Historically, domestic dogs(Canis familiaris)have been documented as central features of Intermountain West and Great Plains Native American camps. Some of these dogs were bred specifically for largeness and stamina to haultravoisand to carry pannier-style packs. Ethnographic accounts frequently highlight the importance of dogs in moving through the Intermountain West and the plains, reporting loads as heavy as 45 kg (100 lbs). We calculated body mass from skeletal morphometric data and used these to estimate prehistoric and historic dog load capacities for travois and pannier-style packs in the Intermountain West, Great Plains, and Great Basin. Specimens of large dogs recovered from sites in the Birch Creek Valley in Idaho and on the Great Plains indicate the animals could carry weights comparable to ethnographically recorded loads. Further, direct dating of the Birch Creek dog specimens indicated that dogs of this size have been present in the Intermountain West for more than 3,000 years. These data have important implications for our understanding of prehistoric mobility in the Intermountain West and the plains and suggest that the use of dogs in transporting cargo may have begun as early as 5,000 years ago.