Translocated Rocky Mountain wapiti (Cervus elaphus nelsoni) released in the southern Cascade Range of Washington State in the early twentieth century hybridized with resident Roosevelt wapiti (C. e. roosevelti). Archaeological remains of wapiti from this area dating between 1400 A.D. and 1835 A.D. are significantly larger than both subspecies, but bones of descendant hybrid wapiti killed by the 18 May 1980 volcanic eruption of Mount St. Helens are intermediate in size between the two subspecies. Deer (Odocoileus sp.) remains from the same archaeological sites as the wapiti remains are no different in size than bones of modern deer, indicating size change in wapiti was not environmentally driven. Anthropogenically facilitated hybridization supplemented and likely saved the local population from extirpation but resulted in smaller individual animals.