Lucy Broadwood wrote in Journal of the Folk-Song Society 4 (1913) p.313:

For the majority of readers the obscurity of the versions here printed is increased by the fact that the lover begs "little streamers" to direct him to his true love. That the streamers are people is plain, from the fact that they "walk the meadows gay" and are asked to write to the "true love". Ordinary dictionaries are silent as to this kind of streamer but the word is in constant use, its meaning is given in Wright's Dialect Dictionary, and thanks to its presence in the song and to the preservation of the stream-name as "Nant-si-an" by at least two old people (one of Mr. Baring-Gould's singers in Cornwall and one of the late Mr. H. Hammond's singers in Dorset), I hope to show that the song probably originated in Cornwall, and in the southern part of that county.

No account of Cornish mining omits a description of the ancient practice of "streaming" ore, but Hunt, in his Romances and Drolls of the West of England (1864, etc.) gives a concise explanation of the term. He states that there is scarcely a spot in Cornwall, valley or hill, which has not been worked over by the "old men," as the ancient miners are always called. Every valley has been "streamed" (i.e. the deposits washed for tin). "Wherever the 'streamer' has been ... we are told the' Finician' has been, or the Jew has mined." The Cornish valleys are called bottoms (cf. "At the bottom of this mountain"), and they contain numerous "streamworks" since the running water, by wearing the soil away, leaves the ore exposed and ready to be washed still further. The processes of streaming are dealt with by the late Sir Francis Head who describes the skill with which they are performed by children of seven or eight as well as by older girls and boys (see Murray's Guide to Cornwall, I893, Introduction and pp. 79 and 8o, etc.). This fact at once makes sense of the song-text, corrupt though it may otherwise be. We have here a lover, in some versions apparently home after long absence, asking the young streamers or tinners where his true love now is.