沿
エン, そ.う, ぞ.い

run alongside, follow along, run along, lie along

JLPT1 G6 S8 F1121

As per # (long, winding flow of water) + an obsolete character indicating a ravine (compare 谷) → water splitting into branches as it follows/runs along a winding, prescribed course in a ravine → *run along*; *lie along*; *follow* (a custom/rule).

[Full Kanji Etymology List]