Taking a break from reading Big Two-Hearted River by Hemingway at the airport to take a look at these trout photos, imagining what Nick sees in the river.
Never read it but I did a Wikipedia search and located the part you speak of. It's what trout fly fishermen like to see, "trout feeding in the evening light "making circles all down the surface of the water as though it were starting to rain." (Hemingway) I caught these two as I was watching them feed.