Increasing air temperatures are changing the arctic tundra. Permafrost is thawing, snow duration is decreasing, shrubs are proliferating, and beavers are colonizing the tundra of northwest Alaska. Beaver ponds are warming stream water and thawing permafrost, while impacts to fish and stream biology are unknown. Beavers create dynamic wetlands and are agents of disturbance that appear to be enhancing the ecological responses to warming in the Arctic. In this talk, I will discuss patterns of tundra beaver colonization and consider how this ecosystem engineer might reshape stream and riparian ecosystems.