a dildo that appears to be made of feces. For both Mailer and Barney, feces represent both death and fertility. Barney cites Maggie Nelson's 2011 book Art of Cruelty as a significant influence upon his view of such "extreme images", explaining that "I think my interest in [explicit scenes] has nothing at all to do with political provocation. I think it has to do with taking a kind of fundamental, albeit extreme, action and trying to naturalize it into the context of the narrative, or into what's happening on screen. And so in some way, it's about trying to remove its shock value".

Art critic Sebastian Smee described the film as "disgusting, dazzling and weirdly great" but that, ultimately, the film was "close to unbearable". He argued that people were often uneasy about celebrating Barney's work because it came with "a realization that giving oneself over to his aesthetic was like joining a cult" [...] If you believe, it's like baptism: You are absorbed into an intellectually stimulating subculture where everything makes more and more sense the deeper you delve. If you don't - if you think the work solipsistic, its relationship to reality fragile - you save yourself a lot of trouble. But you also miss out on the fun".