I saw how He fed his fish.

June 15.

I observe to-night, June 15th, the air over the river by the Leaning Hemlocks filled with myriads of newly fledged insects drifting and falling as it were like snow flakes from the maples, only not so white. Now they drift up the stream, now down, while the river below is dimpled with the fishes rising to swallow the innumerable insects which have fallen [into] it and are struggling with it. 

I saw how He fed his fish. 

They, swimming in the dark nether atmosphere of the river, rose lazily to its surface to swallow such swimmers of the light upper atmosphere as sank to its bottom.

H. D. Thoreau, Journal, June 15, 1850

I saw how He fed his fish. See June 2, 1854 ("When we returned to our boat at 7 p. m., I noticed first, to my surprise, that the river was all alive with leaping fish, their heads seen continually darted above water, and they were large fish, too. Looking up I found that the whole atmosphere over the river was full of shad-flies. It was a great flight of ephemera"). June 9, 1854 ("The air is now full of shad-flies, and there is an incessant sound made by the fishes leaping for their evening meal, dimpling the river like large drops as far as I can see..”); June 8, 1856 (“My boat being by chance at the same place where it was in ’54, I noticed a great flight of ephemera”). June 9, 1856 ("Again, about seven, the ephemera came out, in numbers as many as last night, now many of them coupled, even tripled; and the fishes leap as before."); June 11, 1859 ("When I return, about 5 p. m., the shad-flies swarm over the river in considerable numbers, but there are very few at sundown.")