irrigation ditch, trench, river
澮 centers on a man-made water channel: an irrigation ditch or trench that directs water, and by extension a river or stream that serves a similar purpose.
澮 combines 氵 (water) with 會, which likely contributes the sound and possibly the idea of gathering or meeting, as water gathers in a ditch.
Water 氵 meets 會 (gathering) to form a channel: picture water gathering in an irrigation ditch or trench.
For カイ, imagine a kayak gliding along the irrigation ditch: kayak -> カイ, and the water channel carries it forward.
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