Woodchucks are found mostly in the Northeast and Midwest. They inhabit burrows in orchards, fields, sloping woodlands, and maybe under your porch. The burrows reach deep into the ground and usually have more than one exit. A woodchuck's main goal in summer is to eat as much as possible in order to store enough fat to carry it through a winter of hibernation. These herbivores enjoy clover and alfalfa, along with many young garden plants such as peas, beans, cole crops, and lettuce. They'll also feed on tree fruits and ripening melons.
Make your yard less attractive to woodchucks by removing brush piles, old stumps, and wood piles. Lure the critters away from your garden by planting favored foods such as clover and alfalfa in another area. Apply repellents such as hot pepper spray, dried blood, predator urine, and talcum powder on plants to discourage woodchucks from feeding. You will need woodchuck-specific fencing to foil these deceptively agile creatures. Trap and remove problem woodies or, if the situation is dire, poison them with gas bombs or tractor exhaust (if you can find and seal off all their holes).
Photograph courtesy of Flickr Creative Commons, qmnonic