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In terms of length, my guess would be either the Powder or the Big
Horn (which starts as the Popo Agie) in Wyoming and Montana -- esp.
if you also allow for the lower reaches of the Yellowstone after
confluence point.  The total for the Popo Agie-Big Horn-Yellowstone
covers over 5 degrees of latitude--not even the rivers of Alaska's
North Slope can match that.  -- KR
 
Ken Rockwell
"Just another map librarian" at
Marriott Library
University of Utah
Salt Lake City, Utah
 
"Without Geography--you're NOWHERE!"