spring, fountain-head, source
湶 centers on a natural water source: a spring or fountain-head where water emerges from the ground. The idea extends to any origin or source.
湶 combines 氵 (water) with 泉 (spring), reinforcing the idea of a water source. The character likely developed as a specialized form of 泉.
Water 氵 flows from a spring 泉: 湶 is the source of a stream.
For セン, picture a thousand (千, せん) tiny springs feeding one big fountain: 千 -> セン, and the water gushes forth.