Watatsumi no umi ni idetaru Shikamagawa taemu hi ni koso a ga koi yamame

わたつみの海に出でたる飾磨川
絶えむ日にこそ我が恋やまめ

Roma-ji: Watatsumi no umi ni idetaru Shikamagawa taemu hi ni koso a ga koi yamame
English Translation: Into the great sea
the waters pour out,
the Shikama River;
only on the day it fails
will my love come to an end
Meaning: The Shikama River flows out into the great sea; only on the day its waters cease will my love come to an end.
Commentary: Book Fifteen Poems of the Envoys to Silla 3605

Headnote continued from the preceding poem: "Old poems recited to suit the place"
A love poem declaring that only when the day comes that the Shikama River, flowing into the sea, runs dry, will his love come to an end. Invoking the river's ceaseless flow, it vows that the love will never end. The Shikama River is in Harima Province, thought to be the Senba River flowing through present-day Himeji in Hyōgo Prefecture, and the poem was probably recited as a song of a place on the embassy's route. It is the last of the three poems the left note calls love poems.
According to the headnote, 3602 to 3610 of this book are old poems the travelers recited along the way as suited to the place. Poems not of their own making were thus taken into the travel sequence, showing how people of the time shared old poems and adapted them to the occasion.

First line, "watatsumi no": "watatsumi" is the god of the sea, used here as a word leading into "umi" (sea).
Second line, "umi ni idetaru": "taru" is the attributive of the perfective "tari": flowing out into the sea.
Third line, "Shikamagawa": a river of Harima Province, thought to be the Senba River in present-day Himeji. The lines up to here present the river's ceaseless flow.
Fourth line, "taemu hi ni koso": "koso" is a binding particle forming kakarimusubi with the realis "yamame" in the last line. The condition is the impossible day when the river ceases to flow.
Fifth line, "a ga koi yamame": "me" is the realis of the auxiliary "mu," answering "koso": only then will my love end. Turned around, it becomes a vow that the love will never cease.
Source: Manyoshu
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