Nothing in the poem tells us exactly what happens, and that is part of its effect. It begins with calm, small images: ripples at a sluice gate, barges loitering in sparkling light, moments flowing past like water. Time seems to drift here, almost pleasantly.
Then comes the word "Crack," and the scene breaks. The land itself "jitters," and the next image is much harsher: a "gashed-black scalp" that "refluxes," as if something dark is welling up or flowing backward. The poem seems to move from outward scenery to something bodily and painful. The final line turns to "you," who is reduced to a whimper.
Judging by the imagery, the speaker watches this with unease, perhaps with sympathy for the person addressed. The mood shifts from still and glittering to tense and shaken. The short, clipped lines make the interruption feel sudden, like a moment of peace giving way to shock or distress.