Mountains tend to preserve culture, rivers tend to spread culture. Though mountains still shield pockets of tradition, I think that rivers no longer hold the sway that the once had.

Beyond that, I think that that "Great American River of Song" concept is problematic--the great movement of culture in the US has been from East to West. The fact that cities along the Mississippi have had active music cultures at various times likely has as much or more to do with the fact that the river divided East and West, and music spread laterally from them, rather than that the river itself was a conduit for music.

A small example is that the medicine show circuits didn't follow the river--