…if you don’t have a notorized copy of your birth certificate to go with the driver’s license, you may be delayed.
From http://www.cbp.gov/xp/cgov/travel/vacation/kbyg/documentary_req.xml
Travel from within the Western Hemisphere (North, Central or South America)
U.S. citizens:
* Are exempt from passport requirements. However, it is highly recommended that a U.S. Citizen be prepared to present documentation such as valid or expired U.S. passport, U.S. birth certificate, or Naturalization certificate to avoid delays.
…
PCT hikers crossing back into the US often have trouble if they just have a driver’s license.
Goat Haunt (the customs place in Glacier) had this blurb to say about coming into the US from Canada:
From: http://www.nps.gov/glac/activities/goath.htm
North Boundary Trail
Waterton Lake Trail to International Boundary / 5.8 mi.
Initial clearing scheduled for: 6/23/06 per 650 Boundary Creek suspension: Installed 5/19/06 per 952 U.S./CANADIAN CITIZENS NOW HAVE UNRESTRICTED ACCESS (DAY USE AND BACKCOUNTRY PERMITS) TO GOAT HAUNT AND BEYOND AFTER FULFILLING REQUIREMENT OF REPORTING TO CUSTOMS RANGERS AT GOAT HAUNT. YOU MUST HAVE A PICTURE ID AND A BIRTH CERTIFICATE OR PASSPORT. NON U.S./CANADIAN CITIZENS ARE ALLOWED TO ENTER GOAT HAUNT FROM WATERTON VIA THE WATERTON SHORELINE CRUISES BUT ARE NOT ALLOWED TO TRAVEL BEYOND THE RANGER STATION.
Long story short - if you just have a driver’s license, crossing back into the us can be a P.I.T.A.
Good luck!
Paul Mags