The Danube to the Severn gave The darken'd heart that beat no more; They laid him by the pleasant shore, And in the hearing of the wave.
There twice a day the Severn fills; The salt sea-water passes by, And hushes half the babbling Wye, And makes a silence in the hills.
The Wye is hush'd nor moved along, And hush'd my deepest grief of all, When fill'd with tears that cannot fall, I brim with sorrow drowning song.
The tide flows down, the wave again Is vocal in its wooded walls; My deeper anguish also falls, And I can speak a little then.
Tennyson seems to be ruminating again on Arthur's death. He notes that the ship came down the Danube to the Severn, so from somewhere in Europe (where the Danube river flows) and into England (which is where the Severn and Wye rivers are).
As he watches the river flow along, he can't cry because he's in his "deepest grief of all."
This, to him, is like drowning.
When the tide goes out, though, his sadness lessens somewhat and he is able to speak.