Woodchucks or groundhog (Marmota monax)

Size:
20–27″ long, excluding tail
5–12 pounds.

Signs of their presence:
Adults often seen basking in the sun, in a grassy area, on a fence post, stone wall, large rock, or fallen log—always near its burrow.

Sounds: Occasional sharp whistles and low churrs, given at times of danger.

Odor is distinctive. Will often see flies around an active burrow.

Scat: Rarely seen (woodchucks excavate a privy off their main burrow).

Evidence of their feeding: Chewed wood. Chewing on fresh plants similar to that of rabbits; difficult to pin on woodchucks without supporting evidence.

Dens: Will see a large mound of dirt and stones by the main entrance to their burrow; the secondary entrances, which were dug from the inside, generally don’t have a dirt mound by their opening. Well-worn trail from entrance to entrance, or to the garden.
Common nuisance situations:

Time of year: Peak in July and August, although their damage may begin in spring and last into the fall.
Feeding, or just filing down their front teeth, which never stop growing. Woodchucks raid gardens, fields, lawns, orchards, nurseries, and may gnaw or claw on shrubs and fruit trees. Occasionally chew on outdoor furniture, decks, and siding while scent-marking or filing their teeth.

Marking their territories: They may strip off the bark at the base of a tree that’s near their burrow entrance.

Burrowing. Look for burrow entrances among shrubs near vegetable and ornamental gardens; under woodpiles, brush piles, and stone walls; under sheds, porches, decks, and crawl spaces. Burrows in fields may damage agricultural equipment, while those in pastures may trip livestock, resulting in injuries.

Disease risks: Low. Mange, rabies (rarely), raccoon roundworm.
For more information:
http://www.nwco.net/08-AppendixBNuisanceSpecies/8-14-Woodchucks.asp