～沿い
～そい · ～soi · JLPT N2 · noun
along
🏞️ 沿 has the water radical 氵, from following along a river's edge
～沿い comes from the verb 沿う, to run alongside. Attach it to a noun for along that thing: 川沿い means along the river, 線路沿い along the tracks, 海沿い along the coast.
沿 — water 氵 + 㕣 (marsh/valley) phonetic
沿 combines the water radical 氵 with 㕣, depicting a stream flowing through a small valley. Originally it meant to follow along the course of a river, and by extension came to mean 'to follow,' 'along,' or 'beside.' The 㕣 element also gives the sound.