flowing and engulfing, brimming
沇 centers on water in motion that overflows or spreads: flowing strongly and engulfing, or brimming over. The two given meanings are closely related aspects of water surging beyond its bounds.
沇 combines the water radical 氵 with 允, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with water flowing or brimming over.
Water 氵 beside 允, which looks like a person with legs apart, suggests water spreading out and engulfing the ground. Picture a river flooding and brimming over its banks.
For エン, imagine the water engulfing everything, and the sound of the flood is a deep 'en' echoing across the valley: en -> エン.