沿
エン, そ.う, ぞ.い
run alongside, follow along, run along, lie along
JLPT1 G6 S8 F1121
As per 水# (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]