Either Bombus vosnesenskii, Yellow-faced Bumblebee:
http://bugguide.net/node/view/19538
or Bombus caliginosus. I am not sure of the differences so I am leaving it at that. I think there might be habitat and range differences.
http://bugguide.net/node/view/347459 (scroll down for descriptions of range and physical differences).
Hope this helps!
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