weir
栫 centers on a weir, a barrier built across a river to control water flow, often made of wood.
栫 combines 木 (tree, wood) with 存, which likely contributes a phonetic element. The character is used for a wooden weir or barrier in water.
A wooden 木 structure that exists 存 across a river: a weir made of timber.
For セン, imagine a weir made of a thousand 千 (せん) wooden stakes: 千 -> セン, and the wood 木 forms the barrier.
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