The NYS DEC website provides a wealth of information about the beaver, at http://www.dec.ny.gov/animals/63052.html#Ecology:
The beaver’s diet consists almost entirely of cellulose in the form of woody plant material. Woody plants are nearly indigestible to mammals, so digestion is aided by microorganisms inhabiting the small intestine. Beavers eat the leaves, bark and twigs of trees such as aspen, willow and red maple and a variety of herbaceous plants. During summer months, their dietary preferences may shift to aquatic vegetation including water lilies and rhizomes from shoreline ferns. As winter draws near, beaver may collect and store or cache food items underwater near the entrance to their lodge in what is termed a ‘raft’ or ‘feedpile’ for use throughout the winter. This food store is imperative for survival when thick ice prevents access to fresh food during New York’s long winters.