The yān reading is the common one, almost always in 湮没 (to pass into oblivion) and 湮灭 (to be obliterated; also 'annihilation' in physics). It describes things lost to time, not literally drowned.
The water radical in its three-drop left form, from 水. It carries the core image: to be submerged and silted over by water, which extends to being buried and forgotten by time. It groups 湮 with 沉, 没, and 淹.
phonetic
垔yīn
phonetic component
Supplies the sound (yīn shifting to yān). It also faintly evokes blocking up or filling in, suiting the silting sense, but it works mainly as a pronunciation marker.
