flowing of tears
沱 centers on the flowing of tears, specifically a profuse or streaming flow of tears.
沱 combines 氵 (water) with 它, which likely contributes the sound. The character is associated with the flowing of tears, but the exact historical development is uncertain.
The water radical 氵 beside 它 suggests a stream of water. Picture tears flowing like a river from the eyes.
For タ, imagine a tap of tears flowing: tap -> タ, and the water radical 氵 reinforces the image of flowing tears.
—