ditch, drain, narrow waterway
沟 centers on a narrow, man-made channel for water: a ditch, drain, or small waterway used to direct flow.
沟 combines 氵 (water) with 勾, which likely contributes the sound and possibly the idea of a curved or enclosed shape, suggesting a water channel.
Water 氵 flows through a curved, enclosed 勾 shape: a narrow ditch or drain.
For コウ, imagine a narrow ditch that curves like a コ (ko) shape, and water flows through it with a long ウ (u) sound: コウ.
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