An ice dam forms when heat escaping from the attic melts snow on the roof. The meltwater then refreezes at the colder eaves. Water backs up behind that ridge of ice and can leak under the shingles. Boise's cycle of freezing nights and sunny days makes this a real risk in snowy winters.
Don't chip at the ice with tools; it damages the shingles. A roof rake used from the ground can pull snow off the lower edge. For an active leak, call a pro, who may use steam to remove the ice safely.