Edward Thomas starts his dream in comfort and then pulls the ground away. The speaker walks "over known fields with an old friend", and both "known" and "old" promise safety. By the end of the same couplet he has come "sudden to a strange stream", and the poem never really returns to that opening calm. Its subject is the pull of something vast and elemental, strong enough to make a man forget the person beside him. The dreamer says so himself: he was so "bemused" by what he saw that he "neither saw nor sought" his friend "till the end".
The stream is the poem's real invention. It comes "bursting out most bright" from "a great mountain's heart", runs "a short course under the sun", and then plunges "into a pit... once more as black / As at their birth". It is hard not to read this as a picture of a life, with a sudden emergence, a brief spell in the light and a return to darkness. The poem never says so, and that restraint is its strength. The water also contradicts itself in a way that feels truly dreamlike. It is "dark" yet "bright", and the dreamer imagines "how white, had the day shone on them, they were", even though the stream has just been running "under the sun". The brightness seems to belong to the water's own violence rather than to any light falling on it. The verbs "heaving and coiling" make it half-alive, like something muscular and serpentine.
The form is plain, which suits a poem about being overwhelmed. It runs to fourteen lines, the length of a sonnet, but it is built from seven rhyming couplets rather than any sonnet pattern. The lines are loosely iambic and five beats long, and most words are short and everyday: "stream", "light", "black", "roar", "hiss". The couplets could feel tidy, but Thomas keeps breaking the sentences across them. "They ran a short course under the sun, then back / Into a pit they plunged" is the clearest case. The line ends on "back" and the next begins with the drop, so the reader's eye makes the same plunge as the water. Elsewhere the rhymes close each statement with a small click, as in "dream/stream" and "bright/light", and the effect is of a calm voice describing something that is not calm.
Sound carries a good deal of the experience. "The roar and hiss" is onomatopoeic, and "the mighty motion of the abyss" lets the repeated m sounds roll forward like the current. "Abyss" is one of the few longer, more abstract words in a poem of short ones, and it arrives at the moment the dreamer is most absorbed. "Bemused" does double work too. It means bewildered, but it also suggests a man held in a spell, with his attention taken from him.
The last couplet changes the poem's direction. "I awoke from waters unto men" sets the dream's solitude against ordinary company, and the one line of direct speech, "I shall be here some day again", closes the poem in quotation marks. The sentence can be read several ways. It might be a dreamer's simple promise to revisit a marvel. It might be a quieter acknowledgement that the black pit awaits him, and everyone. The poem refuses to choose, and the ambiguity is deliberate. It is also notable that the speaker says this "unto men", to the waking world, rather than to the friend he lost track of. The friend slips out of the poem as quietly as he slipped out of the dream, which carries a faint, unspoken sense of guilt.
The poem is worth reading for exactly this lack of explanation. It doesn't interpret the stream, and it doesn't tell us what the friend means or why the dream matters. It trusts a handful of images, the bright-dark water, the plunge and the forgotten companion, to leave a lasting unease. A reader comes away with the feeling of having been somewhere familiar that turned out to be much larger than expected, which is what many dreams are like.