flowing water
洊 centers on water in motion: flowing water, a current, or a stream. The single given meaning is straightforward and concrete.
洊 combines 氵 (water) with 存, which likely contributes the sound. The exact historical development is uncertain, but the water radical clearly ties the character to flowing water.
The left side is water 氵, and the right side 存 looks like a person 亻 beside a child 子, suggesting water that keeps moving like a parent guiding a child along a stream.
For セン, picture a flowing stream with a thousand (千, せん) tiny ripples: 千 -> セン, and the water keeps flowing.