I also noted that he used a lot of words that began with the letter S. He used words like stream, slow, sliding, and skimming. This gave me the sense of tranquility and peacefulness. The last and second to last stanza seemed to end those feelings and jar me back to a reality. He used words like plunge, drown, dry and ache.
The choice of words made it more formal to me. I had to look up some of the words like foundered, precipice and slake. He could have chosen to use fallen, abyss or quench instead. Although the words he chose do sound better to the ear and of course add to the rhyming pattern within the poem.
The poem itself seems soft and slow (with the exception of the last two stanzas, which I think is done on purpose.) The words he uses to describe the scene make it very vivid and you can almost see it as though you are the one in front of that sliding glass. I felt like I could see that trout swimming gracefully through those serene waters and the dragon flies minding their own