In those lines and elsewhere in the poem, we're meant to imagine a strange and lush landscape, an otherworldly one in which the very ground seems to pant exuberantly. For that reason, it's worthwhile to read this poem aloud and hear the sound devices that help convey the startling images of nature in the poem's setting.
Line 17 reads: "And from this chasm, with ceaseless turmoil seething." You'll hear the alliteration in "ceaseless" and "seething," a repetition of sibilant sounds that echoes the rumbling of the chasm itself, which is about to explode in a fountain of rock and water. This chasm is supposed to be savage, magical, holy, and romantic all at the same time, nearly an assault on the senses and the imagination, so it's logical for the poet to mirror all this chaotic cacophony in harsh, repeated consonants.
Here's line 25: "Five miles meandering with a mazy motion." Again, the chief sound device used here is alliteration: the repetition of the consonant "m" sound in "miles," "meandering," "mazy," and "motion." Very unlike the harsh sibilant "s" discussed above, this "m" sound is smooth, voiced without a hiss but with both lips, easily protracted, and indicative of the back-and-forth motion of the labyrinthine (mazelike, or "mazy") river itself.
Both of the lines above also feature jagged, uneven meters, with repeated waves of accented and unaccented syllables toward the end of the lines: hear the capitalized syllables as slightly louder and more emphasized as you read "CEASEless TURmoil SEETHing" and "WITH a MAzy MOtion." The effect is dizzying and cacophonous, appropriate for portraying the loud rumbling of the chasm and the spurting of the river.
Taken all together, these sound devices emphasize the surreal, chaotic, grand nature of the "romantic chasm" and the "sacred river," and they also help establish the poem's imaginative, lavish, dramatic mood.