Beaver are the largest rodents in Yellowstone and can weigh up to sixty pounds. They are primarily nocturnal, so they are difficult to see. They build mud and stick dams across small. slow moving streams and den in the banks of faster and larger streams. Their primary food is the bark of willow, aspen, and cottonwood. They also eat other trees and brush and even rely on pond lilies. Alder is their favorite food if it is abundant. Location: North America.