She comes and goes. Each morning it is her face that replaces the darkness (15-16)
Now that the mirror is a lake, we actually see one of those faces that the mirror referred to in the first stanza. It seems that this woman's face has replaced the pink wall in prominence now that the mirror is a lake, though it's not quite a part of the lake's heart. This quote shows that time has passed in this poem, as the woman comes and goes from the lake and returns each morning – but we don't know how long she has been coming.
In me she has drowned a young girl, and in me an old woman Rises toward her day after day, like a terrible fish (17-18)
The entire poem has been building up to this image: the woman seeing time pass in her reflection in the lake. Her youth is dying, and her old age is rising, day after day. Time will pass for this woman, whether she likes it or not, and her reflection will show it.