Aspen (Populus tremuloides) on the northern Yellowstone winter range has declined over the last half-century. Beaver (Castor canadensis) were reintroduced in Eagle Creek in 1991 in an attempt to reverse this trend. In 2005, we assessed the efficacy of this project by quantifying the long-term effects of beaver on aspen stands and the riparian area in this drainage. Between 1990 and 2005, the canopy cover of mature aspen decreased more than 62%, whereas immature aspen cover more than tripled, resulting in a total aspen canopy cover decrease (p < 0.05) from 43 to 25% (a loss of 7.25 ha). Willow canopy cover increased from 10 to 14% during the same period. The impacts of beaver on aspen stands were estimated by comparing vegetative changes among control sites with less than 10% beaver use (n = 5), active beaver sites (n = 6), sites inactive for 1–3 years (n = 7), sites inactive for 4–6 years (n = 4), and sites inactive for 7–11 years (n = 5). Aspen sprout and sapling densities were greater (p = 0.01) on sites which were active and inactive for 1–3 years compared to the other sites. Aspen ramets were not able to grow taller than 2 m on sites without beaver activity for 4–11 years due to ungulate herbivory. Although beaver stimulated the growth of aspen sprouts and saplings, ungulate herbivory prevented successful aspen recovery in the Eagle Creek drainage of the northern Yellowstone winter range 14 years after beaver reintroduction.