The term postmodernism is a complicated one, and according to this website
http://www.colorado.edu/English/ENGL2012Klages/pomo.html "postmodernism only emerged as an area of academic study since the mid-1980s." The term postmodernism is hard to define, because the concept or idea appears in a wide variety of disciplines and areas of study. Also it is hard to track it down temporally or historically because the exact time when postmodernism began it's not clear. According to this website "http://www.iath.virginia.edu/elab/hfl0242.html", postmodernism is "a rejection of the sovereign autonomous individual with an emphasis upon anarchic collective, anonymous experience." In other words, postmodernism rejects what has been established and makes emphasis on combined revolutionary experiences. Postmodernism can be said it's the "derivate" of modernism; it follows most of the same ideas than modernism but resist the very idea of boundaries.
The story "Winter Count, 1973: Geese, They Flew Over a Storm" written by Barry Holstun Lopez is an example of postmodernism.
Barry Holstun Lopez was born in Port Chester, New York and graduated from the University of Notre Dame in 1966. To explain better why this story is an example of postmodernism we should take a look at the principal ideas of modernism because like I said earlier postmodernism comes from modernism and they share a lot of the same ideas, but postmodernism rejects boundaries between high and low forms of art and also rejects rigid genre distinctions. A list with some of the principal ideas of modernism can be seen here: "http://www.colorado.edu/English/ENGL2012Klages/pomo.html".
Things like the freedom on which the author breaks away from traditionally writing format and puts dates and events, and also sometimes omits some character's names, are the ones that make this story a postmodernism one. As well we can see sentences like "it is too dangerous for everyone to have the same story because...