Peter, apologies for making you groan with the accumulated frustration of enduring 200 years of this topic. But re your query:
Looking at the OED, there is indeed a riverine background (=principal current) but it’s probably not very present in most users’ minds today.
Nor, regretfully, is the notion of “mainstream jazz” – originally a fifties style between big band and modern jazz, approximately. But evidently still in use, e.g. on the cover of – contrived excuse for a plug - Joan Davis’ magnificent album “Promiseland” (2005).
You can see the metaphor in development in Arnold: “Byron and Shelley will be long remembered‥for their‥Titanic effort to flow in the main stream of modern literature.”
Ford Madox Ford (1938): “The very considerable influence that Mr. Pound‥exercised on literary mainstreams.”
What the OED entry doesn’t really show is how the word exploded in use around the 1970s (?), mainly in the sphere of mass media and culture, then extended in every direction (politics, economics, cinema...) . That, no doubt, is the direct source of its reintroduction into the poetry world.
At some stage around the 1950s, the idea developed of identifying yourself with the perennial outsiders, and consequently conceiving the mainstream in intrinsically negative terms. I think (though I can’t really prove this right now) that this was a pretty significant change in common conceptions of culture, and it happened in all sorts of cultural fields at around the same time.
1958, E.Crispin: Best SF Three 9 “Main~stream fiction‥has been almost uniformly catatonic in its withdrawal from environment.”
Yet Arnold’s idea of a mainstream not so much as a nasty conventional style but as the desirable and natural destination for any original artist who hopes to truly engage with his society still remains relevant.
There’s a pretty good discussion of this two-sided aspect of “mainstream” here (in connection with Asian cinema):
http://books.google.co.uk/books?id=MCc0DbyLaq0C&pg=PA7&lpg=PA7&dq=mainstream+OED&source=bl&ots=-gjvQnuQVV&sig=eIWbZkVR6md-7Jfi3mHu2A5laec&hl=en&ei=1aeWToarJcqO8gOR0si4BQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCYQ6AEwAA#v=onepage&q=mainstream%20OED&f=false
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