The water radical on the left, the side-form of 水, points to the channel's purpose: carrying water. Ditches, gutters and trenches are essentially small artificial waterways, putting 沟 in the same family as 河, 渠 and 流.
Carries the sound gōu unchanged. The hook-shape sense even helps a little: a drainage ditch curves and hooks across the ground much like the strokes of 勾 itself, which makes it an unusually neat phono-semantic pair.
