Abstract. Tridactyl bird footprints preserved in Lower Eocene sandstone of the Chuckanut Formation in Whatcom County, Washington, USA, were made by a species of giant ground bird that walked along the subtropical lowland riverbank. The morphology and age of the tracks suggest the track maker was Diatryma (? = Gastornis). Although these birds have long been considered to be predators or scavengers, the absence of raptor-like claws supports earlier suggestions that they were herbivores. The Chuckanut tracks are herein named as Rivavipes giganteus ichnogenus and ichnospecies nov., inferred to belong to the extinct family Gastornithidae.