water gate, lock
閘 centers on a controlled passage for water: a gate or lock that manages flow, like in a canal or river.
閘 combines 門 (gate) with 甲, which likely contributes the sound and may relate to the idea of a shell or armor-like closure, suggesting a gated water passage.
A gate 門 with a shell-like 甲 inside: picture a water gate that closes like a clam shell, locking the water in place.
For コウ, imagine a water gate that makes a deep 'koh' sound when it locks shut: koh -> コウ, and the gate controls the flow.