• 03.026 as mountain-streams do of a summer evening,
• 06.039 and he drank from a small mountain-stream that crossed the path,

This word has a proper sub-entry in OED,  where it is both hyphenated and not.

“mountain, n. and adj.” OED Online, Oxford University Press, June 2017, http://www.oed.com/view/Entry/122893. Accessed 19 September 2017.